buell_cyc

1996 Grand Sport coupe; C68 electronic climate control; 16,300 miles

All A/C controls work perfectly, all settings on head unit work as they should, blows ice cold a/c

Then one day took for a drive and battery light came on, turned everything off (including a/c, radio) got it home and checked the voltage output on the alternator before I shut it off. Voltage output was 11.8 volts.

Disconnected negative battery cable, removed alternator. Didn’t want to buy a rebuilt or non original alternator, so bought a rebuild kit from Alternator Starter Parts Wholesale for Delco CS144 late production alternator for 124-145 amp (kit for GM alternator 10480200 or 10480202 among others, believe this applied to 1996 corvette). Replaced bridge rectifier, voltage regulator, brushes and capacitor. Put back on car reconnected negative side to battery and it worked great – 14.7 volt output no battery light, all ok. Except now my A/C doesn’t work. All the controls on the head unit still work fine - lights up, readout works, can adjust temp, switches between low vents, high vents, defrost, auto, recirculate, fan speed etc – it moves air just does not blow cold.

When calling for air, the compressor clutch does not engage. First thing checked fuses – all ok. Also checked codes on the a/c head unit (none) and with an OBD II (no codes). Disconnected battery negative to try to reset any codes in PCM even though none shown. Did not work

With car running and calling for A/C, checked for voltage at the compressor clutch connector – no voltage.

To verify compressor clutch/diode ok, applied 12 v to compressor side of connector, clutch engages no problem.

So I checked a/c low pressure switch – it’s closed; checked voltage on high pressure switch (ign switch on, calling for a/c) – had 3.05 volts (believe this is ok since reference is 5 v). Replaced a/c compressor relay – probably unnecessary but did it anyway.

FYI Checked static pressure in a/c system – low side 100 psi, high side 110 psi (almost equal, where it should be in static condition). Not sure what else other than PCM doesn’t respond to call for A/C and is not sending ground signal to A/C relay to close it to send 12 v to compressor clutch.

Not sure what else to check or how to test the PCM, any advice would be appreciated.

grandspt

Does the auto fan work? Also if you turn the heat up on high (like 90) does the fan (when fan is in auto mode) blow high (10 on the display). If not the temperature display is probably flashing and set a code.
Let me know what happens when you do this.

Hooked on Vettes

Start the engine.
If AC is requested you should measure 12 volts on the Dark Blue wire at the Low Pressure Cycling Switch. If the Low Pressure Cycling Switch is closed the 12 volts passes thru the switch to the Dark Green/White wire which goes to the PCM as the AC request signal.

If no 12 volts reseat the connector on the back of the HVAC Control Head.
If still no 12 volts it points to a bad HVAC Control head or wiring problem from the Control head to the Dark Blue wire at the Low Pressure Cycling switch.

If you do measure 12 volts when AC is requested, reseat the PCM connectors.

You can verify the PCM is not the problem by disconnecting the Low Pressure Cycling switch two pin connector and apply a fused 12 volts to the disconnected Dark Green/White connector pin.

Start the engine which I'm not sure is necessary. This will simulate the AC request signal and the compressor clutch should engage assuming no DTC's are set that would prevent the AC Clutch from engaging.

buell_cyc

Thanks for the reply, appreciate the help. Set temp to 90 and hit auto, blows hot air on high (10). No flashing and no code

buell_cyc

Thanks to you as well for the reply, appreciate the help. I started the engine and set for 60. Measured voltage on both sides of low pressure switch - measured 6.55 volts. Any ideas?

antfarmer2

Jump the wires on the switch if compressor kicks on replace switch. If not check connections on compressor.

buell_cyc

Thanks for the reply. Tried that originally when the clutch didn't engage. Jumper in the low pressure side switch does not help. I did jumper the relay (across internal switch pins 30 & 87) and a/c blows cold as normal.

grandspt

Ok, sorry it doesn't have the same issue that I had when I disconnected my battery for a week and then reconnected it and lost my AC.

antfarmer2

Sounds like you found it.

buell_cyc

I wish, only works with the relay jumpered out of the circuit. Still not getting the 12 v to the compressor clutch otherwise. Probably either head unit or PCM issue

WVZR-1

The FSM mentions that Programmer Logic Can be lost doing most of the things that you'd customarily do when replacing the alternator. I thought for a Programmer Logic Reset you just with the KEY OFF disconnected negative cable and the FSM mentions a 1 minute duration.

