Danger: RESISTOR SPARK PLUGS
 

:book:Resistor spark plugs can NOT be used in many MB engine applications..
Follow Mercedes Benz specification on what spark plug to use.

:eek:What spark plug is in your engine at this moment, please remove one to check the number personally, if it has an "R" = resistor.

Do not trust service records on this.
Bad wires, cap, rotor and/or wrong spark plugs can wreck your ignition module, most applications this = $2,000.00 - $3,000.00 for a new one. :eek:

If you know your MB part number, here are links to verified Bosch Non Resistor Spark Plugs
http://www.peachparts.com/shopforum/mercedes-parts-reference-library/181047-bosch-non-resistor-spark-plugs.html


Call or e-mail your VIN# to check application.

whats wrong with them?

As the name ( Resistor ) implies, the resistance & load that is placed on all ignition components, with these plugs, could seriously damage the ignition module.
In simple terms, if you overload an electrical circuit in your house, you'll end up blowing a fuse/breaker.
In this case the ignition module is your fuse/breaker, but a very expensive one.;)

That is not cool... Does this also apply to older mercs? ie '77 and back?

It sure does. No older Mercedes should have a resistor plug - resistance is built into the spark plug boot.

I have to challenge the thinking in this thread.

The previous statement:

Is incorrect as it applies to ignition circuits of Mercedes Benz vehicles. I can prove it. As the resistance in a circuit goes up the "load" or current goes down.

Would the originator of this thread care to share with us the scientific or factual basis for his statements;

If anybody wants to get into technical discussions on why I don't agree with this thinking I would be happy to engage them.

Well, I for one don't KNOW why resistor plugs would or would not ruin an ignition module, but I can certainly dream up a technical argument for the damaging scenario...

The coil output must drop across the spark plug gap with enough initial voltage to jump the plug gap. A plasma is then created which drops the resistance of the air / fuel vapor to near 0 Ohms, allowing a lower source voltage from the coil to push enough current across the gap to complete the ignition process. If the series resistance of the wired path is too high, the lowered energy flow rate to the gap could be insufficient to sustain the plasma, and the spark will die. For the remainder of the spark cycle, the full output of the coil will then appear across the coil terminals, risking internal breakdown over time. Now I don't know that this would actually occur if resistor plugs are used, but I would like to hear the technical reason refuting this possibility.

Steve

Answer:
 

Personal/professional experience and MB TSB/public announcement data on this issue is fact.

I do not have time to dig up all of the data.

If you desire to discuss your opinion, please start another thread.

Well, if you or someone else can get some time to dig up the TSB, I would love to see it.

I called 4 MB dealers in Houston and none of them could remember anything describing the TSB mentioned here. This does not prove that it does or does not exist, just they had no memory of anything related to the use of resistor plugs being detrimental to the ICU.

Owning 5 vehicles, and having resistor plugs and wires, I am concerned about the possibility or risk to the ICU. I would like to know for sure. I think everyone else would also.

Certainly someone here has access to the TSB's and can confirm this one way or the other. Is the TSB specific to all cars or only to certain years / models? We need to know.

For me personally, I am not concerned. But I would like to know the official word on the subject.

Presence of a TSB (supposed) indicates Mercedes has a solution and you can get proper plugs or plug/wire combinations at your dealer.

It's patently ridiculous to think anything else.

And, I'd be surprised if Mercedes' solution doesn't include Bosch plugs.

The first time ever changed plugs on any Mercedes was with my first MB, the 97 C280. I put resistors in. Mistake. Did not run well. I thought I broke an insulator or something (kinda hard to do when the things screw straight down from the top). So I bought another set! Then I figured something was wrong with my plug choice.

Then I found mercedesshop, read all about as a lurker back then, and went to the dealer and bought the correct plugs. Ran as it should after that.

As for plasma, etc, I don't know, but the correct plugs make things right. There isn't too much you can do on these cars cheaply that will result in improvement, as I have found out after owning these things for 10 years now.

sbourg

You explained it pretty well.

