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  #1  
Old 08-21-2007, 12:25 AM
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Spark Plugs: Resistor vs. Non-Resistor

OK, I’m game. Whunter wrote a sticky entitled “Danger: RESISTOR SPARK PLUGS” in which the recommendation is to stay away from resistor spark plugs. Some discussion began and it was requested that further discussion continue in a new thread – so here we are. There has been a lot of emotion over whether resistor spark plugs are applicable to Mercedes, et. al. Opinion is sharply divided. This thread will explore people’s viewpoint.

I actually broached this question a couple years ago in another MB website and was ceremoniously crapped on. At the time, Bosch had just released their Platinium+4 product line:
http://jobs.boschusa.com/AutoParts/SparkPlugs/PlatinumPlus4/
…and I was interested in others views/opinions.

The ‘compelling’ argument against resistor plugs has always been one centered on this logic: MB Engineers chose Bosch non-resistor plugs… MB Engineers are smarter than us… so Bosch non-resistor plugs must be the *only* plug that will work, otherwise the MB Engineers would have chosen something different or given us options. There are other arguments, but this one seems to always boil to the surface.

This logic is unadulterated bull. Bosch non-resistor plugs are good quality plugs – no question – but they are also the least expensive for any auto manufacturer to use. The savings per automobile adds up pretty quick, as opposed to relatively more exotic, resistor plugs. MB Engineers do *not* engineer around a spark plug; they simply use a spark plug that is inexpensive to the manufacturing process. In other words, MB Engineers have other things to worry about than a simple US$0.50 (that’s 50-cents) per unit part bought in bulk through inter-company purchase agreements. My professional background as an Engineering Manager to a manufacturing company provides me a unique opportunity to understand the ‘drive’ of companies to save pennies in this manner.

I believe the use of Bosch non-resistor plugs is driven more by bottom-line profits than electrical engineering excellence. I welcome objective data that supports the claim that only non-resistor plugs are to be used in Mercedes motorcars. Folks claim to have read MB documentation that clearly indicates *only* non-resistor plugs are to be used. But as of this writing, no one seems able or willing to produce the objective information. Lots of theory… lots of ‘history’… lots of emotion… very little objective data.

I have run resistor plugs in various Mercedes over the years and seem to have had suffered no ill effects that can be attributed to resistor spark plugs. Performance differences have been, unfortunately, subjective. Thus, I do not profess to know one-way or another. But I am always interested in reviewing objective data. This thread is to address the question of whether resistor plugs (of any brand) can safely be used.

So, what do you think of the subject?
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  #2  
Old 08-21-2007, 01:36 AM
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IMHE, I've had more customers coming in who had recently switched out to resistor plugs only to find their fresh cap/rotor/wires in a sad state.. Namely lots of arcing in the cap/rotor and wires that are no longer w/ in spec.


The WIS states on many occasions that 5Kohms is the maximum total ignition resistance, beyond that they state that damage to the ignition componentry will result.

Were it really a non issue and simply a cost matter I do not think the service literature would state the above...

Just my 2.

Jonathan
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  #3  
Old 08-21-2007, 01:52 AM
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Have you considering doing a search and reading one of the 1 million threads on the subject? Honestly if I had a dollar every time this was topic was explained to somebody...
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  #4  
Old 08-21-2007, 02:35 AM
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All I know is this.I have a 1996 E320,Car ran like crap.Changed plugs to Bosch Platinum 4's Ran worse.Changed to Bosch Copper Non Resister Plugs F8DC4,about $2.00 each,runs like new again.Did nothing but changed to non resister plugs.
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  #5  
Old 08-21-2007, 11:46 AM
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How about for my m103,Mercedes dealer says only RESISTOR plug available,and it will be okay to use them.?????This apparently is sanctioned by Mercedes Canada.

So before they say it's not okay to use resistor plugs,now they say it's okay?

I searched high and low and found some non-resistor plugs in the ebay.co.uk.Now they are in my car.
Next time around,there wont be anymore?
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  #6  
Old 08-21-2007, 12:15 PM
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Quote:
Originally Posted by jhodg5ck View Post
IMHE, I've had more customers coming in who had recently switched out to resistor plugs only to find their fresh cap/rotor/wires in a sad state.. Namely lots of arcing in the cap/rotor and wires that are no longer w/ in spec.


The WIS states on many occasions that 5Kohms is the maximum total ignition resistance, beyond that they state that damage to the ignition componentry will result.

