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Discussion Starter #1 (Edited)
Hello,

I am trying to diagnose and AC issue. I found a post on here that said all wiring diagram PDFs were uploaded to a resources section but I can’t seem to locate that section on the forums.

My 2004 wouldn’t blow cold air so I started with the r134a level, relays, and fuses. Fuses are good, swapped the relay with no change, jump the pins at the relay and clutch kicks in and AC blows cold. Went ahead and added some r134 with compressor running as it was low. Low side settles around 45 with ambient temps at 85. Put relay back in place but clutch doesn’t kick in. Next I jumped the low pressure sensor but clutch still doesn’t kick in. The only time the clutch will kick it and run the compressor is when so first start the car (runs for about 10-15 seconds) or manually jump the pins at the relay.
 

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Does the engine idle OK? Everything is controlled by computers these days and if the idle isn't as smooth as they want it to be the computer will not run the AC.

Does the engine cooling fan seem to be running OK? If it doesn't and the high side pressure goes too high that will also disable the AC. By your account of the low side pressure though this seems unlikely.

Check engine light on? That too might disable the AC.
 

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Discussion Starter #3
Engine idles perfectly. Slight drop in RPMs when the clutch does kick in but what I feel is pretty normal. Only check engine I ever get is the P0014 for the camshaft actuator on long drives but it goes away on it's own and I haven't taken the time to pull and clean it. Truck has 178k mi and no current check engine lights or codes. The cooling fan seems to be strong and pulling a lot of air. I would like to find the high pressure switch to check the wiring on it. When I bought the vehicle, it had been sitting for a year and a half and some mice had gotten into some wires. I soldered and heatshrinked all the wires and gradually replaced/fixed any issues. Aside from the AC, everything works perfectly now but I know there's still a chance I have a bad wire somewhere that may be the cause. I can't seem to locate a wiring diagram though to start putting my multimeter to work.


Does the engine idle OK? Everything is controlled by computers these days and if the idle isn't as smooth as they want it to be the computer will not run the AC.

Does the engine cooling fan seem to be running OK? If it doesn't and the high side pressure goes too high that will also disable the AC. By your account of the low side pressure though this seems unlikely.

Check engine light on? That too might disable the AC.
 

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The high pressure sensor is located down behind the compressor. It is a 3 wire sensor consisting of a 5 volt reference, a 0 volt reference and a varying return signal.

You can pick up a $12 ELM327 clone and use that with the Torque android app or the OBD Fusion App for android or iPhone. With this you can read your AC high pressure. I can point you to custom parameters that do not come with the apps, for things like engine cooling fan speed, cam phaser operations, accelerator pedal and throttle position sensor data and so on.


I don't think there are any wiring diagrams here on this site but you can find them here...along with a lot of GMT360 enthusuiasts...

 

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Discussion Starter #5
Thanks, I picked up a gauge set and with the clutch relay jump I am getting around 45 psi on the low side and 205 psi on the high side with an outside temp of 89F. Seems to be ok to me.

I do have a WiFi OBD adapter that is based on elm327 I believe already and normally use the InCarDoc App on iPhone to pull codes. I will look and see if I see anything related to high side pressure in the meantime.

The high pressure sensor is located down behind the compressor. It is a 3 wire sensor consisting of a 5 volt reference, a 0 volt reference and a varying return signal.

You can pick up a $12 ELM327 clone and use that with the Torque android app or the OBD Fusion App for android or iPhone. With this you can read your AC high pressure. I can point you to custom parameters that do not come with the apps, for things like engine cooling fan speed, cam phaser operations, accelerator pedal and throttle position sensor data and so on.


I don't think there are any wiring diagrams here on this site but you can find them here...along with a lot of GMT360 enthusuiasts...

 

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So the parameters I was referring to are live data streams, not codes.

An advantage to live data is you can see how things operate in real use conditions like what hi pressure is rolling down the road.

Screenshot_20190718-130113.png



Screenshot_20191030-142126.png
 

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Discussion Starter #7
Ok, so the iPhone app OBD fusion will do that? If so I’ll go ahead and buy it.
 

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I can only guess that the iPhone version has the capabilities that the android version does. I have no iPhone so cannot say definitively.

Also, the parameters for our platform will need to be loaded. Those I can put together in a .csv file format for import. That assumes the iPhone version uses that file format for PID import.
 

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So I remembered I had purchased Torque Pro previously so I reinstalled it on my kindle fire (Android) and have it connecting to the car. Will this work?
 

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So I did some more googling and discovered how to go in and add the predefined PID sets in torque for GM. This added a lot of new commands and values and I can see a lot of new data now however the [GM]AirCon High aside Pressure sensor is not showing a value. If I go into the PID Editor and run the test I get the following...

Result for equation: [No Response]
Command: 221564
Response: 7F22156412

I’ll wait on your response in case maybe this sensor is not defined correctly by default. Either that or my high side sensor is not working correctly which is why the AC isn’t working correctly.
 

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wonder if or when they changed that pid, or has it always been that way. Anyway, edit thusly:


change that 221564 to 221144.

the equation should read:
(A*1.83)-14.7

make sure the header field reads 'Auto'.
 

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There are a number of additional pids for our 4.2 TrailBlazers/Envoys/etc I have discovered in the last year or so. I need to put together a file for import so they don't all have to be entered by hand. There's a thread about it over here...

 

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Discussion Starter #13
Ok, I updated the PID and equation.

I am getting a value of -12.87 when vehicle is running with the AC on (and cold) with compressor relay pins jumped. The test shows...

variables used in equation:
A=1

=(A*1.83)-14.7
=1.83-14.7


wonder if or when they changed that pid, or has it always been that way. Anyway, edit thusly:


change that 221564 to 221144.

the equation should read:
(A*1.83)-14.7

make sure the header field reads 'Auto'.
 

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The pressures you had on your gauge set seem about right but I would still pursue the pressure sensor as time allowed.


OK, let's review. This is the 6 cylinder 4.2 right?

The 221144 PID works on my 2002 4.2 and also my 2005 5.3 Yukon.

You mentioned some mice had chewed some wiring. Where was that? When I got my TrailBlazer a previous owner had done some work on some underhood wiring beneath the PCM. In doing so he crossed up 4 gray wires. He had two 5 volt reference wires connected together and two sensors connected together!!

I would say it is time to verify power, ground and signal down at that high pressure sensor.

Have you picked up the wiring diagrams yet, I'll post a partial here.

Screenshot_20200628-190148~01.png
 

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Discussion Starter #15
yes it’s the 4.2l. And it was in the wires below the 3 pcm connectors near the intake manifold. There was probably 8-10 wires that I had to solder and heatshrink. It cleared up several codes I was throwing for the cooling fan speed, IAT sensor, and a few others.
I’ve not found the wiring diagrams. I was looking here and found a thread saying they were uploaded as PDFs so a resources section but I’ve not found the resources section on this site and the old links I found to it are dead now.
 

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Look back in this thread to post number 4. I put a link there to service literature on another site.
 
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