Waybomb

FWIW, my 2000 C 230 K had Bosch F7 KTRC sparkplugs OEM.
Notice the " R ", it's a resistor plug.
Life just ain't that simple anymore. ;)

Guys, I found you a great site: http://www.nightrider.com/biketech/faq_ignition_wires.htm

All I was saying is use the correct plugs for your car. If your car's service manual indicates the use of a F7KTRC plug, then use that plug. Don't try to make it "better".

Awww, I think this thread is still the place to discuss the subject. As an EE I would like to know the reason for an announcement, to see if its based on technical or other basis. It could be a simple warning to keep sales up on MB instead of aftermarket parts. I think its pretty rude to try suggesting starting another thread. Tech help matters are open to discussion too aren't they????

Show us the details, Mr Hunter

Yes and NGK's work better in older models that can accept the equivalents. And Champion resistor plugs are indeed very unreliable and have high failure rates even out of the box, based on my experience.
But we aren't talking experience here its what Benz has to say in some obscure bulletin that is hearsay at the present time.

Lets have some technical reasons for resistor vs. non resistor plugs. Its going to be sticky because the types of ignition systems differ radically over the past say 5 decades. I know some ign systems fault when they see a short, and putting plugs with platinum coated tips cause faults or fault codes to come up because the pt flakes off over time (bad idea IMHO, just a way to sell more expensive plugs to fools who are eager to part with their money).

Are you seeing what is involved here, its more than just whether or not a plug may have a resistor in it or not.

I have also been wondering about what models/years fall under the no resistor rules. All I know is my 94 e420 gets a smooth 27-29mpg with the Bosch P4s, and a rough running, hard starting 23-24mpg with the "recommended" copper bosches.

If the P4s are both a gimmick and a resistor type, how is what I am experiencing possible? It should be the other way around. My car also came with P4s when I bought it at 82K. It now has 112K.

Shouldn't my ignition module have broke by now from the extra resistance?

That's right, and the only experience that's worth two cents is that of Mercedes/Bosch engineers and I'm still waiting to to what they've published.

Yeah, use non resistor plugs. Itll run sooooo much better.


My car had a slight miss at idle, new non resistor plug fixed it.

I believe the M119 and the late M104 engines are less susceptible to the resistor issue. Both engines are a double overhead cam design with 4 valves per cylinder and variable valve timing on the intake side with fully electronic and integrated ignitions. One coil for two cylinders. There is a lot of adjustability in these engines, not as much as the newer engines MB has now but I would say enough computer control to compensate and benefit from platinum resistor plugs.

The M103 and M102 engines on the other hand are traditional single coil designs with no variable timing. Resistor plugs do cause issues especially if the coil is an old (original) one. But this is just based on my experience, no technical document or lab test was performed lol :D

This makes for some interesting reading & it's in layman's terms:
http://www.ultralightnews.com/enginetroublshooting/resistorcapsandplugs.htm

The problem with resistor plugs is not the current but the voltage. The firing voltage may get too high and damage ignition system components.

Huh?!? Wanna run that one by us again? Please explain.

When resistor plug used, you add the resistance of the plug to the total resistance already there - in the plug connector and in the rotor or cover (not sure were the resistor on the distributor side is). The resistance of the path from the coil to the plug will be higher than normal. This high resistance puts additional demand on the secondary voltage to initiate and support the spark. It is like having a bad high voltage wire.
Mike

True.

True.

False. I’m sorry Mike, but the addition of resistance “in series” to the secondary side of any transformer (which is what a simple automotive coil is) does not place additional demand on the secondary voltage. This simply causes less current to flow from the secondary side of the coil. In essence, less spark energy would be created. If anything, it would be less stressful on the coil because the increased resistance from secondary through the high voltage wire through the resistor spark plug allows less current to flow. Less secondary current flow equates to less energy being coupled through the coil from the primary side. The coil runs cooler and less stressful.