Were it really a non issue and simply a cost matter I do not think the service literature would state the above...

Just my 2.

Jonathan
I, too, have heard of this situation through legend and folklore. Obviously, as a mechanic by trade you can speak from direct experience. Though I’m not sure how the arcing can be worsened in the distributor/rotor interface simply by using resistor plugs.

However, your point about the WIS stating a maximum of 5Kohms resistance is interesting. While I do not have the WIS available to me, to does provide another piece to the explanation puzzle.

Quote:
Originally Posted by danwatt View Post
Have you considering doing a search and reading one of the 1 million threads on the subject? Honestly if I had a dollar every time this was topic was explained to somebody...
Yes, I have searched… and searched... and searched. The problem is the sheer amount of emotional flotsam on the subject. But more importantly, there is the possibility that as time goes on, opinions and capabilities change. MB Engineers change direction as any other engineering team does. They and their vendors make advances that allow backwards compatibility. Further discussion may bring forth a new revelation or hard, objective data that explains the old method. Continual questioning is a natural tendency for us engineers. Sorry. At least this thread gives the Diesel guys something to chuckle about!

Quote:
Originally Posted by jodiedog View Post
All I know is this.I have a 1996 E320,Car ran like crap.Changed plugs to Bosch Platinum 4's Ran worse.Changed to Bosch Copper Non Resister Plugs F8DC4,about $2.00 each,runs like new again.Did nothing but changed to non resister plugs.
Interesting. My experience – anecdotal like yours – was that when I installed Bosch Platinum+4 plugs in an already good running SL600, that the engine ran better. I freely admit my experience is a subjective statement. But it does show that people can have different experiences with the same product. For the record, I did take out the Bosch Platinum+4 plugs and installed standard, off-the-shelf OEM parts. It was an action based upon emotion, as opposed to data.

Quote:
Originally Posted by petaling View Post
How about for my m103,Mercedes dealer says only RESISTOR plug available,and it will be okay to use them.?????This apparently is sanctioned by Mercedes Canada.

So before they say it's not okay to use resistor plugs,now they say it's okay?

I searched high and low and found some non-resistor plugs in the ebay.co.uk.Now they are in my car.
Next time around,there wont be anymore?
Hmmm… Playing the Devil’s Advocate, dealers do not always provide accurate information. Then again, they may know something we don’t. Regardless, are you saying the dealer could not supply the ‘factory’ spark plug for you vehicle? Doesn’t make sense.

Again everyone, I’m not stating one plug is better than another or that resistor plugs are better than non-resistor. I only want to find conclusive, objective data that says one way or another. This is the same discussion that many of us had when synthetic oil was first introduced. Now look – it’s the standard! And how many of us poo-poo’d synthetic when it first came out, simply because it was ‘different’. Same could be for plugs. Discussion is always healthy.
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  #7  
Old 08-21-2007, 12:47 PM
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Quote:
Originally Posted by danwatt View Post
Honestly if I had a dollar every time this was topic was explained to somebody...
It sounds like Larry Luffer. "I wish I have a dollar everytime I say that" LoL
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  #8  
Old 08-21-2007, 02:29 PM
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Well these days they use super expensive platnum plugs.

I doubt that when building the M119, or the M120, or the 600 Pullman's, they gave a flip about saving a couple cents on spark plugs, when they spent so much money on wires! Or in the case of the Pullmans $1,500 seat switch's!

I think some engineer got a little to much freedom back in the day and built an ignition system around these cheap plugs, and they just followed his lead for years. AFAIK all gas MB's called for non resister plugs until the M112/113 came out in 1998.

The FSM is pretty specific about what you can use, I prefer not to backyard engineer things. More so when the part they want you to use is the cheapest! Why should I go out of my way to research if a $5 plug will work, when they tell me to use the $1.50 ones in the manual?

You can do whatever you want with your car, and if from your experiance platnum plugs run fine in these go for it. But from my experiance I have never had problems doing what the FSM says to.
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  #9  
Old 08-21-2007, 02:59 PM
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well, now you folks are getting me paranoid... our 300e ran the last 30k on R plugs, so i put new ones in.

put new ones in the 260e, too.

i put my last set of Bosch NR plugs in the CE, which is the car I bend over backwards to keep original.

my beef has been the unavailability of NR plugs...but now if this site has them for 3 or 4 bucks, i may as well pick up a set... at least for the 300e.
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  #10  
Old 08-21-2007, 04:23 PM
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I'm glad someone started a continuation thread about this subject. I think that things might be getting off track at bit. See if you agree.

