So my A/C had been working in the car just fine and recently stopped working. The control unit in the car seems to be working for the other functions such as the defroster and the blower fan speed. A quick visual check tells me that the clutch is not engaging. Below is a summary of what I've checked. Does anyone have an ideas on what I should be checking next? Pulling the diode and testing/replacing it?

I checked the 25 AMP fuse (pictured) and it is fine.


I put a probe on the clutch connector with the diode and I never see any power under any circumstances. The same with the other connector on the back of the compressor.


On the accumulator I never see any voltage on the green or light blue wires. I only ever see voltage on the red wire coming into relay thing on the left. There is refrigerant in the system.
Pulling the connector off the switch on the accumulator and jumping the connectors doesn't change any behavior of the system.


There is a mess of relay looking things on the drivers side corner of the engine bay. I don't know what any of that is.

 

When the system is low enough on refrigerant the system pressure is below safe operating level. The pressure switch opens so the compressor clutch will not engage.
This is a safety to keep the compressor from overheating

"there is refrigerant in the system" is a pretty general statement.
How many # are in the system and what are pressure readings at the HI and LOW service ports?

If the compressor does not cycle when the defroster is turned on, this would indicate the issue is with compressor (low refrigerant) or the pressure switch.

 

Sounds like low refrigerant level.

Take off the connector to the switch on the accumulator, and jumper it by sticking a piece of wire in the terminals. If the compressor runs, then that's its problem. DO NOT leave it like that, or even let it run for more than a few seconds. If that makes it work, then find the leak(s), fix them all, evacuate the system, and recharge it. Add acoupla ounces of oil along with the cold juice.

 

Sounds like the A/C compressor relay may be bad. On the TTA the ECM normally holds the relay closed to allow the clutch to engage. At WOT/boost the ECM opens the relay to dis-engage the compressor clutch. I'm not sure which of the relays it is.

The wires to the relay are as such:

A - LT BLU (power from the A/C pressure cycling switch, also goes to ECM)
B - DK GRN/YEL (from ECM to control relay coil)
C - PNK/BLK (IGN feed via the gages fuse for relay coil)
E - DK GRN (power to compressor)

RBob.

 

Re-reading your post, since there is no power on the green (LT GRN) wire on the pressure cycling switch, the issue wouldn't be the relay. It is either the fuse, or the dash switch. Here is the A/C control wiring. Note the wire labeled 66, that is the LT GRN wire going to the pressure cycling switch.

Note that the relay on the HVAC unit (next to the accumulator) is likely the high speed blower relay.




RBob.

 

Not my add, but if you need a recharge, I recommend you go with the correct refrigerant, R12.

https://houston.craigslist.org/pts/d...059532424.html

 

Already tried that like mentioned in the first post. The clutch does not engage like that.

I don't find anything with the colors everyone keeps citing. The only relay I have near the accumulator has red, black, orange and purple wires. I find it strange that I cannot find where a specific relay is located in the car by searching this forum, lol. I don't have an owners manual as it wasn't included with the car, but I'll keep digging and look for those wire colors. It has to be somewhere.

The entire A/C system was redid last year with R152a.

 

.

 

So by process of elimination, all of the relays near the driver's firewall are not it. Those are for fog lights, fuel pump, spark control and something MAF related. The large relay in the 3rd picture next to the pressure switch controls the highest speed on the A/C blower motor fan. There are two relays near the radiator cap on the passenger's side. One is for power to the cooling fan and the other one also looks like control related functions to the cooling fan.
The only one I haven't identified yet is between the battery and coolant overflow (now Meth) tank mounted to the inner fender.

 

As for the cooling fan, if GM used the same basic setup as the turbo Buicks, it is a two speed fan. Which uses two relays, low speed is A/C and ECM controlled. While high speed is A/C and coolant temperature controlled. The fan would have a big resistor on it for it to be the Buick two speed fan.

I looked at other info to see if I could find out where the A/C control relay is, but no luck.

You may have missed my other post with the A/C control wiring diagram in it. The LT GRN wire at the accumulator pressure switch should have power with key-on and the dash HVAC control in the A/C position. The power then goes on to the control relay.

RBob.

 

Thanks. That was helpful. I was able to confirm voltage there.

I did some more playing around and with the car running, A/C set to on, I shorted the connector going to the low pressure switch and the clutch turns for about 1/4 of a sec then stops. Removing the short and reconnecting it causes the clutch to start again but stops just as quick. That sort of behavior doesn't sound like a bad relay to me. Does anyone know what the symptom of a bad diode is on the clutch connector?

 

un plug the A/C clutch harness connector and attach a test light to the + terminal of the connector and turn the AC on. (engine running)

If the test light turns on and stays on the pressure switch is good. You have isolated the problem to the compressor (dirty terminals, broken wiring, worn-out clutch, etc)

If the test light is off then try by-passing the pressure switch; if the light turn on and stays you have isolated the problem to a bad pressure switch or low refrigerant.

Also what was the last thing you did to the car before the A/C "just stopped working" if converted to R-152 was it, and you have ruled out a bad compressor by troubleshooting, odds are you have slow leak in the system.
I would top off the system and check the low and hi side pressure valves for leaks. Most people don't know how to screw in a light bulb let alone make sure the adapter fittings are threaded on properly to the original R-12 valves.

 

if you think its the compressor it could be the clutch plate/clutch coil

https://www.gmpartsonline.net/oem-pa...bC12OC1nYXM%3D

 

Just for clarification, I haven't done anything to the car. I bought the car about 8 months ago. The previous owner replaced the A/C and had it charged with 134a but wasn't happy with the cooling performance. He said he then switched it over to 152a and was much happier. Since I've owned the car the A/C was working fine. I went on vacation during the holidays and offshore afterwards so the car sat for a good 3 weeks without moving (in a closed garage). When I drove it earlier last week I noticed the A/C wasn't cooling; that is when I looked under the hood and noticed the clutch wasn't turning.

Bypassing the pressure switch results in a quick turn of the clutch and then it stops. Every time I put the bypass wire in place with the A/C on it does this; turns a bit and then stops. With that behavior, a bad pressure switch or low refrigerant would not be my first guess. In the case of bypassing the switch, the clutch should stay engaged and keep spinning right? I will break out the meter and test light and see what is happening on the clutch connector when I bypass the pressure switch. It is obviously getting some juice, I just don't have the full picture yet.

 

Your PO is not too sharp ;134a and 152a have similar properties. difference is 152a has less GWP (global warming potential) than 134a which is why Europe switched in 2017.
Both of these have "0" ODP (Ozone depleting potential) which is the reason R-12 was replaced by R-134A. Funny the tech instructions for my 1972 Cutlass simply says 'to service the system discharge the refrigerant to atmosphere". live and learn !

He probably had similar issue so re-filled the system and added some seal conditioner (maybe) and flipped the car to you.

So it sat for almost month then decided to not work... I say bad compressor (low/no oil) / clutch or it is out of gas. You need to check.

You can try to add a small can of R-151a with compressor oil/seal conditioner to see if you can wake it up.

 

When I pull the connector going to the low pressure switch, I see 12 volts on both wires (blue and green) with respect to ground. Is that normal? If you bypass the low pressure switch, what else stops the clutch from engaging? This is quite frustrating as all of the resources for wiring diagrams, especially pertaining to colors that I've found, do not match what I see on my car. That tells me someone has hacked it apart before, more likely, or the turbo trans am was somehow different. I still cannot definitely say I've found the A/C clutch compressor either.

 

The diode is there to simply kill the voltage spike of the clutch cycling on and off. If it went bad as in shorted, you'd blow the fuse that powers the compressor, if it went bad as in open circuit, there wouldn't be any problems other than the system getting a 4000 volt spike when the compressor cycled.
Since the compressor doesn't stay on when you jump the low pressure switch, I'd lean towards something being up with the compressor clutch itself. Unplug it and measure across the terminals and see what the coil resistance is..

 

A search shows the coil should measure 3-5 ohms..

 

Post #1: Your second photo: The power connection to the compressor clutch is at the front part of the compressor (the green and purple wire) unplug that and test that harness connector, refer to my post #12.