To illustrate through an extreme case, imagine removing all the high voltage wires from your engine and attempting to start it. Of course it will never fire, but if the battery could stay charged forever and you kept cranking the starter, you would never see current flow from the secondary – it’s impossible… no path for the current. The coil would last forever because there is no current being generated in the secondary. In fact, the primary side is also protected, but without getting into electrical/magnetic theory, you’ll have to accept my word on it.

All automotive coils generate energy by allowing 12v current to be pulsed through their primary windings. The pulsing action (timed to the when the spark needs to fire) transfers energy to the secondary windings of the coil. The ‘voltage’ created on the secondary side of the coil is in direct relation to the ratio of wire windings of the primary to the secondary. The amount of energy transferred is completely dependent upon the "resistance to ground" of the secondary circuit.

In a perfect zero resistance scenario, the energy builds in the coil and the output voltage of the secondary rises rapidly. When sufficient energy is achieved to overcome the spark gap, the gap arcs and suddenly there is current flow until the energy in the coil drains enough to not sustain an arc. The spark gap – only when arcing – is considered a resistor and has a certain limiting effect on the high voltage current. It is important to remember that other than when arcing, the spark gap offers infinite resistance. The current flow is indirectly proportional to the resistance – higher resistance to the spark gap, less current. Thus, resistor plugs should create a weaker spark, but be less stressful on the coil. It is this 'weaker spark' that greatly reduces the infamous radio interference noise.

However, your example of a bad high voltage wire is valid for coil stress, but for a very different reason. When a high voltage cable begins to fail, it allows current through/across its insulator jacket. This equates to current flow directly to ground, so a portion of the secondary energy bypasses the spark plug gap. This means that the coil will produce more current (i.e.: work harder) to generate sufficient energy for the spark. In extreme cases, enough secondary energy may be drained off through the insulating jacket to the extent that the sparking arc may not be sufficient for initiating a complete fuel/air mixture burn – i.e.: misfiring. This scenario can cause stress on the coil, secondary side, in particular. Whether that stress causes ignition module issues is debatable and is very dependent upon the electrical engineering design of the ignition module.

It is a common misconception that increased spark plug resistance causes coil stress. High voltage cable breakdown does. A partially shorted spark plug does. But spark plug internal resistance should lower spark current, not increase it.

Simple.
The resistance is the airgap between the rotor and the distributor cap. ;)

No, there is a resistor in the rotor, about 1.2k. Just checked it on my old rotor from M120 engine.
Mike

This is missing the point - less current flow at some point will not sustain the plasma at the plug tip, where the resistance will jump from near 0 Ohms to no connection - open circuit, infinite Ohms. The coil voltage output at this point of the discharge cycle may be high enough to damage its insulation dielectric, but insufficient to re-create a plasma. With less series resistance initially, plasma would be sustained due to higher current flow, until coil output voltage falls to a safe level.

This speculation is simply to show that increasing secondary resistance at some point will likely cause a problem. Don't screw with the engineered parameters!

Steve

This is not a good example. You won't get any sparks in this case. Now if you remove the wire from the plug a move it away a little to create a second spark gap, you'll see secondary voltage demand increase to bridge both gaps. In your example the secondary voltage on the coil will be very high and could damage the coil insulation.
I'm not talking about bad insulation. The bad wire in terms of increased resistance. This is very unlikely with MB copper wires, but could happened to wires with distributed resistance.
I agree the the resistance has no effect on initiating the spark, but it will put higher demand on the secondary voltage to sustain the spark.

With all due respect, I'm not sure I agree with your line of reasoning. The secondary winding will have stored the maximum amount of energy for that duty cycle, immediately at the moment of spark ignition. Then, the energy is drained off the secondary at a rate that is defined by the total resistance to ground. Assuming for this discussion that the high voltage wires do not have insulation leakage, the energy in the secondary winding drops for the duration of the spark until there is not enough energy to sustain a spark. In essence, the secondary winding 'sees' maximum voltage and energy potential at the instant before spark ignition - regardless of type of spark plug used. In other words, once a spark starts, there is always less energy remaining in the secondary.