I guess I may have intrepreted the original thread a bit differently. What I read was;

"Bad wires, cap, rotor and/or wrong spark plugs can wreck your ignition module".

I agree that there is a lot of subjective information about using different type of plugs in different engines. I have my own opinions based on my personal experiences with different ignition components. I agree it does make a difference what you use where. But the original thread was not about that. It was about;

Quote:
"Bad wires, cap, rotor and/or wrong spark plugs can wreck your ignition module".
I have a lot of experience of what happens in the ignition system of these cars on a microsecond level. Without getting into all the techno mumbo jumbo the bottom line for me is this;

If "Bad wires, cap, rotor and/or wrong spark plugs can wreck your ignition module" then you have one crappy ignition system. The engineer who designed this thing should undergo appropriate punishment.

It is my opinion that the ignition systems in these cars is another great example of robust German engineering just like the rest of the car. I dont believe the statement ""Bad wires, cap, rotor and/or wrong spark plugs can wreck your ignition module". If it were true then ignition modules would be dropping like raindrops in a hurricane. This is not the case. Just pull the high voltage wire off the coil - that certainly represents total failure of everything that comes after it. The ignition control module does not break. If it did, the designer of the ignition module, you would be forced to make the module survive all sorts of failures in the other components.

What can go wrong with the various components? Lets take a few examples from an electrical perspective and assuming everything else is OK mechanically.

1. Spark plugs;

- Leakage resistance from tip to ground due to carbon deposits - Result - weak spark.
- Resistor has high resistance - Result - weak spark.
- Resistor has low resistance - Result - stronger spark.
- Resistor has open condition (Almost impossible to occur) - Result - no spark

2. Ignition wires (resistive type)

- Wires leaks or arcs current to ground (insulation failure) - Result - weak spark.
- Wires have low resistance (Almost impossible to occur) - Stronger spark.
- Wires have high resistance (carbon conductor failure) - Result - weak spark.

3. Distributor Cap.

- Cap leaks or arcs current to ground - Result - weak spark.
- Cap has corrosion on terminals - Result - weak spark.
- Cap leaks current between the various terminals - Result - weak spark & misfires

4. Rotor

- Rotor leaks or arcs current to ground - Result - weak spark.
- Rotor has corrosion on surfaces - Result - weak spark.

None of the problems listed above, (I think I got them all) has any impact to on the ignition control module. The module is not affected in any way by these conditions with the exception of "open circuit secondary". The one component that has any possible chance to damage the ignition control module is the ignition coil itself. The coil has the following failure modes;

- Coil primary is open - Result no spark, no primary current
- Coil leaks or arcs current to ground - Result - weak spark. Same as leaking wires.
- Coil has internal shorts in the windings - Result varies depending where the short occurs.
Possibilities include Weak spark and or high primary current.

It is the shorted winding scenario that represents the most danger to the ignition control module. High current resulting from a shorted primary will the stress the ignition control module switch and produce a lot of heat and failure. But ---- if the control module is properly designed it should protect itself from that condition. I do not know if the MB ICM's have this feature or not. The protection feature known as "current limiting" and is often designed into control electronics to make them more robust and resistant to failure.

Now when the coil output terminal (High Voltage) is not connected to any load (the wire removed) the voltage goes very high. Much higher than it does when it is connected to the distributor. The voltage will find a path to ground if it can. This condition places the most stress on the coil and to some extent the ICM. The ICM has an internal voltage clamp which prevents this over voltage condition on the secondary of the coil which is reflected to the primary from damaging the switching element. They are designed to be able to withstand "open circuit secondary" conditions.

The issue of resistance vs non-resistance components has such a small impact on the current and voltage waveforms that it is difficult to see. The resistance was put into ignition circuits in the early days to prevent noise in the AM radios of the day. It really has nothing to do with the arcing and sparking of the plugs.

So thats my opinion. And thats why I don't believe that the resistance of plugs / wires or rotors and distributors that leak or arc can cause damage to the ignition control module. Now for which brand / type make the engine run best - thats another chapter.

