Posted by David G on May 05, 2019 at 18:59:12 from (209.181.208.169):
The AC compressor does not come on.
There are two sides to the circuit.
The first is a permissive circuit that starts out as an input to the ECM, then ends up on the heater control with the AC button pushed in, This circuit is grounded to tell the ECM it is OK to run compressor if it wants to. I believe this circuit is the problem. I would expect 5 volts on the good side of the problem and ground on the bad side of the problem. I am working my way through the circuits, from the heater control towards the ECM, as this increases in difficulty to get to the connectors. It appears I have 0V on both sides of the pressure switch, which is first device after the heater control. I believe the problem is closer towards the ECM, or I would have voltage there. The next thing in the circuit is a thermal overload mounted on the compressor and it is a bugger to get too. Those devices also appear to fail, according to google.
The second circuit is an output from the ECM through a relay to the compressor clutch, I jumped out the relay and clutch pulled in. I was getting about 40 PSI on the suction side, and about 150 on the discharge side. We were getting cold air coming, but the pressures did climb as the condenser fans did not come on because the computer was not telling them to run. I think I might have pre charged it a little too much, once running consistent I will check that out. It is close, but might be just a little high. It is hard to check this without the condenser fans running.
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