96 GS A/C not working after alternator replacement
08-22-2016, 04:12 PM
#21
08-22-2016, 06:48 PM
#22
Melting Slicks
I would be testing the Control ground at the Relay with a quality [High impendence] DVOM next.. this separates the control circuit from the Power circuit..
Just seems awful Odd that this popped up after an Alternator fix..
08-22-2016, 10:12 PM
#23
Le Mans Master
Except now my A/C doesn’t work. All the controls on the head unit still work fine - lights up, readout works, can adjust temp, switches between low vents, high vents, defrost, auto, recirculate, fan speed etc – it moves air just does not blow cold.
When calling for air, the compressor clutch does not engage. First thing checked fuses – all ok. Also checked codes on the a/c head unit (none) and with an OBD II (no codes). Disconnected battery negative to try to reset any codes in PCM even though none shown. Did not work
With car running and calling for A/C, checked for voltage at the compressor clutch connector – no voltage.
To verify compressor clutch/diode ok, applied 12 v to compressor side of connector, clutch engages no problem.
So I checked a/c low pressure switch – it’s closed; checked voltage on high pressure switch (ign switch on, calling for a/c) – had 3.05 volts (believe this is ok since reference is 5 v). Replaced a/c compressor relay – probably unnecessary but did it anyway.
FYI Checked static pressure in a/c system – low side 100 psi, high side 110 psi (almost equal, where it should be in static condition). Not sure what else other than PCM doesn’t respond to call for A/C and is not sending ground signal to A/C relay to close it to send 12 v to compressor clutch.
Not sure what else to check or how to test the PCM, any advice would be appreciated.
When calling for air, the compressor clutch does not engage. First thing checked fuses – all ok. Also checked codes on the a/c head unit (none) and with an OBD II (no codes). Disconnected battery negative to try to reset any codes in PCM even though none shown. Did not work
With car running and calling for A/C, checked for voltage at the compressor clutch connector – no voltage.
To verify compressor clutch/diode ok, applied 12 v to compressor side of connector, clutch engages no problem.
So I checked a/c low pressure switch – it’s closed; checked voltage on high pressure switch (ign switch on, calling for a/c) – had 3.05 volts (believe this is ok since reference is 5 v). Replaced a/c compressor relay – probably unnecessary but did it anyway.
FYI Checked static pressure in a/c system – low side 100 psi, high side 110 psi (almost equal, where it should be in static condition). Not sure what else other than PCM doesn’t respond to call for A/C and is not sending ground signal to A/C relay to close it to send 12 v to compressor clutch.
Not sure what else to check or how to test the PCM, any advice would be appreciated.
Your reading of 3 volts off the high pressure sensor for the A/C if you back probed it correctly should be 1.0 to 1.5 volts. That would indicate a very high pressure and the PCM might not like it and not activate the compressor relay. Your 100 psi static reading should relate to about 1.25 volts.
If the compressor does not start when requested and the PCM does not see a change in pressure in about 10 seconds, it will lock out the compressor and not start the compressor ever during that ignition cycle. The engine has to be turned off so PCM can reset (10 seconds) the then you can try to start the compressor again. No MIL lamp is lit during this event or activity.
I would check the connector and for the compressor. Make sure the connections are good since that is close to the area you first worked.
If you back probe the low pressure switch and measure the voltage as you did, you should have 0.0 volts when the A/C is not requested, and about 4-6 volts when you select A/C and request cooling. The engine does not have to run for this test. If this tests OK, then the control head and programmer are working OK and the problem is PCM related (not saying the PCM is bad). The PCM closes the clutch relay with the request from the low pressure switch (5 volts) however the sensor inputs to the PCM have to be OK for the PCM to activate the relay.
08-27-2016, 02:46 PM
#24
Cruising
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Realizing the 96 is different than my 94 with obd2, the thinking and operation of the PCM and sensors might be pretty much the same. So here is some specific information according to my 94 and how things work. Hope it's helpful.
Your reading of 3 volts off the high pressure sensor for the A/C if you back probed it correctly should be 1.0 to 1.5 volts. That would indicate a very high pressure and the PCM might not like it and not activate the compressor relay. Your 100 psi static reading should relate to about 1.25 volts.
If the compressor does not start when requested and the PCM does not see a change in pressure in about 10 seconds, it will lock out the compressor and not start the compressor ever during that ignition cycle. The engine has to be turned off so PCM can reset (10 seconds) the then you can try to start the compressor again. No MIL lamp is lit during this event or activity.
I would check the connector and for the compressor. Make sure the connections are good since that is close to the area you first worked.
If you back probe the low pressure switch and measure the voltage as you did, you should have 0.0 volts when the A/C is not requested, and about 4-6 volts when you select A/C and request cooling. The engine does not have to run for this test. If this tests OK, then the control head and programmer are working OK and the problem is PCM related (not saying the PCM is bad). The PCM closes the clutch relay with the request from the low pressure switch (5 volts) however the sensor inputs to the PCM have to be OK for the PCM to activate the relay.
Your reading of 3 volts off the high pressure sensor for the A/C if you back probed it correctly should be 1.0 to 1.5 volts. That would indicate a very high pressure and the PCM might not like it and not activate the compressor relay. Your 100 psi static reading should relate to about 1.25 volts.
If the compressor does not start when requested and the PCM does not see a change in pressure in about 10 seconds, it will lock out the compressor and not start the compressor ever during that ignition cycle. The engine has to be turned off so PCM can reset (10 seconds) the then you can try to start the compressor again. No MIL lamp is lit during this event or activity.
I would check the connector and for the compressor. Make sure the connections are good since that is close to the area you first worked.
If you back probe the low pressure switch and measure the voltage as you did, you should have 0.0 volts when the A/C is not requested, and about 4-6 volts when you select A/C and request cooling. The engine does not have to run for this test. If this tests OK, then the control head and programmer are working OK and the problem is PCM related (not saying the PCM is bad). The PCM closes the clutch relay with the request from the low pressure switch (5 volts) however the sensor inputs to the PCM have to be OK for the PCM to activate the relay.
08-27-2016, 04:01 PM
#25
Le Mans Master
I thought the 3 volt reading on the high pressure switch would be ok and I hate to just change parts, but I put a new pressure switch on the high pressure side and it seems to have fixed the problem, A/C works again! Big thanks to all who commented and provided advice - this forum is an extremely valuable tool!
Not many of these pressure sensors go bad I've heard. You might be one of the selected few.