I am still waiting for someone to produce a copy of the TSB mentioned in the original thread.
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Last edited by dpetryk; 08-21-2007 at 04:30 PM.
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Old 08-21-2007, 05:34 PM
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Quote:
Originally Posted by petaling View Post
So before they say it's not okay to use resistor plugs,now they say it's okay?
Please don't generalize.
M-B specifies resistor plugs in some engines & none-resistor plugs in others.
So, if you want to make your own rules go for it ( and live with the consequences ).
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  #12  
Old 08-21-2007, 10:50 PM
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Say, can I use synthetic oil with resistor plugs?

Besides, real men don't need spark plug wires (et.al. 119/980)
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  #13  
Old 08-21-2007, 10:57 PM
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this ***** is confusing. heres a question: why doesn't someone just ask mercedes? like their engineering team. heck I'm gonna email them and see what their official response is.

roar.

email sent to the Classic Center. We'll see what they say...
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  #14  
Old 08-22-2007, 02:05 AM
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A simple question: I have installed Bosch platinum plugs #4020 in a 110 engine - there is no "R" in the number. (I could not care less about radio interference as all staions in the SF Bay Area now suck anyway). So are these plugs resistors or not?
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  #15  
Old 08-22-2007, 02:25 AM
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Quote:
Originally Posted by MB-Dude View Post
OK, I’m game. Whunter wrote a sticky entitled “Danger: RESISTOR SPARK PLUGS” in which the recommendation is to stay away from resistor spark plugs. Some discussion began and it was requested that further discussion continue in a new thread – so here we are. There has been a lot of emotion over whether resistor spark plugs are applicable to Mercedes, et. al. Opinion is sharply divided. This thread will explore people’s viewpoint.

I actually broached this question a couple years ago in another MB website and was ceremoniously crapped on. At the time, Bosch had just released their Platinium+4 product line:
http://jobs.boschusa.com/AutoParts/SparkPlugs/PlatinumPlus4/
…and I was interested in others views/opinions.

The ‘compelling’ argument against resistor plugs has always been one centered on this logic: MB Engineers chose Bosch non-resistor plugs… MB Engineers are smarter than us… so Bosch non-resistor plugs must be the *only* plug that will work, otherwise the MB Engineers would have chosen something different or given us options. There are other arguments, but this one seems to always boil to the surface.

This logic is unadulterated bull. Bosch non-resistor plugs are good quality plugs – no question – but they are also the least expensive for any auto manufacturer to use. The savings per automobile adds up pretty quick, as opposed to relatively more exotic, resistor plugs. MB Engineers do *not* engineer around a spark plug; they simply use a spark plug that is inexpensive to the manufacturing process. In other words, MB Engineers have other things to worry about than a simple US$0.50 (that’s 50-cents) per unit part bought in bulk through inter-company purchase agreements. My professional background as an Engineering Manager to a manufacturing company provides me a unique opportunity to understand the ‘drive’ of companies to save pennies in this manner.

I believe the use of Bosch non-resistor plugs is driven more by bottom-line profits than electrical engineering excellence. I welcome objective data that supports the claim that only non-resistor plugs are to be used in Mercedes motorcars. Folks claim to have read MB documentation that clearly indicates *only* non-resistor plugs are to be used. But as of this writing, no one seems able or willing to produce the objective information. Lots of theory… lots of ‘history’… lots of emotion… very little objective data.

I have run resistor plugs in various Mercedes over the years and seem to have had suffered no ill effects that can be attributed to resistor spark plugs. Performance differences have been, unfortunately, subjective. Thus, I do not profess to know one-way or another. But I am always interested in reviewing objective data. This thread is to address the question of whether resistor plugs (of any brand) can safely be used.

So, what do you think of the subject?
Just my two cents worth. I have a 1984 500SEC with now 265,000 miles on it. Car runs great. I have always used resistor plugs. No real reason other than I just thought they were better.

Now with this car and resistor plugs, I would notice carbon debris on the bumper when I was washing the car. Not much. But enought to turn the sponge a little black. I thought with the high miles, I was seeing evidence of blowby.

Then I read MB spark plug wires offer resistance in themselves. So by using resistor plugs, I was actually not getting a full burn on my fuel.

Made enough sense to me to replace my plugs with non resistor.
It's been about 4000 miles now and I have no more carbon deposits on the bumper. Same gas, same oil. Just no deposits.

For what it's worth...
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Last edited by cdplayer; 08-22-2007 at 02:25 AM. Reason: spelling
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