PCM guru's-a little help, please! Very difficult A/C issue

[FirebirdTN]

For the past few months my a/c has been cutting out. Its getting worse. I am starting to think its a PCM issue.

It took me a while to back probe around during the problem, since it seemed like everytime I pulled over and grabbed my multimeter, it would start working again!

Anyway, I finally was able to do some probing. Here is the deal:

It might run for 20 seconds or 20 minutes at a time and then cut out. No real pattern or ryme or reason to it; it just shuts on and off when it wants to.

When it is in the non working state, I have battery voltage present on pin 21 of the CCRM which indicates the head unit in the passenger compartment is good, the low pressure switch is good, and the high pressure switch is good. I do realize the high pressure switch actually contains two separate switches, one that feeds the CCRM, and one that feeds the PCM. Backprobed both sides and it is working as it should.

Now, back on the CCRM I of course do not get my battery voltage at pin 23 (clutch coil), and pin 22 (WAC from PCM) is at 0 volts.

I have removed pin 22 from the back of the connector and just left it floating, and so far a/c blows nice and cold and does not cut out. That pretty much rules out the clutch coil and CCRM. Although prior to doing that I went so far as to open the CCRM and reverse engineer the schematic for the WAC circuitry (electronics is a former career field of mine).

Bottom line is, the compressor is cutting off because it is being told to do so by the PCM!!! But my question is WHY???????????????????

So, under what conditions does the PCM cut our the ac compressor? I know of only two: WOT, and an overheat condition that causes the PCM to kick out the compressor in an attempt to reduce engine load and cool the engine down.

There is one more thing I have not checked yet, and that is the presence of battery voltage at pin 10 of the PCM (basically a/c request from head unit in the passenger compartment).

I did a KOEO test and the only code that came up was 564 I believe it was (it has to do with engine cooling fan circuit failure, but that code has ALWAYS been present since I have owned the car even though the cooling fan works perfectly, both low and high speed). No other codes came up.

I am at a loss. Although I have the PCM a/c clutch control line floating and its working fine now, it does bother me that the PCM now cannot kick it out at WOT.

Anyone have any thoughts?

Car is a 94 vert gt, no mods.

Thanks,

-Alan

 

[rz5.0]

Have you checked the pressure.

 

[FirebirdTN]

Yes. If the pressure was low (low refridgerent), the low pressure cutout switch would interrupt the battery voltage to pin 21 of the CCRM. If the pressure was too high (too much refridgerent) the high pressure cutout switch would so the same. But besides that, back in the heat of the summer I actually took my car to the stealership to look at this problem, and of course it worked fine for them and they could not duplicate the issue, but they did say refridgerent level was fine.

-Alan

 

[rz5.0]

Good luck all I had

 

[CC'S95GT]

wish I could help also.
it sounds like you have more electronics training than most of the guys on here.
there is a super tech on stangnet.com nemed hisson5.0.
gool luck let us know what you find

 

[mikey94gt]

I wonder if the return signals for your fan hi/low is actually getting to the ECM, as you do have a code for it. The only other thing I can think of is that signal from the HVAC panel in the dash not getting to the CCRM. I cant remember if the AC request signal from the HVAC control unit is tied into the ECU or not, telling the ECU when AC is on..... been a while since I have even had AC.


Btw, Kudos for posting an intelligent "help" thread.....

 

[FirebirdTN]

I wonder if the return signals for your fan hi/low is actually getting to the ECM, as you do have a code for it. The only other thing I can think of is that signal from the HVAC panel in the dash not getting to the CCRM. I cant remember if the AC request signal from the HVAC control unit is tied into the ECU or not, telling the ECU when AC is on..... been a while since I have even had AC.


Btw, Kudos for posting an intelligent "help" thread.....

Good point on the fan return not making it to the PCM. I never looked at it since the fan works fine-I just shrugged off the code since its always been there.

You are kind of correct-the hvac signal (battery voltage when ac is on) come from the HVAC panel in the dash, and then goes thru the low pressure switch, the high pressure switch, then the CCRM. So it is making it there. HOWEVER you are correct that the ac request also comes from the HVAC panel and goes to pin 10 of the PCM to tell the PCM the ac control is turned on. That is the one thing I haven't tested yet (PITA to back probe the PCM-I need to get it out where it is accessible). I'm just wondering if there is anything else I might have missed.

Thanks,

-Alan

 

[extreme_21]

Good point on the fan return not making it to the PCM. I never looked at it since the fan works fine-I just shrugged off the code since its always been there.

