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AC won't come on

Launch on Sun June 15, 2008 3:33 PM User is offline

Year: 1994
Make: chevy
Model: Z28
Engine Size: 5.7
Refrigerant Type: 134a
Country of Origin: United States

Told by local ac shop that pcm was problem so I replaced it and ac worked well for about a week. What could be causing this problem. I'm at a loss on this one.

Brett

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Brett

mk378 on Sun June 15, 2008 4:42 PM User is offline

Check the cycling switch, if you have a two wire switch on the accumulator, unplug it and jumper between the wires in the plug. If it comes on and gets cold put a new switch on, this can be done without removing the refrigerant.

Intermittent operation also frequently results from the clutch gap being too wide. Measure if you have voltage at the compressor clutch when it is supposed to be running, if so then hit the clutch plate carefully with a suitable tool and see if it snaps in and starts cooling.

Some GM's will "lock out" the compressor after the PCM detects a potential problem. That can be reset by disconnecting the battery for a time with the key off. Or replacing the PCM, but that is overkill.

GM Tech on Mon June 16, 2008 8:01 AM User is offline

Well- replacing the PCM was a bad guess-- very, very rarely the cause of no A/C-- what you probably have is a code 46 being set- this will NOT light the service engine light-- but will be read by any good scan tool-- unhooking the battery will reset the code-- much like unhooking the battery to replace a PCM-does-- that is why it worked for awhile-- but if the conditions are not fixed the car will shut down the compressor again to save the compressor-- it counts the cyclesa and know the evap temp and if the algorithm deems necessary- it will set the no a/c code 46-- my guess from here is that you have lost enough refrigerant to set the code-- you need to find and fix your leak and recharge- and quit replacing uneccessary parts- did your a/c guru read the codes? did he try unhooking the battery first?

also- there is no cycling switch on F-cars of that generation to jump- they use an evap temp probe to determine when to shut off compressor for evap freeze protection-- and there is NO A/D on this TXV R/D system anyway-- so be aware of that fact.....the system uses a pressure transducer (3-wire) that decides if there is enough pressure to turn on the compressor-- never jump these transducers- you could damage your PCM-- perhaps that is why they recommended a PCM change.....just a thought

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Launch on Mon June 16, 2008 3:15 PM User is offline

I had a spare pcm. I also reloaded my tune on the pcm removed and the compressor came back on but does not last that long. I believe you are correct that it is loss of freon causing this problem. Perhaps if I reset it for them they can find the leak. Thanks for the help. If you can think of anything else that will help me please let me know.



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Brett

GM Tech on Mon June 16, 2008 3:30 PM User is offline

Compressor shaft seal is most common leak location-- is it slinging oil on the frame rail right next to the compressor? is compressor clutch oily/grimey? the seal can be replaced if you find someone with the tools and who has done a few......

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Launch on Thu June 19, 2008 8:09 AM User is offline

Thanks again for your help guys.

Brett

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Brett

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