In the real world, there is leakage through high voltage cable insulation. In this case, energy begins to drain from the secondary winding immediately, through a continual resistive path in parallel to the spark gap, even before the spark is initiated. Thus, less energy is presented to the spark gap. This is why old high voltage cables cause misfiring (i.e.: weak spark).

In an additional real world scenario, aging rotors/caps increase the resistance from the secondary winding to the spark gap – just like adding resistor plugs. As we know, the carbon build-up and erosion of the rotor/cap interface can add substantial resistance to the spark circuit. Now we know that aging rotors/caps are not good for performance, but I have never heard of old rotor/cap taking out a coil. The vehicle may run like poo because of a really bad rotor/cap, but the coil is basically undamaged. I am not a professional mechanic and invite real examples, if they exist.

However, you are correct in that adding more resistance would create a weaker spark. (If I mis-paraphrased your thoughts, please correct me.) But the secondary windings have long since seen the maximum amount of energy by the time the spark extinguishes. Since the elusive TSB cannot be presented for whatever reason, I believe this is the most probable line of reasoning for not using resistor plugs – a weaker spark. How that can cause damaged ignition modules/components I do not know. But I’m keeping an open mind and continue to think about it and question it.

Cheers,
Jeff

I have a little sence on this, I know a german mecanic that works on volvos and Mercedes exclusively, and he told me to run AC A8,s So I tried them and my 92 300E runs so quiet I cant even hear the damn thing run.

You are correct; except for one point... the coil is nothing more than a transformer. The voltage generated on the secondary is related to the ratio of primary windings to secondary windings and the voltage supplied to the primary. Thus, the secondary voltage can only reach whatever the coil's electrical design is. Running voltage is something less because the spark gap fires before the secondary voltage maximum is reached. We cannot exceed this UNLESS the primary voltage is increased. And the maximum secondary voltage is what the coil is rated for.

You are also correct in that moving the high voltage cable to create a third gap will 'force' the secondary winding to generate more voltage - no question. But the voltage can never exceed what the coil is designed for. What you are seeing is normal. But the coil (all things being equal; see below) will easily be able to handle this scenario. Remember, coils are designed well in excess of worse-case scenarios anyway.

It is, of course, possible for coils to fail as you describe, but not for the reasons you describe. The coil may have a minor manufacturing flaw in the internal insulating material, or wire used in the windings may be at the lower end of the specification, or a poor internal connection, etc. Any of these could be susceptible to premature failure, under the right conditions. But a good, solid, well-in-spec coil should not fail under your example.

Cheers,
Jeff

Yes, and the airgap is an additional resistance. ;)

Anybody read this thread?

http://www.peachparts.com/shopforum/tech-help/180933-why-non-resistors-plugs.html?highlight=resistance+waste+spark

Also, relative to Platinum plugs, I did find this:

In the Technical & Restoration Forum in the March / April 2003 The Star Magazine, there is a question about platinum plugs and an interesting reply. It doesn't say who exactly wrote the reply, but it was probably Stu Ritter:

After I put Bosch Platinum +4 spark plugs in my 190E 2.6, a dealership advisor reprimanded me, saying they aren't recommended by Mercedes-Benz. He said that a service bulletin indicated that they should not be used. Is this so?

Some time ago a Mercedes-Benz service bulletin recommended against using fine single-wire platinum electrode spark plugs in their engines. The electrode on these plugs was so thin that the spark could be blown off the plug, causing misfires. The new Bosch +4 spark plugs are not covered by that old bulletin. Thin-wire plugs could cause poor idle quality and misfires at high rpm under full throttle. Mercedes-Benz has not recommended against the newer +4 plugs, which have four electrodes so are not about to have the spark blown off. You might tell your service advisor that serval versions of Bosch platinum plugs exist.

