98 s70 strange ac problem... Not gap ect.

[Ozark Lee]

Sure, just go around it and see if things return to normal. In many ways it was a solution in search of a problem in the first place. If I recall you can just unplug the bullet connectors to go around it and you don't even have to cut and splice.

...Lee

[Heath720]

Lee... I have zero voltage running to compressor. Fan relay and low voltage all testing fine. If I apply 12v directly to compressor it runs. Here's new strange...If I reset the ecu by pulling battery cables, and jump the low switch the compressor will kick on and run a second. I cant duplicate unless I reset the ecu.

[Ozark Lee]

I dug through the schematics and located the correct version for your car.

On your setup the compressor is more directly controlled by the ECU than on the 850s. The compressor is directly controlled by relay 2/22 which is the small relay, just below the fuses, in the kidney shaped auxiliary fuse box. That relay gets its signal to turn on from the ECU. The relevant inputs to the ECU are the MCC control head which should have + battery voltage on MCC pin B-22 when the AC is turned on. From the control head the control voltage flows through the low pressure pressostat and provides a + battery voltage to pin B-25 on the ECU. Since you have battery voltage at the pressostat it is getting to the low pressure switch just fine. Back probe the wires at the low pressure switch, while it is plugged in to make sure that you have battery voltage on both terminals.

The other input to the ECU is the high pressure switch. The ECU sends a signal to the high pressure switch from ECU pin B-29 (Violet/White wire) and receives a response from the high pressure switch on ECU pin B-9 (Blue/Orange wire). There is also a common terminal from the high pressure switch which goes to ECU pin B-28 (Brown/Pink wire). I think that the high pressure switch is a double pole switch that toggles the response from +voltage to common depending on its state.

When the ECU is satisfied that everything is fine it drops pin B-40 (Blue/Black wire) to common which turns on relay 2/22. When relay 2/22 is energized then battery voltage flow to the compressor and turns it on.

This would be much simpler if I could post the schematics but the Volvo lawyers get all bent out of shape if I post them.

Given what you have done I would first verify that you have + battery voltage on both sides of the pressostat when it is plugged in. If that test is passed I would then turn to the compressor relay and work it back from the other end.

The relay should have terminals marked 85, 86, 87, 30, and 87a. The coil for the relay is between pins 86 (+ battery, Red wire) and 85 (Should go to ground when the AC is turned on). The actual voltage to the compressor comes into the relay on pin 30 (Green/Yellow wire) and leaves the relay when it is energized on pin 87 (Green wire). Pin 87a is not used but battery voltage should be present there when the relay is not energized.

The decisions that the ECU makes are all in firmware and I know of no flow charts that explain what it is doing internally. The ECUs are very robust and an actual ECU problem, while not impossible, is unlikely.

...Lee