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Old 04-12-2003, 08:59 PM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
resistance for silicone spark plug wires?

Hi everybody,
I am trying to get rid of a slight rough idle and have already run injector cleaner through the engine with still a tad of roughness left, particularly in wet weather.

I read the archives regarding the resistance of spark plug wires as well as the MB CD.
As per CD, the resistance of the spark plug wires should be between 700 and 1300 ohm for a 1987 420 SEL.

My readings are:
Zyl: Ohm
1 1320
2 1322
3 1302
4 1303
5 1351
6 1370
7 1335
8 1342
The wire has “Bosch 7 mm Silicone High temperature” written on it.
As per my records, the PO had the wires, distributor, rotor and plugs replaced in 2/1999 with 36,735 miles on it. Car has now 48,500 on it.

I guess the original “indestructible” copper wires have been replaced with Bosch silicone wires by the dealer in 1999 and my reference of 700-1300 ohm might not apply to these.

1. Is the resistance for these wires “within spec.” or do I need to replace them (with the copper core) because they are all over 1300 ohm? Could those few ohms make a difference in the idle?
2. Is it likely that distributor and rotor cap need replacement after 12,000 miles or 4 years?

Right now, I am thinking of replacing the spark plugs only with a little wider gap to see whether it improves on the idle.

Thanks for your help

Reinhard Kreutzer

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Old 04-12-2003, 09:19 PM
Registered User
Join Date: Feb 2003
Location: Ontario, Canada
Posts: 3,160
I would not replace these wires.
If you want to give them a little test, start the engine, let it idle, and spray the wires with a fine mist of water ( Windex bottle will do fine ).
Any noticeable misfire or change in rpm may indicate suspect wire (s ).
Your idea of increasing the plug gap by a few thousands is reasonable & worth a try.
2007 C 230 Sport.
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Old 04-13-2003, 12:00 AM
Registered User
Join Date: Jul 2002
Location: S. Texas
Posts: 1,237
I agree with Manny. Basically your wires and cap ought to be good for 50-100k miles.

As young hot rodders on damp nights (all nights were damp since I lived near the water in S. Texas) we used to raise the hood and rev the engine. If there was the slightest high voltage ignition leak you could see the spark dance all over the wires.

I don't know about a grown man spraying water on a high voltage ignition system. Isn't that kind of like pissing on an electric fence? The idea seems interesting after a few beers, but it doesn't take you long to figure out that it really wasn't that clever. I sound like I speak from experience.
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Old 04-13-2003, 11:10 AM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
Thanks Manny and Kip,

I'll look under the hood the next time it rains here...

I just wonder what the resistance of these wires is when they are new.

Reinhard Kreutzer
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Old 04-13-2003, 01:06 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
My guess is that the term silicon refers to the insulative coating around the actual wire not the wire itself. I am of the opinion that all Bosch wires are copper with resistors on the ends as MB did it in the first place.

They ought to be fine, but so were the originals. The wires themselves very seldom fail. The resistors are another story and could be bad out of the box. The only way to tell is to watch the firing voltages on an ignition scope. Oh.... the other way is to replace them and if there are no bad ones in the box; substitution is a diagnostic process. (the most expensive of course).

I haven't measured the resistance of a plug wire in at least 20 years. It may be good for writing a thesis but plays now use in modern diagnostics. Too many ways for the results to infer the wrong thing! Both good and bad.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 04-13-2003, 05:58 PM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
Thanks Steve,

I looked at the pictures FastLane is providing in their parts catalog and it looks like the Bosch wireset has the word "copper"
on it, which is missing on mine.
As I have no scope, I can only do limited testing. Based on the MB test routine (15-540 on the CD) the spark plug connector should have resistance of 700-1300.
I just find it "surprising" that after only 12,000 miles all are above 1300 with the worst only 5% above that limit.

Of course, I am glad that you think they ought to be fine.

Thanks again

Reinhard Kreutzer

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