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1983 380 SL
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ICM has arrived, a HUCO. It came with a very small tube of a white material that I am assuming is a dielectric grease for the contacts but before I start slathering an unknown material on this new part I thought I would ask to be certain.
The problem is not to smear or not to smear... the problem I had was how to mount it. The new ICM is a different size than to old one and I couldn't find any way to mount it so the heat sink lay flat on the mounting area. I mounted it the best I could and decided I would make a heat sink from extruded aluminum some day.. but I never did.
 

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Discussion Starter #22
The problem is not to smear or not to smear... the problem I had was how to mount it. The new ICM is a different size than to old one and I couldn't find any way to mount it so the heat sink lay flat on the mounting area. I mounted it the best I could and decided I would make a heat sink from extruded aluminum some day.. but I never did.
I mounted it using two of the screws from the old module. Sadly, the new ICM doesn't correct the no start issue. What next?
 

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Discussion Starter #23
I went through the troubleshooting guide for the ignition and everything reads good -- coil, green wire, ICM. I'll go to the other end and verify spark at a plug.
 

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1986 560 SL
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That's likely a heat sink paste which you apply to the mounting surface to help dissipate heat.
 

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1983 380 SL
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When you say you went through the troubleshooting guide what does that mean? Did you do voltage checks or just resistance checks?

Did you check for voltage on the coil? Basically you're looking for 12 volts at the high side of the coil (should be marked with a + sign) with the ignition switch in the "on" position. The job of the distributor is to send pulses to the ICM through the Green wire that tells the ICM when to short the bottom of the coil to ground. If the ICM is grounding the bottom of the coil to ground... and the coil has 12 volts on the high side (+), then the coil has no choice but to produce a spark every time the ICM lifts the low side of the coil off ground (in a old system with points the time between when the low side is connected to ground and then lifted off ground was called "Dwell")

A simple manual test you could perform is to short the low side of the coil to ground yourself, but I caution you to do so with great care and only very briefly (split second) connect the low side to ground and be absolutely certain it is in fact the low side you are grounding... and only for an instant.

You could do this with a jumper wire connected to the low side, with the ignition switch on, and 12 volts present at the Plus (+) side of the coil, very quickly tap the jumper to ground and when I say very quickly I mean for a fraction of a second. Each time you do you should get a spark from the top of the coil. If you plug the coil wire on the top/Center of the coil directly to a spark plug (instead of the rotor on the top of the distributor) and position the threads of the spark plug so they are touching ground (any metal surface attached to the chassis), you should see a spark every time you flash the bottom of the coil to ground.

If you get spark it will verify that the coil and all it's connections are good. If you don't get a spark then the coil is bad... or you're not grounding the low side... or the high side doesn't have 12 volts on it.
 

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Discussion Starter #27
Along with the resistance checks on the coil and the green wire to the ICM I verified 12V present at the coil with the ignition switch to the on position. I have not checked output from the coil itself so it is possible that the coil is bad. And I'm wondering about the rotor although it appears clean. The distributor cap is new.
 

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1983 380 SL
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Along with the resistance checks on the coil and the green wire to the ICM I verified 12V present at the coil with the ignition switch to the on position. I have not checked output from the coil itself so it is possible that the coil is bad. And I'm wondering about the rotor although it appears clean. The distributor cap is new.
We can ignore the rotor and even the distributor cap for that matter while we determine if the coil can produce spark. Try the test I suggested but after further thought I would suggest you first take a resistance check from the low side of the coil to ground with the ignition switch off. If the ICM can't open and close the low side of the coil to ground then the coil won't produce a spark.

Look at the picture in #10 post. Note the green alligator clip laying on the bench connected to nothing. When testing the coil for spark that green alligator clip is connected to the positive (+) terminal of the battery. Indeed "all" the thin green wires are connected to the 12 volt battery source. The yellow wires are all connected to the negative (-) terminal of the battery and the white wire connects the low side of the coil to the ICM.

The thick "Green Wire" connects the distributor pulses (pulses 8 times per revolution) to the ICM which in turn responds to each pulse by closing and then opening the path to negative off the battery (yellow wires). When the ICM lifts the path to ground is when the coil fires the spark. The way I have it setup in the photo is essentially identical to the way it is setup in the car. In my case when I spun the distributor gear by hand (bottom of the distributor), the coil fired the spark plug I had connected to the coil (just out of view to the right).

By manually flashing the low side of the coil to ground (split second) with the high side (+) connected to 12 volts, we are eliminating everything in the circuit other than the coil. If the coil won't produce a spark under those conditions then you have your answer. If it does produce a spark then we'll know to look elsewhere in the circuit.

Although this explanation may appear complicated, the circuit itself isn't. There isn't all that much in the circuit... the distributor pickups (8 of them in the base), the "Green Wire", the ICM and the coil. That's it. If the +12 volts is present at the + terminal of the coil, and the ICM is receiving the pulses from the 8 pickups in the distributor and is flashing the negative side of the coil to ground, then the coil will fire. If it doesn't then it's bad.
 

