My cruise control will only engage at 65mph or above, and at that point it either surges to 80mph or settles in at 71mph. Usually the former, and rarely the latter. When I press the stick to decelerate, it will but then surge back to 80mph. After reading through some other threads, it looks as if a bad amp means that you have no CC at all. Is there any way to service or clean the actuator? Thanks

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'81 300SD

The usual culprit is a failing amp, located beside the steering column.

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83 SD

84 CD

Try these:
Actuator Tune-up
Inside the box: VDO Cruise Control Actuator

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Sam

84 300SD 350K+ miles ( Blue Belle )

I found these websites very helpful: https://sites.google.com/site/mbcruisecontrolrepair/home/diagnostics

Middle-aged Mercedes Electrical Repairs

I ordered a repaired amp from JamesDean, and its worked flawlessly since.

Thanks for the links. I'll start with the motor current test described by JamesDean.

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'81 300SD

Once they start acting up, it's not long before they fail. Have it rebuilt or get one that's been rebuilt.

Sixto
87 300D

By "they" I assume you mean the amps. I should test the actuator anyway since a faulty one can damage a good replacement amp.

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'81 300SD