Mike, you did a great job today !!...well the Cliff Notes version of this is with key OFF pin 85 has a ground at the Relay Control circuit when there shouldn’t have been...when PCM harness was disconnected no more “short to ground” ..so it appears that the PCM driver is stuck on !!... I’d say bad PCM but if other can chime in that would be great !!....also no "short to ground" on the circuit to double check and good continuity between pin 43 at PCM to Pin 85 !!

 

Again, thank you to all of you trying to help me out on this. I'm trying to grasp what's happening here. IIRC, I had continuity at female #43 (the harness), and pin#85@fusebox. With the key off and everything plugged in, I have a ground@Pin#85, and there shouldn't be with key in off position. So, and I'm certain I did the following test: With PCM unplugged to access Male Pin#43@PCM, I DID NOT get the test light to come on with one end of light at battery positive, and key off. If the PCM has a circuit that is providing ground to Pin#85 with key off, why should I not get the test light to illuminate with key off when probing Male Pin 43@PCM? But, when PCM is plugged in, test light shows ground at Pin#85. Is something somewhere else in the harness providing a ground? I know this is a lot to absorb, let alone read!! But I'm kinda confused at this point. Not kinda-VERY!!!

 

I have an appointment with a shop (not an A/C specialist shop) that has a Tech 2, or equivalent, on Wednesday, 8-21@8:00A.M.. Will post any new info then-Hopefully they will figure out exactly what it is. As always, thank you very much for any and all help suggested. This forum is the BEST!!!!

 

I suggest you bring the car to a Chevy dealer ship when it needs servicing that has exceeded your ability to diagnose the main issue. It will roughly cost maybe 169-179 you will know exactly what the issue is at the dealer. Then it’s your choice to let them repair or you fix it ! At least you’ll know what the issue is and you won’t get suckered into making repairs that weren’t really needed past experience dealing with some outside garages!

 

@grinder11 ​​​​​As mentioned before, I have never tested the status of Pin 85 from Relay 34 when ignition switch is OFF. Although I would have expected it to remain open, the fact that having-or-not-having ground applied to one end of the coil with key OFF does nothing to the relay because the other end simultaneously gets power removed, had me wondering what was normal or not until C5 Diag verified it in his car.

If as it seems, your PCM A/C Clutch Relay Control circuit is defective (not stuck ON because it goes OPEN as soon as the ignition switch goes to ON), the inability to replicate the problem when the harness connector is unplugged could be explained by the fact that you are not only disconnecting Pin 43, but the whole connector, affecting all circuits carried by it and probably "killing" the PCM. If you want to do this (while being able to say "all other things being equal" ) you would need to remove female Pin 43 from connector, plug the connector back to the PCM and repeat the test, managing somehow to touch male Pin 43 from PCM through the connector (or just cut the DK GRN/WHT wire to do the test and splice it back afterwards).

With all the excitement about the mysterious ground to Pin 85 when ignition switch is OFF, it seems that the main symptom fell through the cracks: Engine running - Pressing A/C button: no ground to Pin 85.

Of course that once is established that your PCM A/C Clutch Relay Control circuit is defective, both symptoms could be the result of that, but the main symptom shouldn't be completely forgotten.

Since you're already pondering the replacement of your PCM, a couple more tests won't hurt. If I were in your shoes, I would like to rule out the possibility of a sensor telling the PCM not to engage the A/C clutch, or the HVAC Controller not being able to communicate with the PCM via Serial Data Bus (do you have any "No Comm" codes involving the HVAC Controller?).

In a previous post I suggested testing these 2 sensors:

• Outside Temperature Sensor
• Refrigerant Pressure Sensor

The first one will prevent the clutch to engage if the outside temperature is reported as below approximately 35°F to 40°F, and the second one will do the same if the refrigerant pressures are reported as below or above certain thresholds.

You could test the Outside Temperature Sensor using the table posted before, a multimeter, a thermometer and water. A mix of water and ice should provide 32°F and if you remove the sensor from the car and submerge its tip in the water, it should read around 32.6k Ohms. Using progressively warmer water, test it at several points and check its resistance against the table below.





