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Marc 11-20-2002 04:32 PM

Coil Test
83' 500SL
My wife starts it up, gets to the end of the driveway and stalls, has never started since.

There seems to be no spark from the coil. I have placed a test light on the + and - terminal of the coil (primary) and turned the key, the test light stays on while cranking. My understanding is that if the test light blinks the coil is bad. Am I testing correctly?

I understand that other possibilities are the ICU, the green lead from the ICU to the distributor, and the crank sensor.

The green lead I will test with a continuity tester.
Is there a way to test the ignition control unit? Also, where is the crank sensor and is there a way to test this also. I do have a DMM.

Had no other symptoms. We don't drive the car that often but it was running fine.

Any thoughts would be greatly appreciated.



stevebfl 11-20-2002 05:05 PM

If your test light is continuously lit while cranking on both sides of the coil then the ignition control unit is not switching. The + side should stay hot but the - side should be ground to connect the coil. The field is built during the brief time the coil is energized. During the energized time the light will be off on the b=negative side.

The control unit reads the speed signal from the inductive coil within the ignition distributor. If you have another car handy it would ease the diagnostic by having a good control unit to plug in.

To diagnose properly the speed signal should be monitored with a scope to verify. If the controller is powered and the inputs are there then it would look like a bad controller. Fairly common on that era controller.

Marc 11-20-2002 05:30 PM


I appreciate the quick response, but I'm confused.

When you say both sides of the coil are you referring to the primary and secondary or do you really mean the + and - as stated in your post?

I believe I understand how the coil works. The primary windings are energized, when power is cut to the primary the more dense secondary windings become "induced" stepping up the voltage.

If you don't mind, would you explain the proper method for testing this coil with a light or DMM. (where to connect the leads for each test)

Unfortunately I don't have another car to test with, nor do I have a scope.

Is there a test point on the ICU so I can test that it is powered and has the inputs?

Since you mention that the ICU is a common failure on cars of this era can I rule out the crank sensor? (does this car have a crank sensor?)

Thanks again


stevebfl 11-20-2002 06:20 PM

No the car doesn't have a crank sensor for the ignition. The control unit reads the induced signal from the impulse transmitter in the distributor. You pretty much have to have a scope to view its output.

By placing your test light on the negative side of the coil you should see a blinking light while cranking. if you see a solid light then the coil is never being connected to ground.

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