If the test light stays on with the A/C turned on you are getting power to the clutch ( assuming the system has refrigerant and pressure switch is letting current pass to the clutch) if you have power going to the compressor clutch then the problem is a bad clutch/compressor.

The clutch is electro-magnetic.. A/C power on engages the clutch and the compressor runs. the clutch assembly can be removed and replaced.

 

I've used both a test light and a meter, and I never see sustained voltage at the plug to the clutch. I say it that way because during some combination of bypassing the pressure switch, I would see the clutch make a partial turn then stop. Almost like it would receive a bit of power and then it would disappear.

 

Did you test it while bypassing the pressure switch? if you did not get constant power at the plug to the clutch with A/C turned on but you do see power at the plug to the clutch with the AC turned on and bypassing the pressure switch you are low on "Freon". End of discussion.
if you don't have power at the clutch plug when you bypass the pressure switch the problem is the wiring between the two devices.

More importantly: Did you even check the "freon" pressure on the low and high side ports? if you want to work on and troubleshoot any A/C system you need the right basic tools otherwise you are guessing and wasting time.
You need a manifold gage set to test and re-fill the system. Doing this one simple thing will tell you 100% if the system is charged or not.

I am trying to explain best I can and I assume you have mechanical aptitude to figure it out..

 

It sounds as though the cut-out relay is being de-activated by the ECM. Or it is bad. Note in the wiring diagram that the relay is between the low pressure cycling switch and the compressor clutch. If the power is OK at the cycling switch (stays on when it should), and only for a moment at the clutch, it is the relay that is between them.

And that the A/C active input to the ECM is before the relay, it comes off the pressure cycling switch.

If you have a Scanmaster or scan tool check the airflow and TPS values, along with the other sensor values. It may be that an incorrect sensor input is confusing the ECM to de-activate the A/C.

RBob.

 

Scanmaster looks ok. Car is running fine and values seem within normal expected ranges. Will maybe dig into the wires a little more tomorrow.

 

I see 3.4 ohms there.

Since I was just chasing my tail, I decided to take things back to basics. One of the relays, more specifically the connector, looked to be in pretty rough shape and I couldn't really see the wire colors through the crud. It was where diagrams say the A/C clutch relay is supposed to be (smacks head!). I decided to pull it out and separate out the wire from the loom and give everything a good spray and clean with some Lectra-Clean, and low and behold I found wire colors. These are very close the the wiring diagram. The middle pin C has what looks to be an pink (discolored to almost orange) and black that has power when ignition is on. I don't really see any dark green/white wires but there is certainly dark green and yellow. Perhaps it has also discolored over the years.

Regardless, I feel like this is the clutch relay at least and the connector has obvious issues. The terminals are no longer being retained up towards the top of the connector. You can see the exposed copper on A and B below. When I unhooked it the exposed copper of the two wires to A and B were actually touching and a bit twisted with how the harness was positioned. I imagine I'll need to get a replacement connector before continuing to troubleshoot. The only other thing I did was put a test light on the other wires to see if I could see anything while the car was in various states, iginition on/running/A/C on, etc. but I was unable to see any juice anywhere but terminal C with the pink/black.

 

(3) possible loose terminals and exposed wiring making contact with each other... nothing wrong there

you can try to remove the pins and replace them.
meantime try to re-seat those wires and wrap the exposed copper with some electric tape, plug her in and see if it rips.

 

I think you found the issue. BTW, where was the A/C relay located?

RBob.

 

Where the manual said it was located.... Well, almost. In the picture below the closest representation would be #8 right against the driver's side firewall. #3 would be my fuel pump relay but that is almost centered between #13 and #8. You can see this better in picture 4 from the first post. Back to the below image, #13 for me is not the cooling fan. I have 3 other relays near the passenger side radiator for that.



I think I was looking for wire colors and I kept glossing over it because I couldn't see them through all the grease on that connector.

 

so is it fixed now?

 

No. It is still confusing me. I replaced the connector today and bench tested the relay.