To check the Refrigerant Pressure Sensor, connect your A/C gauges and start the car. Read both pressures at equilibrium (they should be the same) and read the sensor's output at that pressure. Then jump Relay 34 as you did before, and read your high pressure and the sensor's output at that pressure. Compare those 2 sets of values to the ones C5 Diag has posted before to verify if you're within the "safe zone" (where the clutch is not prevented from engaging) and if the sensor's output matches the expected values at those pressure points.

These tests won't cost you a cent and they will only take you a few minutes. I would definitely perform them before replacing the PCM, if for nothing else, just to be thorough

​​​​​

 

Hopefully when Mike brings his car in the shop can check some HVAC Data PIDS with their Tech 2...was taking to Paul Danner (Scanner Danner) a few days ago and initially he thinks the PCM may be bad...he needed to know the year of the car so hopefully I’ll be hearing more from him...also the shop can look at the pressure sensor and ambient air temp voltages with a scan tool...Mike, have them look at the temp sensor on the Generic OBD2 side with whatever other scan tool they have !!

 

SO SORRY GUYS, for the late post. I took the car to C&S Corvettes in Sarasota, Florida, about 3 months ago, while I was down there. Great service and very knowledgeable people. Car even stumped them!! BUT-We have a better idea of what's going on now. When I left, they had found a major refrigerant leak at the high pressure port. Refilled it, and blew cold when jumping the pins 87 and 30 as C5Diag told me to do. That was the only way it would work. As I was leaving, they said they eliminated many things it WASN'T. They tried 3 different dual zone head units, and 2 different BCMs, all of which were pre-tested and were good. They believe it's something in the PCM. But it still wouldn't work. Now for the good part-As I was driving down the road at about 40mph, I thought "What the Hell, I'll try it again". Pushed the A/C button, and PRESTO-A/C worked all the way back to Tampa!!!!! I've since figured this much out;
A/C works fine.....UNTIL you stop. It still works fine, until you take tranny out of Drive, then the A/C light goes off, and it wont work again until you're moving, then it's as good as it was off the showroom floor!!! Until the next time you put it in neutral or Park. Then it quits, and wont work again until you you start moving again. Since I have an LS7 in the car now, I'm thinking....Could it be something in the tune? Again, sorry for such a late addition to an 8 month old thread, but I guess better late than letting it just hang in cyberspace with no update. Still dont know for sure what it is. Many, MANY thanks to C5Diag, and you Forum members for all your help. At least I have A/C now, as long as car is in drive. The only reason I put it in neutral at lights is because Floridas red lights are long enough to get out of your car at a red light, fetch a soda from a cooler in the trunk, and have it half gone before the green light!!!!!

 

Hey Mike !!...I just can’t believe C&S couldn’t fix it...they are a respected Corvette “specialist” !!...glad you got it somewhat fixed...if you jumper 87 and 30 while driving see if the AC light stays on when you come to a stop and AC stills pumps cold air...if so the PCM is not keeping the AC clutch relay control circuit grounded !!...did they check that like you had with the test light ??...I should be retired (July 4th) and moved down to Florida in a month or two so you can bring it by me !!...LOL !!...that AC “request” circuit has to be grounded to allow the relay control circuit to work...do you know if they had a Tech 2 or similar looking at the AC Request data PID while driving to see if it was saying “yes” or “no” when coming to a stop ????

 

Hi Rob. Bottom line at C&S was $$!! ME! I had over a grand in it at that point, and asked them to stop. They wanted to go farther with it, and were zeroing in on PCM connections. I think it could be something that was in the custom tune. Could be the tuner accidently, or purposely had the A/C clutch disengage when in Park. After all, when most people are putting the trans in Park, they are exiting the vehicle very shortly. I have a medium cam in it, and the tuner may have had the A/C disengage when in Park to help stabilize the idle. I don't know. But when you're in Drive, and/or moving, A/C works great! C&S is a great shop. I'd recommend them to anyone. Not a good year for the C5. As you know, it has an LS7 engine I installed myself. You may be aware the LS7 heads have a serious machining flaw when brand new, and has a reputation for dropping valves, which destroys the engine. This has been traced to valve guides wearing out prematurely. My heads are shot at 22,000 miles. It will cost me another $1,100.00-$1,200.00. to have new guides installed, and have the seats cut concentric to the valveguide bore. So this brings this years totals to around $2,500.00 in the old C5!! Wife has been very good about it. But she's reached her limit!! I'm very fortunate I can R&R the heads myself, and install the valves and valvesprings myself. If this was done at a shop, probably looking at $5,000.00+. Also thankful I caught it before the engine self destructed. Anyway, have a great 4th of July, Rob. Are you moving to Naples area, or?? We'll be back end of October. We'll have to get together then......

