The warm up compensator, powered by pin 87 of the fuel pump relay, which as I understand has power with RPM input, and constant ground.....how does it enrich the mixture only when the engine is cold? How does it know?

The electrical hook-up has nothing to do with its hydraulic function. The pressure control which is its job, is varied by temperature due to a bimetal spring holding the valve shut in the regulator. When cold it is further open and leaks down the pressure above the piston in the fuel distributor (low control pressure).

The point of the electrical circuit is to warm up the device faster than the piece would do if waiting for engine heat. When hot the function is that of a warm device because of engine heat keeping it warm. If it waited for the electrical heat the car would be way rich for that 20 seconds till electrical heated the bimetallic.

If one leaves the electrical off the pressure stays at the cold setting until the regulator heats from engine heat and would be way rich beyond whats necessary for cold running.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thank you, Steve. Ctaylor told me the comp can be checked by measuring resistance across the terminals. Should this reading change cold/after a few minutes of current, or do you have to check fuel flow/pressure at the lines? Thanks for your help.
Paul

The resistance of the heater circuit would only be of use to tell if it still heated. I would presume it to be stable over the various temperatures and it probably is less than an ohm.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician