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Discussion Starter #1 (Edited)
On a 1999 EX with 160K miles purchased a couple of years ago with 130K miles. (A bit of hopefully irrelevant information - the van needs its EGR plumbing cleaned (codes, lights, rough starting unless warm). The van has a rebuilt transmission and an add-on cooler with the hope of longer life.)
My problem is the air compressor clutch does not engage when the A/C switch is turned on (with the vehicle running).
On the 'downstream' side of the PCM, if I jumper across the switched pins in the clutch relay socket, there is a current of about 4A and the clutch engages as expected and the compressor runs normally. I have not tried grounding the PCM side of the relay coil but the pin to ground circuit remains open at all times indicating the PCM does not pull this to ground to close the relay switch.
On the 'upstream' side of the PCM, the line from the climate control unit to the pressure switch is pulled to ground and the pressure switch is electrically closed. When the connector is plugged into the pressure switch, the condenser fan starts (turns on without intervention from the PCM). So I assume the input to the PCM is also being pulled to ground.
Does anyone know what other inputs the PCM uses to determine whether or not to engage the A/C clutch? Is there a potential the EGR issue prevents the PCM from engaging the A/C clutch? My next task is to clean out the EGR ports, but was hoping to address the A/C while waiting for gaskets.
Thanks to anyone who has some insight on this.
 

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The most common cause of compressor not engaging is low coolant. It's a feature to protect the compressor. Have you checked the AC pressure?
 

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Discussion Starter #3
Thanks for the reminder davedrivesody. The pressures are good and the pressure switch conducts from pins 1 to 4.
The wire leading to pin 4 is connected to ground. So I believe the request reaches the A/C ON Input (ACS) in the PCM. But the PCM does not seem to ground the ACC pin to pull in the clutch relay. Which led to my question about what logic the PCM uses to determine whether or not to engage the compressor.
Thanks again for making sure I was not missing the obvious and trying to find an electrical solution for a mechanical problem.
 

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Still verifying the obvious stuff...is the fan switch set higher than OFF? The AC won't engage if OFF.
 

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Discussion Starter #5 (Edited)
Good question. Fan is actually on auto but temperature is set to minimum which seems to be interpreted as max A/C (the re-circulation switch comes on automatically and the fan goes to full speed). Definitely not 'Off'. The other confirmation of this is the call for A/C coming from the CCU and passing through the pressure switch. (On the other hand, I have not traced the circuits to make sure there is not an independent fan signal going into the PCM.) Thanks for checking.
 

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Sorry, it's on the manual system (LX) that the fan set to OFF will disengage the AC. Ignore that comment.

Back on track, the signal from the PCM through the pressure switch goes to the climate control unit, not directly to ground. Looks like the CC unit has the final say whether the compressor clutch relay is energized or not.

The evaporator temperature sensor also communicates with the CC unit. If the evaporator is too cold, the AC will not operate. Maybe test that sensor?
 

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Discussion Starter #7
Thanks again. As far as I can tell, the CC unit is grounding the signal so the CC unit is happy and is trying to tell the PCM to switch on the compressor. But the PCM does not switch on the compressor but leaves the connection with high impedence. I will resolve the EGR issues, get rid of the associated codes and see if the PCM is a bit more cooperative.
 

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This is where a good pro level scan tool is very helpful. Have you done the on board diagnostic of the HVAC system? You turn the ignition switch to ON, press the Auto and the Off button for one minute. It will light up certain segments of the temperature indicator if there is a malfunction.
 

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There is also a way to display sensor inputs to the HVAC system. to run the sensor input display mode, turn the ignition switch off, press and hold the Auto and Recirc buttons and start the engine. Release the buttons when the engines starts. It will flash the sensor number then the value for that sensor in degrees C. If it says Er then that means there's an openor short in that circuit or sensor.
 

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Discussion Starter #11
Tried both tests but neither seemed to work. Display kept showing 68 Full Auto. Tried with the system initially off and initially on but nothing, Are these diagnostics perhaps third gen (or later 2nd gen) features?
 
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