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.

That may be true if the original wires are used.

But, Bosch on their own site specify their part 09027 wire set for our '72 350SL 4.5L . Almost all on-line parts suppliers sell the same wires for 4.5L M117 engines.

These wires are not copper wires, but are what Bosch call Opti-Layer Mag wires - They say it is a nickel alloy (stainless steel). These wires measure about 1.6 kohms end to end and do not appear to have any resistors built in, at least on the distributor end.

The original 1972 vintage MB wires that I just removed measure 8 kohms and have an integral 1.6 kohm resistor at the distributor end (molded into the ceramic plug) (I know - I cut one apart!)

So now we have a Bosch recommended wire set that has 6.4 kohm less resistance than the original MB wires.

So the question comes up as to which plugs to use.

I was sent plugs with following designation:

7500
0 242 235 663 -74U
WR7DC+

Old plugs, which were W7DC, also had the 7500 designation? Does this mean that Bosch consider these "R" plugs a replacement for the non-resistor plugs? WR7DC used to have 7501 number. Can anyone verify this change and explain the thinking?

If I used non resistor W7DC plugs, would my overall resistance not be lower and could this mean a very short lived hot spark? How much resistance would the 7500 WR7DC plugs add?

There are two of us with this dillema, being discussed in a separate thread. But the experts are here, so I would really like to hear any comments.

BTW, the car is running OK with new wires/plugs , but always interested in improvements!

So everyone is saying that by using a Resistor plug like the autolite r103 can short out a coil?

There are two sides to this excellent discussion - one side believes the use of resistor plugs can damage coils and ignition systems, while the other side believes the use of resistor plugs is acceptable. Also, this is not a one-size-fits-all sort of discussion; meaning there may be certain special mitigating circumstances in a particular model/year.

However, without additional conclusive and objective data from MB Engineering, we are sort of at an impasse and have to approach the subject with theory, personal experience and gut-feel. Therefore, you must use your own judgment based upon what you learn as to whether resistor plugs will work in your vehicle. Personally, I’ve enjoyed this thread immensely.

Cheers,
Jeff

Ignition Module
 

Please read post #1.

The coil is cheap, and should be replaced every ten years = I don't care about it...

The ignition control module is at risk...

Ouch,

Still its getting so hard to find non resistor plugs

I think I;m OK
Got non resistor the last 9 they had
NGK BP5ES which is one hotter than the equiv BP6ES that my engine
was spec at when new

Bosch W7D...no C on mine its a Euro

So I'm ok on plugs

What about the wires?

Problem is now they have Bosch wires & we don't know how the resistance of the new type wires in combination with the spec plugs will be off & possibly dangerous to the system.

It has been said the resistance in the new wires is lower than the oem solid core copper. Copper core has more resistance by nature than the new Bosch replacments & they don't use copper core now as it messes with the radio magnetic fields & hence the computer systems vital to the newer cars.

So even if you get the non resistor plugs
what about the new ignition wires & how does this affect the system?

I have a feeling that there is a difference between resistance of a wire
& a resistor built into a plug.

They might not be of the same units so there fore you can't add apples with oranges & compare it with the origiinal resitance of the oem wires.

Guess thats a new thread
Low resistance wires...

I got those new Bosch opti layer nickel wound SS & just wonder if its hurting anything when paired w the spec non res plug?

I should hope to try to understand the physics
as I once was a physics major
but alas time has a way of clouding the mind
& I'm trying my best to if not figure it out
at least know the system is not compromised...
:confused:

Resistance is a measure of the force that reduces the flow of energy
less resistance would mean a higher end voltage.
What happens when it gets to the plug?
less resistance would mean a more efficient flow thru the plug
sometimes resistance creates additional heat as electrons have to work harder & bump each other more

:eek:

Help, I'm loosing it again

:D

I believe you feel strongly about this subject and appreciate that you are trying to be a good soul to help people prevent problems. But a number of us would like to understand 'why', as opposed to simply and blindly accepting information without substantiation. You have much experience, but the logic/theory does not necessarily mesh with your experience. I, for one, would immediately adopt the ‘no resistor plug’ stance if I could read about why there is the requirement. We hear of the perfection of the MB Engineering Department, but cannot find any data/information from them on this subject. Remember, MB has reversed course on a number of initially taboo subjects, such as gas additives (Techron is now authorized), etc. Could they reverse course on resistor plugs, too? Some of us simply want to know ‘why’, or we have to figure it out ourselves.
Cheers,
Jeff

Even although I am sure your experience says this and we should take note, I would still like to know WHY resistor plugs will cause an ignition module to fail.

I would also like to know more about wire/plug combinations. Bosch, who are not new at this game, say that we should use non-resistor wires 09097 and resistor plugs WR7DC+ on our early 107,s or other M117 powered cars.

Are they wrong? I have called them and the confirm their recommendation. It's hard to know who we should believe.

The only thing the resistance does in an ignition circuit is reduce noise for the AM radio band. It does not matter where the resistance is located because its a series circuit. Resistance of wires or plugs will not affect the ignition control module, coil or anything else in the ignition system The voltage loss across the resistance is insignificant and is difficult to measure. It sure as he?? will not cause the ignition module or coil to burn out or fail.

Well, I'll give you my 2 cents worth ( from experience ) on this subject.
When you measure the plug resistance, it can be all over the map ( even varying with temperature).
When you measure the resistance of a sparkplug wire or better yet, the connector at the end of the wire, they are pretty consistent.
Long story short, my choice would be non-resistor plugs with resistor wires.;)