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Old 04-22-2006, 11:53 AM   #1
tvoss265
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Default A/C Problem: does PCM use other sensors than low pressure switch?

I have a used 92 SL1, A/C never worked. If I jumper the low pressure switch, the compressor still does not come on. But if I activate the A/C relay everything seems to work normal (compressor on, fan on, idle boost) and it blows cold air.

I'm just trying to figure out if I have to take it to the Saturn dealer. Or Can I take it to a normal A/C mechanic for 1/2 the cost, but probably doesn't know anything about the Saturn PCM.

Are there other sensors the PCM monitors before turning on the A/C?

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Old 04-22-2006, 12:09 PM   #2
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Default Re: A/C Problem: does PCM use other sensors than low pressure switch?

When you say activate do you mean turn on the a/c from the dash? And is this when you bypassed the low pressure switch? And is your a/c manual, not auto a/c? And lastly when the a/c was on did you have some cool air coming from the outlets? If yes to all then you may just be low on freon and can go to your local a/c mechanic to service the system. If no cold air then check for burst hoses, leaks around fittings, telltale refrigerant oil leaking anywhere.
I'm going to take a scientific wild a** guess that if you have a manually operated a/c then the PCM has nothing to do with controlling your a/c.
BTW you seemed to have ruled out close to 95% of your problems!?

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Old 04-22-2006, 01:18 PM   #3
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Default Re: A/C Problem: does PCM use other sensors than low pressure switch?

No, that is not it. When I push the dash button, the light comes on, but nothing else. When I bypass the low pressure switch, it still does not come on. When I force the relay to power the compressor, then it seems to work normal.

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Old 04-22-2006, 01:48 PM   #4
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Default Re: A/C Problem: does PCM use other sensors than low pressure switch?

Power to the relay missing? can you switch relays w/another? Checked fuses?

Last edited by fdryer; 04-22-2006 at 01:54 PM..

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Old 04-22-2006, 05:12 PM   #5
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Default Re: A/C Problem: does PCM use other sensors than low pressure switch?

Fuses and relay are fine. The PCM is not sending the signal to activate the relay. (The wire is fine also)

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Old 04-22-2006, 06:15 PM   #6
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Default Re: A/C Problem: does PCM use other sensors than low pressure switch?

The computer will interrupt AC for Wide open throttle, low voltage, and engine overheat conditions. Also you probably know this but you need the inside blower fan on at least "1" and you've likely tried "4" which has no resistors at all and would certainly effectively signal A/C demand.

You can check your coolant temp sender, in case it is having issues. Some bozo could have tampered your check engine light to go with this.

Also check your alternator for good output under load. Turn on the high beams, rear defroster, and an AM radio station and listen for a buzz mixed in the radio that changes with motor revs. Monitor for 14.something volts at 1500 rpm with all the stuff on and at idle with few accessories on.

You should find an AC shop will do an ok diagnosis. Probably the dealer sneaks cars out the back of the service bay to an AC shop down the street at a markup anyway.

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Old 04-22-2006, 07:13 PM   #7
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Default Re: A/C Problem: does PCM use other sensors than low pressure switch?

I've searched around and it appears that you posted this problem back in 5/05 and found that when you grounded pin 86 of the relay that you were able to have a/c. If this is the same vehicle then w/o the schematic for your model I would think that the ground is either a signal provided by the pcm or is connected directly to the car body. You seem to have gone this far to troubleshoot your a/c so it should be just a matter of figuring out if a ground is missing/disconnected/corroded or something. Will tying the wire from relay pin 86 to car ground allow you to switch the a/c on/off from the dash? The suggestion from ssicarman to eliminate the pcm from the problem(past post reply) is that it would be costly to replace and not likely to be the source of the a/c problems. I'm assuming that pin 85 is the hot 12v source.

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Old 04-24-2006, 12:05 PM   #8
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Default Re: A/C Problem: does PCM use other sensors than low pressure switch?

Thanks, that's some good info. I did try to fix this last year, but ended up going the summer without. I thought about adding a switch to ground the relay and force the A/C, but really wanted to fix the problem, not just bypass it.

I think it may have something to do with the voltage, as I did some other work, cleaned up the alternator wires, and now the compressor does come on when I bypass the low pressure switch.

Found out that the PCM is not bumping the idle, so I had to get someone to hold the idle at 1500 rpm to get the compressor to pull in the freon.

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