You mentioned all of the appropriate diagnostic procedures I believe BUT .....

Maybe? I'd maybe try.

Do you have the FSM? If yes I think it's covered in several areas.

Did you try the "fused jumper Dave (Hooked on Vettes) mentioned"?

buell_cyc

Still wouldn't mind hearing what happened and how you fixed it. Never know still might be of help. Thanks

antfarmer2

Try the reset.

buell_cyc

Thanks for the help, I've tried the reset procedure a couple times with no difference. Haven't had a chance to try the fused jumper yet.

WVZR-1

I'd do the "fused jumper" and I'm sure you have BUT did you actually check maybe the programmer fuse? It seems silly to ask but ....

grandspt

Sorry, after reading my reply it looked like I was hiding something (not my intention).
I pulled my HVAC programmer out and checked the electrolytics but they were fine. My car had a problem that turned out to be the Blend Door Actuator.
The plastic gear inside the blend door actuator is press fit onto the metal sleeve which slides onto the blend door. The plastic gear engages another small gear (blue in the picture) which is the potentiometer that tells the HVAC programmer what position the blend door is in. So the plastic gear cracks (and it was probably cracked for a long time but still worked because it never had to do a complete cycle) but when you disconnect the battery for a few days and reconnect it, the HVAC programmer sends a signal to the Blend Door Actuator to cycle completely. But the gear is cracked and now the potentiometer cannot send the correct postion information to the HVAC controller. This caused me a lot of grief because I changed the water pump and disconnected the battery, everything worked fine before I disconnected the battery to replace the water pump.

grandspt

Oh another thing GM does not sell the original part and all the replacements are made by Dorman. Dorman does not fit physically! I also measured the resistance of the Dorman and it seemed like the potentiometer was of a lower impedance 5Kohm instead of 10Kohm and if you read the shop manual for diagnostics it should be around 10Kohms.
I ended up repairing mine by removing the metal sleeve and gear from the housing, I heated up the metal sleeve with a small butane torch so the crack in the gear would close up. Then I epoxied the gear making sure the epoxy would not interfere with anything. I aligned the timing marks on the gears and reinstalled it. I did have to disconnect the battery again for a day or so to get the HVAC programmer to do a full cycle of the Blend Door Actuator. But it has been working fine now.
Good luck I hope you track your problem down quickly.

Hooked on Vettes

I gave you some bad information.
The AC request signal comes from the HVAC Programmer when the HVAC Control head requests AC.

That's the unit bolted to the firewall to the right of the gas pedal.
You could try and reseat the electrical connector that goes to it.

The service manual troubleshoots a no AC clutch engagement by using a Tech 1.

It can simulate the HVAC control head to determine if there is a problem with the Control head or the Programmer.

You said you measured around 6 volts at the pressure cycling switch.

What voltage do you measure with no AC requested? The voltage should change when you request AC and don't request AC.

If the voltage stays at 6 volts with AC or no AC requested.

Unplug the two wire connector plug at the Low Pressure Cycling switch.
Ground the Dark Green/White wire on the disconnected plug.

Start the engine and verify if the compressor clutch engages.

buell_cyc

Thanks for the run down and the photo, interesting stuff. Appreciate you taking the time, might come in handy at some point.

buell_cyc

Thanks, no worries. I did check voltage with no call for A/C and it just reads a nominal voltage ~ 0.17 volts. Will keep digging. Thanks again for help!