You are kind of correct-the hvac signal (battery voltage when ac is on) come from the HVAC panel in the dash, and then goes thru the low pressure switch, the high pressure switch, then the CCRM. So it is making it there. HOWEVER you are correct that the ac request also comes from the HVAC panel and goes to pin 10 of the PCM to tell the PCM the ac control is turned on. That is the one thing I haven't tested yet (PITA to back probe the PCM-I need to get it out where it is accessible). I'm just wondering if there is anything else I might have missed.

Thanks,

-Alan

When you did all this testing, was the concern present?? Otherwise these voltage reading are just showing you how it works when everything is working. I don't have the diagram in front of me now to go about, but to properly diagnose this fault you need the high and low gauges hooked up to the a/c system, then you will be able to see what is going on when the conern is present..this is really one of those K.I.S.S faults, you've diagnosed all the wiring of a system that runs on pressure, but have yet to diagnose the pressure readings.. if the temp outside rises/drops this will affect the r134a pressure, one minute the a/c will be good, 5 mins later it shuts off...1 hour later comes on...works for 2 days, shuts off 5 days later..comes on 40 mins later..blah..blah..etc...aka, borderline low refridgerant

If what I understand is, you bypassed the compressor clutch from turning off, your going to overheat the system and possibly damage the compressor or clutch. When the a/c is turned on in the car, the a/c clutch engages but only stays engaged till the evap temp required is reached and the line pressure isn't to high.

I don't remember if are cars have a evap temp sensors, but i yet to date have not seen a system that does not have one..so are's should have, the compressor clutch cycles from readings of the amount of pressure in the lines and by the evap temp. if the pressure is to low and evap temp is to warm, clutch is engaged to cool system providing the right amount of r134a is in the system. If pressure is to low in line with the right amount of r134a and evap temp is to cold, the clutch will be disengaged because this would indicate the evap is frozen up.

The evap will freeze up due to a faulty temp sensor or blocked air passage either to or from the evap. Before checking any more voltages and wiring, like I mentioned before, get the gauages hooked up and check the pressures as to what is going on when the clutch is engaged and when the fault occurs.

Unless you, yourself have seen the pressures of the amount inside, I wouldn't believe the garage if your A/C is still not working...depending on the gov. law in your area, the garage that touches your car is responsible for the r134a that leaks if they repair anything, this can result to a 2,000-25,000 fine for the garage and 500-2,000 fine for the mechanic that touched the car..a lot of garages now shove the problem away saying they couldn't find anything due to the fines involved..hook the gauges up and let me know the readings of when it works and when the fault occurs, I'll then tell you everything I know

Hope this Helps..

 

[FirebirdTN]

When you did all this testing, was the concern present?? Otherwise these voltage reading are just showing you how it works when everything is working. I don't have the diagram in front of me now to go about, but to properly diagnose this fault you need the high and low gauges hooked up to the a/c system, then you will be able to see what is going on when the conern is present..this is really one of those K.I.S.S faults, you've diagnosed all the wiring of a system that runs on pressure, but have yet to diagnose the pressure readings.. if the temp outside rises/drops this will affect the r134a pressure, one minute the a/c will be good, 5 mins later it shuts off...1 hour later comes on...works for 2 days, shuts off 5 days later..comes on 40 mins later..blah..blah..etc...aka, borderline low refridgerant

If what I understand is, you bypassed the compressor clutch from turning off, your going to overheat the system and possibly damage the compressor or clutch. When the a/c is turned on in the car, the a/c clutch engages but only stays engaged till the evap temp required is reached and the line pressure isn't to high.

I don't remember if are cars have a evap temp sensors, but i yet to date have not seen a system that does not have one..so are's should have, the compressor clutch cycles from readings of the amount of pressure in the lines and by the evap temp. if the pressure is to low and evap temp is to warm, clutch is engaged to cool system providing the right amount of r134a is in the system. If pressure is to low in line with the right amount of r134a and evap temp is to cold, the clutch will be disengaged because this would indicate the evap is frozen up.

The evap will freeze up due to a faulty temp sensor or blocked air passage either to or from the evap. Before checking any more voltages and wiring, like I mentioned before, get the gauages hooked up and check the pressures as to what is going on when the clutch is engaged and when the fault occurs.

Unless you, yourself have seen the pressures of the amount inside, I wouldn't believe the garage if your A/C is still not working...depending on the gov. law in your area, the garage that touches your car is responsible for the r134a that leaks if they repair anything, this can result to a 2,000-25,000 fine for the garage and 500-2,000 fine for the mechanic that touched the car..a lot of garages now shove the problem away saying they couldn't find anything due to the fines involved..hook the gauges up and let me know the readings of when it works and when the fault occurs, I'll then tell you everything I know

Hope this Helps..