 

[QUOTE=grinder11;1601777890]Hi Rob. Bottom line at C&S was $$!! ME! I had over a grand in it at that point, and asked them to stop. They wanted to go farther with it, and were zeroing in on PCM connections. I think it could be something that was in the custom tune. Could be the tuner accidently, or purposely had the A/C clutch disengage when in Park. After all, when most people are putting the trans in Park, they are exiting the vehicle very shortly. I have a medium cam in it, and the tuner may have had the A/C disengage when in Park to help stabilize the idle. I don't know. But when you're in Drive, and/or moving, A/C works great! C&S is a great shop. I'd recommend them to anyone. Not a good year for the C5. As you know, it has an LS7 engine I installed myself. You may be aware the LS7 heads have a serious machining flaw when brand new, and has a reputation for dropping valves, which destroys the engine. This has been traced to valve guides wearing out prematurely. My heads are shot at 22,000 miles. It will cost me another $1,100.00-$1,200.00. to have new guides installed, and have the seats cut concentric to the valveguide bore. So this brings this years totals to around $2,500.00 in the old C5!! Wife has been very good about it. But she's reached her limit!! I'm very fortunate I can R&R the heads myself, and install the valves and valvesprings myself. If this was done at a shop, probably looking at $5,000.00+. Also thankful I caught it before the engine self destructed. Anyway, have a great 4th of July, Rob. Are you moving to Naples area, or?? We'll be back end of October. We'll have to get together then......[/QUOTE

Mike,
I retired on Tuesday and my house is due to close on July 31st...will be moving north of Naples to Cape Coral...have a great 4th too !!...yes, get together !!

 

[QUOTE=C5 Diag;1601777909]Hi Rob. Hey, congrats on the retirement!! I'm happy for you. I saw several guys I worked with go out on a stretcher, and never got to retire. Now that this virus is going crazy again, BE CAREFUL! We are being extra careful, too. Talk to you later.......

 

The other day I was wondering if you were able to finally solve this.

Great catch, by the way! Looking forward to see what you can find regarding the tune

 

Congratulations on your retirement! Not exactly neighbors (Miami), but close enough to have a beer next time I pass by your area

 

Thanks !!...yes, stop by for a brewski !!

 

Thanks for all your input too, GCG. Our son lives in Ft. Lauderdale. Us 3 can hopefully get together someday when they finally figure out this virus. As for my A/C, Im sure the tuner did something to disengage the Compressor when shifting out of drive and into neutral, or park. I just remembered this exact thing happening when wife an I took it to Las Vegas 14 years ago. Also, I've recently found another leak. Worst place it could be-The EVAP inside the dash. Bought a sniffer, and ALL the vents are showing a small leak. Now that I'm certain the tune is what's causing the compressor to disengage, I can live with it. But these 4 minute Florida red lights are a KILLLER!!!

 

Definitely!


​

 

GCG and C5DIAG-FINALLY-I have a code. It's an intermittent problem, of course!! I'm now getting a PO645H, A/C relay circuit problem. Maybe this will decipher if it's the PCM, in the tune, or an intermittent short. Someone asked recently if the A/C compressor would disengage if I put the car in neutral while coasting. Great idea, so I tried it. A/C stays on when moving, even when shifted out of Drive!! BUT-Shift out of drive when car is parked, A/C compressor disengages the second I move the shifter!!!!

 

If it’s doing that I would look at the tune...maybe repost this is the Scan and Tune section...maybe someone over there may know !!