My wife was driving to work...

She stopped at a red light and the engine just died without any signs of trouble. When I tested for spark I found none, so I assumed a bad coil. This has not fixed the problem. I have been told there are two ignition controls, one in the engine compartment which is reasonably priced, and one behind the glove box, which must be made of solid gold $$$$. Is there a way to test either of these componants without going to a shop?

It is a 1985 500 SEL.

The 85 500SEL only has one ignition control unit. It is located under the hood behind the drivers side (LHD) head light. There are only 3 components in the ignition system. The pickup coil in the distributor, the ignition control module and the coil. With the ignition on, check for voltage at the coil + terminal. Next, disconnect the round plug from the control unit. Set a volt meter to VAC, connect the meter to the center connector of the cable and the other lead to the outer connector. Crank the engine over and see if you get about 1.5 volts AC coming from the distributor. If both of the above test pass then the problem is probably the control unit. If either test fails then investigate the area that failed. Try these things and let us know.
BY the way, when my control unit went bad I installed an MSD ignition system which cost less than the MB module.

Paul

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84 500 SEL (307,xxx miles)

Paul,

Thank you for the fast reply, I was checking on prices and am appauled at the cost for the ignition control. I was not aware of the MSD replacement part, I'll go that routh if need be. I will begin the checks you recomended now and I'll post the results forthwith.

Mark

OK, I've tested the coil and all seems well. When testing the output from the distributer, I'm getting a reading of about .25, not 1.5. I don't know if I'm doing it wrong or not, my main tool is usually a hammer. Could this indicate an issue with the distributer?

There are two plugs on the bottom of the module, one has 4 prongs, the other only one, which goes to the distributer. That is the one I tested when turning the motor over, with one lead on center, the other on the outer portion of the plug.

The .25 volts may be OK. The main thing is there is activity. I can't check mine today as I am driving the Diesel. The 500SEL won't be driven again until the SNOW melts and the temp gets up to a decent level. I don't have a garage at home to get in out of the cold and it is dark when I get home. Check with Fast lane, they have some reasonably priced modules that should be fine.
Paul

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84 500 SEL (307,xxx miles)

Thanks Paul I'll check them out.

I have a weather issue as well, having to dodge the sun's rays when working on the car. I remember well fighting the cold in Minnesota trying to change timing chains and gears without a garage.

I've surpassed my monthly allotment for car parts, so the MB will have to rest untill another payday rolls around. I'll get back with the final results.

Mark

Well, it was the ignition module after all...

Quick trip to the junk yard was all it took.

curious how to convert to MSD? my 90 300 se has a vacuum plugged into it.
So it may have no advance?

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You're welcome Mark. Glad you got it worked out. You have a lucky wife, she drives a 500SEL.

SD300, the timing on your car is computer controled and little would be gained for the amount of work. The ignition and fuel computer work together and if one part is modified then fuel computer would need to modified also so the could work together.

Paul

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84 500 SEL (307,xxx miles)

I was wrong in my last post that the ignition and fuel computer worked together. In a 86 420 the Ignition and fuel computer are separate items and each does it's job without the help of the other. I am still right that the MSD system wouldn't work on the 420 because of the advance curves etc are calculated in the ignition control unit and the MSD is not set up for that. Diagnosing is about the same as the 85 and earlier except the pickup coil is at the flywheel instead of the distributor.

Sorry for the bad information.

Paul

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84 500 SEL (307,xxx miles)