Most cruise problems stem from the amplifier. However you can perform some tests yourself to assess the rest of the cruise control system, the actuator and the inputs to the amplifier.

Here's my write up on diagnostics that includes links to test the things I mentioned above.
https://sites.google.com/site/mbcruisecontrolrepair/home/diagnostics

If you should pull your amplifier, it should be one of the following part numbers:
001 545 79 32
002 545 32 32
002 545 91 32

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(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


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