Good finds! Is it possible through legend and folklore that the real issue – do not use fine-wire platinum plugs – evolved into ‘do not use resistor plugs’. Maybe the logic went like this… MB says ‘do not use fine-wire platinum plugs’. Now we all know that the vast majority of fine-wire platinum plugs are also resistor plugs. So maybe to keep things easy, people evolved the warning to include resistor plugs – if you do not use resistor plugs, you’ll never use a fine-wire platinum plug. Could explain a whole lot.

Cheers,
Jeff

I am not an engineer of any sort, so I can't comment on whether or not using resistor plugs will damage equipment, but I don't see how everyone that has posted before me on this thread can argue that it is not a bad thing to increase the resistance by using resistor plugs.

Could not find any TSB on this subject, other ideas
 

This has been an interesting discussion. Because so many report poor running with resistor plugs, there must be a reason. Whatever this is, I'll add a few statements to the discussion.

1. I did a search in AllData for any Mercedes TSB relating to spark plugs. The only one I saw was for a change in the heat ratings. Nothing relating to resistor plugs in the older MBZ. I used my '86 300E as the search vehicle. This is not to say that there might not be a TSB on this listed for another model.

2. The comment that the spark coil is a form of transformer which has a fixed ratio of windings between the primary and secondary is correct. However, it is not then correct to say that the secondary spark voltage will always be the same. The sequence of operation is this:

a. A 'steady' current passes through the primary from the battery. This current is controlled by the ignition control device, computer, or in older cars, the points. This current builds a magnetic field in the windings of the primary.

b. When it is time for a spark, The current in the primary is abruptly cut off. The magnetic field in the primary winding collapses and this collapsing field generates a high voltage in the primary winding. It is more than 12v.

c. The voltage in the primary induces a voltage in the secondary. The faster the primary voltage collapses, the higher the voltage becomes in the secondary. Of course the ratio of the two winding affects this as well.

d. The voltage in the secondary rises until one of two things happens. Either the magnetic field in the primary has completely collapsed or the spark plug fires. The design is for the plug to fire as the voltage in the secondary reaches XX volts ( I don't know exactly but I think someone in this thread may have it).

Now, the major resistance to the spark occurring is the gap in the plug. The resistance in the wiring, the rotor, and possibly the plug adds to this. I would think that the added resistance causes the secondary voltage in the coil to have to rise higher before the spark occurs at the plug. This higher secondary voltage may then lead to breakdowns in insulation in the spark wires or internally in the coil. I don't know how big a safety margin the engineers build into the insulation but I have read that a too large gap in the plugs can lead to breakdown in the coil. The added resistance of a larger gap must be much greater than the few thousand ohms of resistance in a resistor plug, however.

The original purpose of the added resistance in the spark circuit was to reduce radio interference. The resistance dampens the 'ringing' or voltage oscillation which occurs because there is a resonant circuit in the coil which can briefly radiate an electromagnetic wave following each spark. This wave can be heard as spark noise in an AM radio. We used to add 'RF interference spark suppressors' to the spark plug connectors in the olden times. As far as I know, these never hurt the coils.

I wonder if the argument that the problem lies with the size of the center electrode may be worth following. I looked and I saw that the center electrode in the HR9DC and the H9DC seem to be about the same. I know the platinum plugs have a very small center electrode. The small electrode has advantages in initiating the spark but maybe the spark is more easily 'blown off' the electrode. I sure don't know. :confused:

300holst - Excellent description of coil operation! You’re right on the mark with one small modification. In any given electrical circuit, regardless of in-series resistance, if there is an open in the circuit (such as the gap of the spark plug), the full voltage potential is always felt across the opening, until a spark is initiated.

What this means is that you can have as much resistance as you want between the coil and the spark gap – as long as there is a path for current – and whatever voltage is on the secondary will be felt volt-for-volt across the spark gap. This is because until the gap arcs, no current flow is generated through the high voltage cable. No current flow (until the spark arc) means no voltage drop across the spark plug resistance. This is basic Ohm’s Law.

