Cruise control problem
 

The cruise control on my 300CE only engages at speeds below 50 mph. What is the likely cause? I can use the cruise lever to increase the speed to 60-65 mph and it seems to work fine for a while but eventually shuts off.

Sounds like the cruise amplifier (computer).

Like the doc says, those amplifiers can fail completely (no cruise at all), or the sneaky thing can malfunction intermittently (my E300) or do something like what you experience.

It can be replaced new, or used part that might fail at some point, or people have tried to resolder the interior solder joints.

Where is the amplifier located? What are they worth?

You might want to contact Dave Hendy. He is in the Parts forum and can probably help. I bought one (used) from him and I think it was under $100
there is another item that you might look at. It is attached to the back of the instrument cluster and interfaces with the CCU. It is a hall effect speed sensor. The part number was 007 542 29 17 I think the Cruise/idle control is 124 545 55 32 My car is a '93 300CE and I assume the parts are common.
Anziani
'93 300CE 200K
'95 E420 122K