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Old 10-09-2000, 04:51 PM
J.HIDALGO's Avatar
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Join Date: Jul 1999
Location: Jax, FL
Posts: 1,785
I am trying to find the problem with rough idle in my car. I took it to the mechanic and he scoped the car. He says most likely is the coil? I tested the coil by itself, using an ohmmeter. The results for the primary resistance, in the 20K ohms scale, were moving between 0.00 and 0.03. The secondary resistance was 8.58 steady.
Does this results make sense or is there a better test to check the coil?
I do not want to buy x number of parts until I find the problem.

'86 300E
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Old 10-09-2000, 05:32 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I don't know the spec off hand but the primary side should be less than 1ohm. Testing on the 20k scale is useless.

The proper way to test a coil is dynamically with an oscilliscope. Looking for available voltage and proper oscillations.

I seriously doubt that your rough idle is caused by the coil, but I suppose anything is possible. Idling is one of the easiest situations for the coil. Starting and acceleration tax a coil much more.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 10-09-2000, 07:07 PM
300EVIL's Avatar
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Join Date: Apr 2000
Location: Lake Geneva, WI.
Posts: 1,676
i have an oscilloscope. how do i use it to test my ignition coil? i know it cannot handle that kind ov voltage. plus, what do i look for in the waveform?

87 300E

87 420SEL
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