However when the spark does occur and current flows through the high voltage cable, then there will be a voltage drop across the resistance of the spark plug. Undoubtedly, this would generate a weaker spark since not all of the energy of the secondary would ‘make it’ to the gap – some is lost in the resistance of the spark plug, along with cable loss, etc..

Thus in your beautiful explanation, as the secondary voltage rises – regardless of in-series resistance – the exact same voltage potential is felt at the spark gap – until the instant the spark fires. So again, the secondary coil cannot rise higher than what the spark gap is set for. Therefore, spark gap has a significant impact on how high the secondary voltage rises. But just the spark gap. This is why adding the old school ‘RF interference spark suppressors’ would have no impact on the secondary – as long as the spark gap remains constant. They are just like adding resistor spark plugs. And in the old days, engines only had simple points, rotor and distributor caps for engine timing – no electronics. This is what makes the alleged MB stance and elusive TSB so perplexing… there’s no electronics, etc. to burn-out in the older cars!?!

Further in your post, you describe the effect on secondary coil life if the gap is widened. This makes perfect sense, for the gap distance is directly proportional to the voltage needed to arc. Bigger the gap; higher the voltage needed. Again, regardless of the amount of in-series resistance.

Yes, this thread has brought a number of very valid points (no pun intended) to the table. There are two paths that I think deserve further study:

1. Center electrode size, platinum in particular, and the ability of them to remain intact. This seems to be an underlying item that comes up periodically. And makes more sense than not using resistor spark plugs.

2. Possible coupling back to the primary, from the secondary, when the secondary is collapsing. Is there a possibility that the use of resistor plugs – in SOME applications – causes ringing or abnormal pulsations that the Ignition Modules may not be able to handle? I do not worry about coils, caps and rotors, though it is likely their life could be shortened by having a longer duration spark.

This second item is pure speculation on my part, but electrically, it is possible. I only bring it up as a further discussion point and I look forward to other views/thoughts. Because there is no TSB to read directly – and I really wish someone could/would produce it – we have no alternative but to discuss the subject and piece together ideas.

Cheers,
Jeff

MB Dude, yes, you are right about the full secondary voltage appearing across the plug gap until the spark occurs.

About the plug points. Electrons will collect and jump between the closest sharp points on the plug electrodes. As the center plug electrode erodes though use, it becomes rounded and the spark can be less strong or sometimes no spark can jump ( a miss). We used to pull the plugs, file the plug points flat, and reinstall.

The platinum plugs have a very small center electrode. This should provide a good 'jumping off place' for the spark. Because the platinum is more resistant to spark erosion than the old copper points, the center electrode should provide a good spark longer. This also lengthens the life of the spark plug, a good thing with plug replacement so hard on many engines. The smaller point may also allow the spark to occur with the same secondary coil voltage but a larger plug gap. As I understand it, a longer spark has advantages in igniting the air/fuel mixture. All of this begs the question of why platinum plugs do not work well in some engines. :dizzy2:

As I mentioned before, the HR9D plugs I was sent seem to have copper electrodes which are about the same size as the H9D plugs my 103 engine requires. No platinum. I haven't tried the resistor plugs since I was able to buy a set of non-resistor locally. Maybe these resistor plugs will work well, I don't know.

What I'd like to try is to rig up an inductive pick-up for my oscilloscope which would allow me to compare the spark waveforms on the plugs. Then I wold replace one of the regular plugs with a resistor plug and try to see the difference. I would not have a voltage calibration but I should be able to do a rough comparison the the two waveforms. :D

I don't think that the added plug resistance would produce any ringing in the coil. The resistance actually would serve to dampen oscillations. This is why resistor plugs reduce radio interference.