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Discussion Starter #30
Resistance from the - post on the coil to ground (engine block) reads zero to 3.5 ohms with the ignition switch off.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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ICM has arrived, a HUCO. It came with a very small tube of a white material that I am assuming is a dielectric grease for the contacts but before I start slathering an unknown material on this new part I thought I would ask to be certain.
That is probably heat sink past that goes on the back of the mounting plate. Are there any instructions?
 

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1983 380 SL
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Resistance from the - post on the coil to ground (engine block) reads zero to 3.5 ohms with the ignition switch off.
Resistance measurements often times can be poor indicators. Although the resistance of the negative terminal on the coil to ground should not be zero... even the 3.5 sounds low to me.

The problem with measuring residence in a circuit with other components still connected is that you don't know if you are measuring the resistance of the component being tested or something else in the circuit. For instance... the negative of the coil is connected to the positive of the coil through the coil itself. The DC resistance of the coil is quite low so measuring the resistance at the negative or the positive will yield very similar if not identical results. I know that in many places they suggest taking resistense measurements but I always cringe at the suggestion.

Please try this test... disconnect everything from the negative (low side) of the coil and connect a jumper wire to the terminal leaving the other side of the jumper connected to nothing for now. Apply 12 volts to the plus terminal on the coil (ignition switch turned to on should apply 12 volts to the coil) and then measure the voltage to be certain it is there on the plus terminal. If you can measure the 12 volts on the plus terminal, measure the voltage on the low side (the side where you connected the jumper) it should also read 12 volts. If it doesn't then the coil is open and needs to be replaced.

If the low side of the coil (the terminal you put the jumper on) reads 12 volts then you are ready to continue with the next step of the test. Connect the output of the coil (center) directly to a spark plug and connect the threads of the spark plug to chassis ground any way you can. Now using a very fast motion, tap the open end of the jumper on the negative terminal of the coil to chassis ground. Do not connect it to chassis ground just flash it on and off using a fast motion. Each time you do, you should see a spark jump the terminals on the end of the spark plug. If you don't see the plug sparking then either the high voltage coil wire (the one that usually connects to the center of the distributor) is bad, or the coil is bad or the spark plug is bad.

However, if you do get a spark every time you flash the negative terminal of the coil to ground then the coil is fine and your problem is elsewhere.

Let me know the result of that test and then we can take the next step.
 

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Discussion Starter #34
Jyuma, I read battery voltage from the + side of the coil to ground and also from the low (-) side of the coil to ground. Proceeded with the jumper to the low side of the coil test -- attaching the wire from the coil to the distributor to a known good spark plug and the spark plug touching the chassis. With the key on I flicked the jumper on and off the chassis and I got spark at the plug (along with spark at the point of touching jumper to chassis). Sounds like the problem is elsewhere.
 

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1983 380 SL
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Jyuma, I read battery voltage from the + side of the coil to ground and also from the low (-) side of the coil to ground. Proceeded with the jumper to the low side of the coil test -- attaching the wire from the coil to the distributor to a known good spark plug and the spark plug touching the chassis. With the key on I flicked the jumper on and off the chassis and I got spark at the plug (along with spark at the point of touching jumper to chassis). Sounds like the problem is elsewhere.
Yup. Reconnect everything.
The next most likely cause of your problem is the Green wire or the pickups at the base of the distributor, but we'll continue to work backwards just to be sure.
Referencing the picture I posted, disconnect the plug on the ICM (not the Green wire plug, the other one) and at the connector, not the socket in the ICM, with the ignition switch in the "On" position make sure there is 12 volts present at the pin I have the green jumper connected to and the pin I have the yellow wire connected to goes to ground and that the pin I have the white wire connected to has continuity to the low side of the coil (zero ohms) .
All of these checks should be made at the connector you took out of the ICM not the socket in the ICM. Post your results.
 

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Discussion Starter #36
With the key on I have12V across the green and yellow points you referenced in your picture. With the key off I have 82.7K ohms resistance from white wire to low terminal on the coil
 

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Discussion Starter #37
You've been very patient and helpful Jyuma. From these results do you know what the problem is or what to try next?
 

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1983 380 SL
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You've been very patient and helpful Jyuma. From these results do you know what the problem is or what to try next?
The reason I hesitated is because I am unsure if there is a switch or fuse or some other component that sits between the white wire position on the ICM plug and the negative of the coil.
In order for the coil to produce a spark the bottom of the coil (negative) must be able to connect directly to ground... certainly not 82.7K from ground.
I would check the path from the white wire position on the ICM plug to the bottom (negative) of the coil. If it is truly 82.7K then there is something wrong in that path. I'll look at my 380 later this morning to see if I get the same resistance reading.
 

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Discussion Starter #39
Operator error on my part. I had the plug upside down. Reading from the correct spot on the plug I show zero ohms.
 
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