Controlling a fixed displacement AC compressor with a 99-02 f-body PCM questions...
11-25-2012, 10:43 PM
#1
Controlling a fixed displacement AC compressor with a 99-02 f-body PCM questions... So I'm building an Ls1 240z with a vintage air AC system. I will be using a sanden 508 fixed displacement compressor.
I have some questions regarding how the PCM cycles a fixed compressor vs. a variable displacement compressor?
I'll admit my knowledge on AC systems is sub par. I have been through over 30 discussions online so far and haven't been able to gather much. I've mostly found bickering with minimal facts.
I'm looking to find out if how the factory PCM controls the factory GM variable displacement compressor would be acceptable for a factory PCM/ fixed displacement compressor combination.
If the factory PCM/ fixed displacement compressor combination would not be a good idea, than is it possible to alter the programming in the factory PCM to correctly cycle a fixed displacement compressor?
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If I have to I will allow the vintage air setup to control the compressor, leaving the IAC & fan control up to the LS1 PCM. I would rather have the PCM control everything if possible.
I know this questions requires HVAC experience, but I think it is slightly more weighted to PCM experience, so that's why I posted it here. Thanks for any comments you might have. -Tyler
I have some questions regarding how the PCM cycles a fixed compressor vs. a variable displacement compressor?
I'll admit my knowledge on AC systems is sub par. I have been through over 30 discussions online so far and haven't been able to gather much. I've mostly found bickering with minimal facts.
I'm looking to find out if how the factory PCM controls the factory GM variable displacement compressor would be acceptable for a factory PCM/ fixed displacement compressor combination.
If the factory PCM/ fixed displacement compressor combination would not be a good idea, than is it possible to alter the programming in the factory PCM to correctly cycle a fixed displacement compressor?
---------
If I have to I will allow the vintage air setup to control the compressor, leaving the IAC & fan control up to the LS1 PCM. I would rather have the PCM control everything if possible.
I know this questions requires HVAC experience, but I think it is slightly more weighted to PCM experience, so that's why I posted it here. Thanks for any comments you might have. -Tyler
11-26-2012, 11:11 AM
#2
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The variable displacement compressors have an internal mechanical regulator valve that holds the pressure within a safe limit. It runs al time when ac is turned on. The fixed compressor use a electric pressure switch on the low pressure side that cycle the compressor clutch on and off when ac is activated. The pressure rise and drop during the cycle. The pcm is set up for the variable compressor and use a high pressure sensor to monitor the system.
When you turn on the ac a wire from the switch inside the car give power to a pin on the pcm.
Then the pcm activate the compressor relay, idle, and fan control. If you let the low pressure switch in the fixed compressor system cycle on and off the ac signal to the pcm, I think that the fan would stop when the pressure is high in the system. (The cycle switch is open).This is not good because it is when you have the highest pressure you needing the fan most.
If you take the lead from the stock pcm compressor relay and let it go through the pressure switch I think that the pcm would set a fault code on the compressor clutch, or compressor relay.P1539, P1546, P0645. When the cycle switch opens, and the compressor clutch disengage. These codes can be disabled in the pcm, or you would have to trick the ac clutch feedback circuit. I don’t know if the high pressure sensor would cause any trouble, but I think you can make a workaround for that if it happens.
When you turn on the ac a wire from the switch inside the car give power to a pin on the pcm.
Then the pcm activate the compressor relay, idle, and fan control. If you let the low pressure switch in the fixed compressor system cycle on and off the ac signal to the pcm, I think that the fan would stop when the pressure is high in the system. (The cycle switch is open).This is not good because it is when you have the highest pressure you needing the fan most.
If you take the lead from the stock pcm compressor relay and let it go through the pressure switch I think that the pcm would set a fault code on the compressor clutch, or compressor relay.P1539, P1546, P0645. When the cycle switch opens, and the compressor clutch disengage. These codes can be disabled in the pcm, or you would have to trick the ac clutch feedback circuit. I don’t know if the high pressure sensor would cause any trouble, but I think you can make a workaround for that if it happens.
11-26-2012, 11:44 AM
#3
The cars don't use a high and low pressure switch they use a pressure transducer. If you used a truck OS in the pcm you could use a high and low switch. There are no settings to edit in the pcm for a/c except when the fans come on/off and wot clutch cutoff.
11-26-2012, 04:13 PM
#4
Thanks you both for your replys. Now I understand whats going on here.... I always saw the F body transducer referred to as a high pressure switch. Never saw a low pressure switchmentioned; coupled with not knowing how the variale displacement compressor worked I was drawing a blank as to how the system operated.
Tori, I will put some thought into your suggestions. BTW your signature picture brings back some memories! I had a 94 roadmaster sedan with a sbc and t56. I ended up parting it out, and the last day I saw it it was stripped down like yours!
Flame, is it possible to load the truck OS onto an f-body pcm?
Looks like worst case scenario I could come up with a circuit to have the f-body os control the fans and wot clutch cutoff while leaving the pressure sensitive clutch control up to the vintage air controller via some relays and diodes...
Do the f-body and or truck pcm os' allow adjustment of idle control parameters when the AC is requested to engage? I'm assuming this is done via IAC and small ignition timing tweaks?
Tori, I will put some thought into your suggestions. BTW your signature picture brings back some memories! I had a 94 roadmaster sedan with a sbc and t56. I ended up parting it out, and the last day I saw it it was stripped down like yours!
Flame, is it possible to load the truck OS onto an f-body pcm?
Looks like worst case scenario I could come up with a circuit to have the f-body os control the fans and wot clutch cutoff while leaving the pressure sensitive clutch control up to the vintage air controller via some relays and diodes...
