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)