a/c control question regarding C5 corvette PCM
09-04-2013, 07:42 PM
#1
a/c control question regarding C5 corvette PCM I have installed an 04 corvette LS6 w/ drive by wire into my 85 corvette. I am using an 02 Corvette PCM to run it. I have it running and driving fine. I'm now trying to get the a/c working properly. The a/c hardware is all installed, charged, and functional. But I have to sort out the control system/wiring.
I know the preferred method to run a/c is to let the LS PCM control the compressor. My custom harness is wired with this in mind. I have a 3-pin GM refrigerant pressure switch/transducer which is wired into this control circuit as well.
The problem I've run into, which I was not aware of when choosing my parts, was that the corvette PCM's apparently will not accept a 12v+ activation signal for a/c request, which is what my C4 HVAC dash unit would be sending. The C5 PCM's apparently require a class 2 serial data signal which I cannot produce in my C4 car.
I've also read here that hp tuners cannot change this setting in the PCM to a 12vdc signal preference. So I am stuck.
My options at this time seem to be:
1) use the C4's stock HVAC controller, switches/sensors, and wiring to control the a/c system, while the LS PCM remains ignorant of the a/c altogether. This would allow the compressor to cycle via the C4 stock control switches as it was designed to do when built. But then my fans would not kick into high speed and I could have more difficulties compensating for the load at idle, especially since I have a good size cam and it doesn't always idle well anyway.
For the time being I have experimented by jumpering the a/c compressor with the engine idling and it does not seem to dip down at all and will happily idle for as long as you want. But, that might change with throttle blips etc. during actual starting and driving.
2) Pay about 200 bucks for a C5 HVAC control unit and find some way to wire it into my C4 as a dummy unit between the C4 dash controller and the C5 PCM, so that when the C4 control unit requests a/c, it turns on the C5 unit which will then send out the correct data signal to the PCM. This would give the LS PCM full control over the idle speed and fans.
It might be bad for the a/c system hardware itself though, as I have one of the old radial/r4 pancake compressors that are said to be crappy, and I would say that it needs to cycle on and off periodically to live long-term. I've read that the LS PCM's do not cycle the compressor signal at all, so I fear that letting the LS PCM run this R4 compressor could be bad in the long-term.
I've read on the forum that somebody did theirs this way, but there weren't many details of how they did it.
The reason for this post is to get any ideas I've overlooked. Here is a third "hybrid" control option that I am considering, but I'd like more info on it.
3) Use the C4 stock a/c control setup to operate the compressor as in option 1, BUT backfeed the compressor 12v+ signal through the "clutch status" wire to the C5 PCM to let it know the a/c is running. That would in theory give me the best of both worlds...easy control of the compressor with the stock C4 stuff, but fan engagement and idle control from the LS PCM. I've already tried it this way and my fans kick on with the compressor, but the idle did not seem to kick up (it may not be tuned to do so, though, I'll have to check later). Would doing this cause any damage or issues with the PCM in the long-term?
Thanks in advance for any ideas or help.
I know the preferred method to run a/c is to let the LS PCM control the compressor. My custom harness is wired with this in mind. I have a 3-pin GM refrigerant pressure switch/transducer which is wired into this control circuit as well.
The problem I've run into, which I was not aware of when choosing my parts, was that the corvette PCM's apparently will not accept a 12v+ activation signal for a/c request, which is what my C4 HVAC dash unit would be sending. The C5 PCM's apparently require a class 2 serial data signal which I cannot produce in my C4 car.
I've also read here that hp tuners cannot change this setting in the PCM to a 12vdc signal preference. So I am stuck.
My options at this time seem to be:
1) use the C4's stock HVAC controller, switches/sensors, and wiring to control the a/c system, while the LS PCM remains ignorant of the a/c altogether. This would allow the compressor to cycle via the C4 stock control switches as it was designed to do when built. But then my fans would not kick into high speed and I could have more difficulties compensating for the load at idle, especially since I have a good size cam and it doesn't always idle well anyway.
For the time being I have experimented by jumpering the a/c compressor with the engine idling and it does not seem to dip down at all and will happily idle for as long as you want. But, that might change with throttle blips etc. during actual starting and driving.
2) Pay about 200 bucks for a C5 HVAC control unit and find some way to wire it into my C4 as a dummy unit between the C4 dash controller and the C5 PCM, so that when the C4 control unit requests a/c, it turns on the C5 unit which will then send out the correct data signal to the PCM. This would give the LS PCM full control over the idle speed and fans.
It might be bad for the a/c system hardware itself though, as I have one of the old radial/r4 pancake compressors that are said to be crappy, and I would say that it needs to cycle on and off periodically to live long-term. I've read that the LS PCM's do not cycle the compressor signal at all, so I fear that letting the LS PCM run this R4 compressor could be bad in the long-term.
I've read on the forum that somebody did theirs this way, but there weren't many details of how they did it.
The reason for this post is to get any ideas I've overlooked. Here is a third "hybrid" control option that I am considering, but I'd like more info on it.
3) Use the C4 stock a/c control setup to operate the compressor as in option 1, BUT backfeed the compressor 12v+ signal through the "clutch status" wire to the C5 PCM to let it know the a/c is running. That would in theory give me the best of both worlds...easy control of the compressor with the stock C4 stuff, but fan engagement and idle control from the LS PCM. I've already tried it this way and my fans kick on with the compressor, but the idle did not seem to kick up (it may not be tuned to do so, though, I'll have to check later). Would doing this cause any damage or issues with the PCM in the long-term?
Thanks in advance for any ideas or help.
09-06-2013, 06:44 PM
#6
Many times, you just can't do it with HPT. Its a case of buyer beware, just because they have all the glitter when advertise ir, doesn't mean the meat and potatoes are there.
09-07-2013, 10:33 AM
#7
As Pacer said TC2 only see pic...
Type 3 12v+ ac request, high and low switches
I'm real suprised that EFI or Hpt has not added that.
Those guys are in a constant battle to out do each other.
And thats good no matter which you buy.
Type 3 12v+ ac request, high and low switches
I'm real suprised that EFI or Hpt has not added that.
Those guys are in a constant battle to out do each other.
And thats good no matter which you buy.
Last edited by sreve; 09-07-2013 at 10:40 AM.