This all has my curiosity heated up. I imagine somewhere on the web there will be a good description of all of this stuff. There must be a reason why some engines, and their ignition systems, work well with platinum-resistor plugs and others do not. :rolleyes:

Go, 300Holst, GO! Objective, real-world data – I love it! I really do not care whether non-resistive or resistive plugs should/could be used. What I am REALLY interested in is getting data so that intelligent decisions can be made on the subject; one-way or another. We will have to supply some theory, unfortunately, since MB seems to be taking an ‘arms length’ approach.

Absolutely agree! I have spent much time searching, but have come up with nothing. In addition, JamesDean (seems funny typing that) reports ‘no joy’ in his inquiry to the MB Classic Center. His post is #15 of thread… http://www.peachparts.com/shopforum/tech-help/180933-why-non-resistors-plugs.html

We must separate the legend & folklore, and emotion, from the process and prove – or disprove – the thinking that resistor plugs are OK in certain applications. It simply does not make sense that they cannot - at least with the theory we know. Maybe in some ignition designs, such as the Waste Spark design (as written by Arthur_Dalton, post #5 of http://www.peachparts.com/shopforum/showthread.php?t=180933) resistor plugs are a hindrance. But in the vast majority of applications, resistor plugs should be usable. 300holst, I eagerly look forward to your findings.

Cheers,
Jeff

Wow, I love this site!! No bull.. here. We'll not much anyway. I started waxing so first: I just replaced 12 plugs on a E320. Single electrode Bosche, 88k miles on plugs, engine running poorly, so I guessed and got it right. I didn't notice if resistant or not. But the new plugs were dual prog Bosche. Old plugs had the fine point and single plated wedge ground. Inspecting them I could see worn multiple tiny bumps on the electrode near the ceramic and much ware on the wedge. I wish I kept them to look again. I remember tiny nodules on the electrodes, too. You need a magifier to see them well. Rounder surfaces and wear giving wider gap requires longer time for the voltage to build before it can jump and harder to sustain = shorter spark.

Why Two plugs? A clue is two coils, one for each plug. I'm guessing the computer can fire the coils independently and the best use of that would be to give a longer spark.

We used to file the old plugs to give sharpe surfaces at the edges for the spark to jump from and to, reducing the needed voltage and increasing the time the spark continues. The lower the voltage the longer the gap sustains the spark, given equal losses (gap mostly) accross the gap. The multiple electrodes allows for more wear points for the gap. Spark jumps the shortest distance first. I've never looked to see if multiple sparks occur, but my thoughts are if multiple jumps occur, the gap is broader and can distribute the spark leading to a longer duration spark in the chaotic environment of ignition and more surfaces allow longer plug life. My assumption is the 4 prong plugs don't all fire at once, but share the duty maybe unpredictably. I'd love to see the process captured in a video.


Waxing on ignitions: Excellent explanations of the spark / coil circuit and the issues of getting a spark and sustaining it. Nothing better than presenting factual knowledge as opposed to what people think. Its amazing that people believe its true just because they think it.

I've been in electronics since the 50's and saw the first transitor ignitions come in. Incredibly, mechanics put ice picks through the insulation to measure dwell!! I worked in a ford dealership (1965-66) and the mechs routinely pulled the new transistor point units because they didn't understand them or how to measure new low current dwell signals with old equipment. But they were sure they were doing the right thing without trying to understand.

Keep the high level of technical knowledge flowing, Please!

Hi guys!
Great thread and I am very pleased to see I can get the non resistor plugs for my W124 wagon!

I am really struggling trying to fix this misfire I have. I tried another place which stated the non-resistor but they sent me resistor which was very frustrating.

I just ordered a set of the Non-R's and I am praying that it will fix my misfire! Will post back with the results when I get them! I currently have resistors in.

Except for one thing: The spark doesn't occur instantaneously. As the air/fuel mixture begins to ionize, it does draw current delaying the spark as the secondary voltage continues to rise and the dV/dt drops.