The wiring diagram I found at the Austin thirdgen site for an 89 firebird is not correct for my car. The diagram RBob posted above is the closest match to what I have on my car (wish I knew what manual that was). Going off that diagram, when I apply 12 volts to C and ground B, A and E are connected. That tells me the relay seems to be doing its job. There are things that are not making sense though. I have 5 wires going to the relay, not 4. I have an extra light green wire going into A and I have no idea why or where it comes.

Going from his diagram, I can confirm the pink/blk has power with ignition, but unless I manually ground out B, the relay never closes. Even when the relay closes, the clutch doesn't spin because I never see voltage at A. And even if I take the power from C and manually attached it to E and bypass the relay, the clutch still doesn't spin although I can hear it click when I turn the ignition.

 

The engine has to be running and the system full of Freon for the clutch the spin....if you can hear it click when you turn the ignition that's a good sign.

 

...or you have to bypass the low pressure switch. Yes, I know. The clutch will not spin. I can turn it by hand with some large amount of force, but it just never goes off on its own. I'm starting to think I have more than one issue that occurred at the exact same time. I never get power to the clutch is one issue. The other issue is that even if I direct wire the clutch, it does not engage and spin. It should be that simple right?

 

Note that the high pressure cut-out switch is in the ground circuit for the clutch. Not sure how you are direct wiring the clutch coil, but if you are providing both +12 V and ground, then there is a compressor issue.

It may also be something as simple as too much clearance between the clutch plate and the shaft plate. There should be 0.020" to 0.040" of clearance.

RBob.

 

Would the clearance between the clutch plate and shaft plate be something that could change by itself after working just fine for 6 months?

 

The plates wear due to the constant cycling. Or the retaining nut has been backing off.

RBob.

 

I used a separate 12v power supply directly to the clutch coil and bypassed the entire electrical system. When I do that, with the engine running, and manually engage the clutch, it catches and spins like expected.

If I can back up for a minute, the pressure switch that is attached to the dryer, is that a high pressure switch or a low pressure switch? Is it normally open or normally closed? Mine is open when I check it with the connector unplugged, but just using a paper clip to bypass the switch doesn't change the behavior of my system, so I thought I could safely ignore that. Looking closer at the diagram posted, I don't see any low pressure switch in the system, just a high pressure.

 

In the diagram it is the "A/C pressure cycling switch." That is the one located on the accumulator. Since it shows open either the switch is bad or the system is low (not enough pressure to close it).

The high pressure cutout switch is indicated by the arrow on the right in the picture you posted of the compressor.

RBob.

 

That is what I was thinking. Looks like I'll need to get a set of gauges or borrow some to continue troubleshooting this.

 

On a separate note, anyone care to guess at why I have 5 wires going into my relay instead of the 4 as indicated by the diagram? I even have it pictured. Looks like another light greenish wire going into the same pin (A) as the light blue.

 

It most likely goes to the ECM.

RBob.

 

As a final update to this, as it turns out, the high pressure cutoff switch on the compressor is bad and the refrigerant was gone. So I had a low pressure and high pressure trip at the same time. The system was refilled with the 134a and the high pressure switch was unplugged until I get a chance to do a proper replacement. I still have no idea where the refrigerant ran off to. A vacuum was pulled and it held fine for a couple hours.

 

Oh ya', two things go bad at once and it can drive one crazy. Good to hear you have it going, warm weather is arriving (probably already there in TX). Thank you for the update.

Just remember that the high pressure switch is exposed to the refrigerant charge. Need to relieve that before R&R'ing the high pressure switch.

RBob.

 

Yes, the warm weather is already here. I am aware about the high pressure switch; that is why that stayed broken for now. I will have to figure out a reasonable time to replace that in the future.

 

I believe there is a "Schrader" Valve behind the switch, which allows it to be threaded out, for replacement, w/o losing refrigerant.

 

For the low pressure cycling switch on the accumulator you are correct.

The high pressure cut off switch is mounted on the compressor.

RBob.

 

Right: the HP switch is usually a little round thing about the dia of a nickel and about ½" thick, that inserts into the back of the compressor and is retained by a snap ring. Has a single-pion Weatherpak connector similar to a knock sensor or fan switch. Its housing grounds to the compressor body.

 

Got it, thank you!