Do the f-body and or truck pcm os' allow adjustment of idle control parameters when the AC is requested to engage? I'm assuming this is done via IAC and small ignition timing tweaks?
11-27-2012, 08:27 AM
#5
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It is some air and torque parameter in the F body computer. They stabilize the idle drop and the lag when the compressor engages. It is at least 6 parameters and 11 tables that can be altered with Efilive, to smooth the engagement of the compressor. (12212156 os for f-body).
Some truck os can be used on the f body computer. But I don’t know if some of them use the old low pressure on of switch. The most common os to use is the 12212156 Camaro os.
The Impala in my signature is actually a Caprice.
It has a 2010 GM performance parts LS3 with an e-force supercharger, and have a 2000 f-body 4l60e transmission. It is runs on a “411” 2002 f-body computer with a efilive cos5.
At first an ac guy recommended me to use a Sanden 508 ac compressor because the Caprice had this “on and of” low pressure switch system. I didn’t like the look of this high bracket so I made some minor modification on the cross member to fit a low mounted Camaro variable displacement compressor instead. I removed the low pressure switch, fitted the high pressure sensor, changed the orifice, and made some new hoses. It is Camaro fittings in one end and caprice fittings in the other. The evaporator and the condenser is actually old R12 stuff. The computer runs the ac and I have not altered any of the ac stuff in the pcm.
Tor
Some truck os can be used on the f body computer. But I don’t know if some of them use the old low pressure on of switch. The most common os to use is the 12212156 Camaro os.
The Impala in my signature is actually a Caprice.
It has a 2010 GM performance parts LS3 with an e-force supercharger, and have a 2000 f-body 4l60e transmission. It is runs on a “411” 2002 f-body computer with a efilive cos5.
At first an ac guy recommended me to use a Sanden 508 ac compressor because the Caprice had this “on and of” low pressure switch system. I didn’t like the look of this high bracket so I made some minor modification on the cross member to fit a low mounted Camaro variable displacement compressor instead. I removed the low pressure switch, fitted the high pressure sensor, changed the orifice, and made some new hoses. It is Camaro fittings in one end and caprice fittings in the other. The evaporator and the condenser is actually old R12 stuff. The computer runs the ac and I have not altered any of the ac stuff in the pcm.
Tor
12-03-2012, 01:45 AM
#6
Thinking whats in these two pictures would work. It would let the pcm handle the fans, the idle, and a few small AC compressor tasks, IE wot cutoff and hi pressure disengage.
The vintage air thermostat would have the task of switching the compressor clutch to adjust coldness and keep line pressure correct.
Am I in the right state of mind with this combination? It seems it would be the least troublesome and have all the benefits the PCM has to offer.
Does anyone know what pressure the f body os turns on the fans? And what pressure it will disengage the clutch if a high pressure scenario were to arise? Just curious how it would compare to the safety switch vintage air offers... 380psi safety cutoff and 220psi fan turn on. Would these values modifiable in the f body os if for some odd reason I needed to change them?
The vintage air thermostat would have the task of switching the compressor clutch to adjust coldness and keep line pressure correct.
Am I in the right state of mind with this combination? It seems it would be the least troublesome and have all the benefits the PCM has to offer.
Does anyone know what pressure the f body os turns on the fans? And what pressure it will disengage the clutch if a high pressure scenario were to arise? Just curious how it would compare to the safety switch vintage air offers... 380psi safety cutoff and 220psi fan turn on. Would these values modifiable in the f body os if for some odd reason I needed to change them?
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12-03-2012, 04:12 PM
#8
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Yes i think that would do. But you have to disable some fault codes in the pcm.
When the pcm activate the compressor relay (459) it look for +12v on the compressor clutch supply (59). If your vintage air controller cut the power supply to the compressor relay (139) then the pcm sets a fault code. This fault code "enablers" has to be disabled in the pcm with a proper tune tool or the check engine lamp turns on.
The fan 1 enable pressure is 1483 and the disabled pressure is 1310. Fan 2 is 1710 and 1434 kpa.
When the pcm activate the compressor relay (459) it look for +12v on the compressor clutch supply (59). If your vintage air controller cut the power supply to the compressor relay (139) then the pcm sets a fault code. This fault code "enablers" has to be disabled in the pcm with a proper tune tool or the check engine lamp turns on.
The fan 1 enable pressure is 1483 and the disabled pressure is 1310. Fan 2 is 1710 and 1434 kpa.
12-03-2012, 06:37 PM
#9
Yes i think that would do. But you have to disable some fault codes in the pcm.
When the pcm activate the compressor relay (459) it look for +12v on the compressor clutch supply (59). If your vintage air controller cut the power supply to the compressor relay (139) then the pcm sets a fault code. This fault code "enablers" has to be disabled in the pcm with a proper tune tool or the check engine lamp turns on.
The fan 1 enable pressure is 1483 and the disabled pressure is 1310. Fan 2 is 1710 and 1434 kpa.
When the pcm activate the compressor relay (459) it look for +12v on the compressor clutch supply (59). If your vintage air controller cut the power supply to the compressor relay (139) then the pcm sets a fault code. This fault code "enablers" has to be disabled in the pcm with a proper tune tool or the check engine lamp turns on.
The fan 1 enable pressure is 1483 and the disabled pressure is 1310. Fan 2 is 1710 and 1434 kpa.
Are those fan enabler values editable or fixed?
12-03-2012, 07:15 PM
#11
Just looked through the rest of the thread and noticed you already answered that earlier but thanks again!
Looks like the only thing I have to watch out for is that disabling the fault code doesn't mess anything up. When the PCM doesn't see +12v at the clutch that is.