-Phil

New specs for new Bosch plugs are out,the new "R" plugs add hundreds, NOT thousands of ohms resistance ,the same amount that is added as systems (plugends,wires,caps,rotors)break down naturally.The backstock of old part #s will be depleated,then what will you do??SO....................for those who have nothing but time,ohm out the complete assembly:"R" plug,resistor end,wire,and boot end,WHAT DO YOU GET.............thats right.............not much.Lets get back to fixing Mercedes!!

Curious where you got the info. on the new specs of the plugs. If the resistance levels are that low for the 'new' units, then there should be no worries with using the 'new' plugs.

Wondering if anyone has thought about putting some non resistance ignition cables together and allowing the plugs to do all the work. Or was it the plug end that had the resistance built in? Just my thoughts.

I have read this thread and it seems no one actually narrowed down the problem of too much resistence in the ignition system. Any one who has ever used a lab scope on the ignition knows you look at the primary side of the coil and get a good indication of what is going on in the secondary. Bad plug wires, rotors, fouled plugs, etc are clearly displayed. This is the results of the secondary inducing a signal back into the primary. If the resistance is low (fouled plug) the secondary voltage will be low. If the resistance is to high (open wire) the secondary voltage will be very high. Since this voltage is induced back into the primary, the ignition moduel sees this induced voltage also. This is what gets the moduels. Is adding a resistor plug enough resistance to make the voltage go high enough to damage the moduel? I don't know. I didn't design it. I do know, always use what the designers say to use.
When I first got my 500SEL, it had been used as a parts car and many things were missing. One thing was the ign moduel. I had on hand an MSD ignition moduel and used it to save a little money until iI knew of I could get the car running and it ran great. The first time I replaced the plugs, I ordered NGK plugs for an 84 500 SEL and I got resistor plugs(BP5RES). The engine started missing at idle. I found some NGK plugs with out resistor(BP5ES), installed them and the engine was again smooth as silk. On a bet I put a set of Bosch Plat plugs in and performance went down hill from the start. I took the car to the dude that make the bet with me. He drove the car, we reinstalled the used BP5ES plugs back in and the car ran smooth as silk again. Easyest $100 I ever made. Now all of this was with the MSD ignition which is capable of jumping about 3/4 inch with multible sparks/firing. So the above tells me that something about the resistor plugs makes a difference and the plat plugs are useless in our engins.
The above are my obsavations, your mileage may vary.

Interesting discussion. I would consider the factory engineered ignition module and coil tolerance specs. I'm certain MB would factor H.V. wire breakdown, heat, worn or fuel-shorted plug conditions? Real world failures drive forgiving component design.

I support MB-Dude's contribution of theory. The secondary winding (coil output) is air-gap isolated from the primary and module. Adding 5K resistance in series to the plug load is within a reasonable specification. The additional load would be insignificant and if excessive - 500K, would effect the spark not pull down the primary side. The primary's charge duration before discharge may be extended by a few milliseconds.

It is all subjective to the original statement that certain Merc's shouldn't use R-plugs. Platinum, iridium and current metallurgy technologies may not have existed when Hans wrote the spec in 1976. Whatever is made, can be made better.

might i suggest some reading material:

http://www.bentleypublishers.com/product.htm?code=h009


solid info from direct from Bosch.

Im about to stick a peice of crap in my spark plug hole. cause thats what its acting like

Interesting thread, I have heard this before. When we got our e320 at 39,000 miles, we changed plugs 'just to.' The originals were a German-branded plug I have never heard of. Not sure if they were resistor-equipped or not, the car ran well. In went 6 Bosch Platinum 4 plugs, the car now has 104,000 miles on it, same Bosch plugs, cars runs perfect. The resistor seems to have no effect on the m104 engine as mentioned before. However, early in the thread someone mentioned the m104 and m119 V8 both were not effected as they were 'computer controlled with multi valves and variable intake timing.' The m119 in my recollection is 32V but has two distributors, so no direct computer control of ignition firing?

Does mercedes make a plug with the star logo on it? I ask because I was really surprised to pull a plug on our EuroVan and find no brand on it but the VW / Audi logos.