AC compressor clutch problem

Tiny
IEDWARDS23
  • MEMBER
  • 1996 MERCURY MARQUIS
  • 4.6L
  • V8
  • 2WD
  • AUTOMATIC
  • 175,000 MILES
When turn the AC on in my car the compressor does not come on. To test the compressor I jumped pin 30 and 87 where the relay goes. The compressor comes on with that test. That led me to think that maybe compressor is not getting signal from computer. I started the car and turned on the AC and checked the voltage on pin 86 and it has 11.3 volts. I have tested the relay and it is working properly. I also tried another known good relay just to make sure, but still not working. Pin 85 is supposed to be the ground and appears to good. Need help figuring out what is the problem.
Monday, August 6th, 2018 AT 12:31 AM

15 Replies

Tiny
ASEMASTER6371
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Good morning.

Yes, pin 85 is the ground that is controlled by the PCM. It will ground the relay when it sees the correct signals from the high and low pressure switches.

The high side is a normally closed switch.
The low side is a normally open switch and is closed with 20 lbs of Freon in the system.

Check the pressures in the system as well as the switches themselves before assuming the PCM is the failure.

Roy
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Monday, August 6th, 2018 AT 4:26 AM
Tiny
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The low and high side pressure is good. I also checked the voltage on pin 85 when I turn the AC but I think I will need to check voltage on that pin in circuit. Since high side is normal closed I will close the circuit on low side connector to see if the problem is there. If I am still having the same issue I will bypass the PCM ground to see if I can get the compressor to click on. If I can confirm all the switches are working correctly and bypassing the PCM works, I will try to replace PCM.
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Monday, August 6th, 2018 AT 9:01 AM
Tiny
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Okay.

Did you verify Freon in the system.

Roy
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Monday, August 6th, 2018 AT 9:08 AM
Tiny
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ASEMASTER6371
Yes, I charged the system myself and all the pressures look good.
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Monday, August 6th, 2018 AT 9:31 AM
Tiny
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Okay, check for voltage on both sides of the switches with the engine running.

Roy
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Monday, August 6th, 2018 AT 9:33 AM
Tiny
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I did some testing and the results are pretty strange. With car on and AC on I only get voltage on one side of the low pressure switch (2 pin). This is strange because I replaced the low pressure switch when I replaced the accumulator/ drier. I tried to complete the circuit with a paper clip inside the connector but still not getting voltage on the other pin. Then I basically bypassed the connector by linking the two back probes with test leads, but still do not get compressor to turn on. I thought I had two issue because even after link the connections by connecting test lead to back probe, I am still not getting voltage inside the connector on one side. So assumed I have a bad connector. Then to my surprise I do not see voltage on either pin on the high pressure switch. I apply power to one pin but I do not get voltage on the other one. When trying to complete the circuit on the high pressure connector with paper clip, I still do not get power on the other pin. I highly doubt that I have two bad connectors. I basically bypass all the connectors going to the PCM but still cannot get compressor to turn on. I was also able to get the compressor to kick on if I use the battery ground for the relay instead of the PCM. Anybody have any ideas? I am ready to just get a junkyard PCM and if that does not work I will just rig up a switch. LOL. But it is really bothering me not knowing what is causing this problem.
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Monday, August 6th, 2018 AT 11:44 AM
Tiny
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Power for the switches comes from the control head.

Diagram attached.

Roy
Image (Click to make bigger)
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Monday, August 6th, 2018 AT 11:59 AM
Tiny
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So the switches are not in series they get power independently from the power head? Also, I applied power directly to the line going the the PCM and I am not sure why the compressor would not kick on if the PCM is getting power on that line.
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Monday, August 6th, 2018 AT 12:15 PM
Tiny
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Yes, they are in series if you see in the diagram I attached and that sends power to the PCM. The PCM grounds the control side of the relay when it sees those voltages from the switches.

With the low-pressure switch jumped, does the relay activate? Did you check for power to the AC clutch?

Roy
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+1
Monday, August 6th, 2018 AT 12:20 PM
Tiny
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With low pressure jumped relay does not activate. There definitely seems like there is an issue on the control side since compressor comes on when I use battery ground instead of the PCM ground. It is weird that when I apply power to the PCM the relay is still not activating.
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Monday, August 6th, 2018 AT 12:30 PM
Tiny
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Apply power to the PCM? That is a ground. If applied power to that wire, you fried the PCM.

Roy
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Monday, August 6th, 2018 AT 12:33 PM
Tiny
IEDWARDS23
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Not power to PCM ground. Applied power to the wire on the high pressure switch. According to your picture I applied power to C150.
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Monday, August 6th, 2018 AT 12:37 PM
Tiny
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Okay, makes better sense.

Did you check to see if the control side of the relay has power with the key on?

It looks like you have a bad PCM here.

Roy
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Monday, August 6th, 2018 AT 12:40 PM
Tiny
IEDWARDS23
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With key on and AC pin 85 is grounded and pin 86 has about 12 volts. It is strange. I am going to try replace the PCM. Do I need to pull PCM from a Grand Marquis or can I use one from a Crown Victoria 1992-1996 or Lincoln Town Car 1992-1996?
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Monday, August 6th, 2018 AT 12:47 PM
Tiny
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That is the control side. If it is grounded then the relay should click or activate.

Recheck that.

No, new one. Used one will not work. Needs to be flashed to your car or a VIN mismatch will lock you out. Must be 1996. 1995 and back are OBD1. You have OBD2.

Roy

https://www.rockauto.com/en/catalog/mercury,1996,grand+marquis,4.6l+v8,1199583,electrical,engine+control+module+(ecm+computer),2896
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Monday, August 6th, 2018 AT 1:02 PM

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