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Old 08-06-2002, 08:01 AM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Since there are no self-diagnostics here you are going to have to do the tests. Your goal is to decide why the PBC is not doing its job. In this case grounding the A/C control pin of the Klima relay through the low pressure switch.

It appears to not be influenced by imputs, but you must test each pin leading to a sensor for its proper signal. The other possibility is that the unit is totally dead and not controlling anything. Check other functions, such as heater water valve activity. It is a pulsed ground similar to A/C control. The water valve is powered and it should be grounded by the PBC when not needing heat. If you run the temp to MAX it should remove the ground and the valve will open (it will get hot). Any set of wiring conditions that turns the controller off (bad power or grounds) or changes its decision (open circuit after controller before low pressure switch, open or shorted sensor wiring fooling the PBC decision). If the PBC sees the evap temp sensor as cold it turns off the compressor. In reality not only does the sensor have to be the right resistance, but the controller must be biasing the circuit with a precise voltage to use it in a decision. Usually 5v (but I'm not sure in this case), the voltage is applied across the resistor to ground. The voltage drop that occurs is what the PBC is looking for internally.

Do you have the CD or the writen A/C manual for 124?
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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