To answer your first question, YES concern was present, but WOW, you REALLY did not read my post, did u? I don't care what the pressures are, if they are correct, you will get 12 volts at pin 21 (I believe, not in front of diagram) of the CCRM. If the pressure is too low, low pressure cutout will interrupt the 12 volts to the CCRM. Pressure too high, the high pressure/fan control switch will interrupt 12 volts to the CCRM. Bottom line, if ac is on, and pressures are ok, you will have 12volts at pin 21 of CCRM. If pressures are NOT okay, you won't. This is 100% an electrical issue. I am by no means an ac expert, but electronics are my bread and butter.

But, since I know my post is old....It recently did start cutting out due to low refridgerent levels, so I just dropped $800 on a new compressor, oriface tube, accumulator, high and low pressure service ports due to leaks. (Had a reputable shop do the work since I am out of my element with ac). I went ahead and put the WOT pin back in the PCM and guess what...now I'm back to my issue at the beginning of this thread...its still cutting out, so that pretty much rules out pressure issues. I drove it to work one day and it didn't blow cold air for the first 20 minutes of driving....then all of a sudden it decided to kick it, and worked for the remaining 30 minute drive.

What I do know:

The PCM is sending a WOT cuttout signal to the ccrm to disengage the compressor. My question is why? I am obviously not driving around WOT. And earlier when I said I pulled the wot pin and left it floating, that is NOT the same as bypassing the compressor clutch. That still leaves pressure switches intact. All removing that pin did was relieve PCM control from compressor, but the pressure cutout switches are still intact.

This is really pissing me off. I am not about to drop coin on a new PCM when the only issue I have is this random ac cutout. But at the same time, I really don't want to leave the WOT pin floating either. But, if I have to leave it like that, then I guess I will have to manually turn off the ac when I goose the "go pedal" LOL.

-Alan

 

[extreme_21]

Normally the PCM will ground the circuit when requesting to perform an action(which your probably know)..so if you check on pin 22 and get a grounded circuit, PCM is requesting to shut down the system like your diagnoses show..

The PCM will shut off the system from WOT and high temps, like you already know..but it will also shut down the system if 1 of it's parts is know faulty to the PCM that is involved in the A/C circuit/system of operation..you have code 564 for the cooling fan, if the PCM see's a fault with the cooling fan it will shut down the system for it thinks the condensor will not be cooled..thus overheat the system. I understand how your fan is working but the signal to the PCM must be faulty.

The fan and the A/C clutch are both controled by the CCRM, they both also route through the EDF Control Relay in the CCRM to have comms with the PCM..

I'd diagnose fault 594 for your fan being that the PCM is telling you what it's problem is, and why it's shutting down your A/C..get rid of the fault, then you can diagnose the A/C further if it's still not working

 

[FirebirdTN]

Normally the PCM will ground the circuit when requesting to perform an action(which your probably know)..so if you check on pin 22 and get a grounded circuit, PCM is requesting to shut down the system like your diagnoses show..

The PCM will shut off the system from WOT and high temps, like you already know..but it will also shut down the system if 1 of it's parts is know faulty to the PCM that is involved in the A/C circuit/system of operation..you have code 564 for the cooling fan, if the PCM see's a fault with the cooling fan it will shut down the system for it thinks the condensor will not be cooled..thus overheat the system. I understand how your fan is working but the signal to the PCM must be faulty.

The fan and the A/C clutch are both controled by the CCRM, they both also route through the EDF Control Relay in the CCRM to have comms with the PCM..

I'd diagnose fault 594 for your fan being that the PCM is telling you what it's problem is, and why it's shutting down your A/C..get rid of the fault, then you can diagnose the A/C further if it's still not working

Thanks for the reply. I was pretty harsh above on my previous reply, but I wasn't meaning to be a prick; I was just mad at the situation. You are correct-I am getting a ground at pin 22 when the compressor is off (but supposed to be on). I can't STAND when things don't work right. I am anal to a fault. For now, I have yanked the PCM WOT pin again, just so I don't die of heat stroke LOL.

The thing about my code 564, is its been there since I bought the car. I am not really even sure how to go about troubleshooting is because during self checks you can hear the fan...BOTH speeds, hi and low, yet it seems to insist that there is a issue with high speed fan circuit. Although its possible its related, I remember when I first bought the car that was one of the first things I did (self tests), but back then the ac worked perfectly even though that code was present.

I'll just have to keep at it when I have time. I still have not backprobed the PCM to see if that "return" signal is getting back to the PCM, but its a PAIN to get to!

-Alan