The low speed relay is turned on based on a ground signal from the AC high pressure switch. It is a simple off/on circuit with no electronics.

To verify the fan portion, pull the "B" relay and verify power to the "30" socket position (the positions are noted on the relay match the pin to the socket). If you have power then locate the 87 socket the same way and use a fused jumper wire to bridge 30 and 87 and the fan should work. If it doesn't verify that the new circuit does indeed have power still and then LOOK AT the resistor!!! The power will be on one side. If there is indeed power on both sides then you either have a bad ground to the motor, a bad circuit from the resistor to the motor, or a bad motor!

Continuously monitor what you think is power because as soon as there is a load on the circuit that unloaded voltage may go away. If it does walk the testing right back to the source and find the break. The test points would be the motor input, resistor output, resistor input, pin 87 of "B' relay, pin 30 of "B" relay, and fuse "D".

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