Hey guys, if any of you have saved a good thread for this please point me towards it. Searching for cooling, compressor, relays....is just too difficult with all of the threads and discussions that go on in this board.

I have the C68 automatic climate control in my '92 coupe. When I attempt to turn on the air conditioner, the blower will come on and the blower speed will ramp up to full speed.

The compressor never activates and the cooling fan for the condensor never comes on either.

There are no codes from the control unit display. It seems that the controller does not sense that there is a problem. I'm assuming the problem is in a fuse or relay that would energize the fan and combpressor, beyond the normal sensors monitored by the system.

Although my FSM is for a '92, it is a "white" version which I've been told is an early copy not the more correct "red" version. This FSM states the relay for the AC should be on the driver's side wheel well grouped with two other relays. There are not three relays together, so I guess this is where the "red" FSM would have been useful.

In addition to any trouble shooting advice or threads you may be able to point me to, I need some assitance with locating the relay and any other test points that you may think would be helpful.

Thanks in advance guys.

 

Before ripping things apart, check the refrigerant level first.

Even though the C-68 Controller is supposed to throw a code if the refrigerant is low that doesn't always happen. I know from personal experience!

 

strange that you would not have a code though. Take a meter ,set it to check ohms on the low pressure switch and see if it is infinity or it measures ohms? another check is to see if you are getting voltage to the compressor? undo the connector and while you have a call for cooling see if it is delivering voltage and let us know what you found

 

 

So all the things that have been said already have merit. There are a lot of directions you can go and things to check. Shorting out the cycling switch may be one item. But if the fan comes on at high speed….do you have any control of it at that point to bring it down to a lower speed?

If the programmer or controller is not working right, then the compressor may never get the signal to engage. But since the symptoms are a bit strange, I think a system reset may be a place to start. You can pull the fuses for the controller and head if you know which ones. But the simple way is to just disconnect your battery for a minute and reset it all. The ECM is involved in making the compressor run also.

Do you have any problems with the display of the control head with lighting up, controlling the temperature or fan speed. Can you select the different modes and have your LED lights follow the button push?

 

If you have a low pressure situation the battery disconnect will clear the code but the electricity still has to get passed the low pressure switch on the way to the compressor

 

You can check to see if the compressor clutch is engaging with the engine off. It makes a pretty loud click, so you can hear it easily.

Turn the AC on and off and you should hear the clutch click. If it does not click then it's likely that the system is low on freon. You can test for this by disconnecting the low pressure switch and jumping the connector pins together. In my case the low pressure switch was bad.

There is also a relay and fuse involved, so if the above fails to resolve the problem then look there.

 

Cliff, I'm going to have to disagree with you on this one. I know exactly what you're talking about and on some earlier GM products that’s the way it works because there's nothing but hard connections of switches and relays.

But on the 92 and up the ECM/PCM actually is in the relay circuit for the compressor cultch. The programmer signals the ECM thru the cycling switch to energize (close the ground) the relay. The kicker of this is the engine has to be running. You cannot close the relay by just turning the key to the ON position. So this feature make troubleshooting a bit more challenging.

You can measure the voltage at the cycling switch with engine running or not running but with key on. The voltage should be normally 0.0. but with an AC mode selected, the voltage will be about 5 vdc +/- 1.0. If you get the 5 volts, this indicates the control head is working as well as the programmer and the signal is proper for the ECM/PCM. Then the ECM conditions has to be satisfied to turn on the relay.

** This information is strictly out of the 94 FSM however it is to my belief that it is valid for a 92 as well.

 

My experience is with the earlier cars, so you're telling me things I didn't know. A lot changed in 1990 and I don't know much about the later cars except what I've read here and elsewhere.

I would think that you could get the compressor to engage if you jumped the low pressure switch connector. The functionality of that part (to protect the compressor if the system is empty of refrigerant) didn't change. The system depends on the refrigerant carrying oil through the system to lubricate the compressor. If there is no refrigerant then the compressor gets no oil and seizes.

 

You are correct the cycling switch still does the same job as it always did but just connected differently with the additional electronics. In the case of the 92-96 the cycling switch simply gives a signal to the ECM/PCM to tell it to engage the compressor but it does not have the final word. Then the ECM/PCM decides if it want to do it. The high pressure sensor (not switch) in the high side line has to give the proper voltage to the ECM/PCM. Also the ECM/PCM has to be working properly. And for a variety of (GM) reasons as per the software in the ECM/PCM, the engine has to run to engage the clutch (close the relay).

Also if a problem develops in the control head or programmer and the 5 volt signal is not there (to the cycling switch), then the compressor will not come on even if you jump out the cycling switch (since there is no voltage there if a problem was occurring with head or programmer).

I can post a picture of the diagram (2 pages) if someone wants it.

 

I would think you'd get a trouble code in the HVAC system with these symptoms. Post #4 seem like a good place to start. It's free and should give you more information about what's happening than you have now.

 

Skipping steps is not a good way to start and checking the HVAC codes world be the first thing to do. But he has said he has already done that and no codes were present. I personally don’t find that many of the error codes of real use.

Just on principal I would check the 5 volts at the cycling switch to see if it is there, I don’t think it is. If it is not, you can forget about all the stuff under the hood like ECM, relays and such. Without the signal, the ECM will not close the compressor relay or run the left cooling fan.

The clue that’s important is the blower fan ramps up when turned on. The programmer controls the fan speed with a control voltage sent to the fan module in the evaporator. But I don’t think the fan module is bad because that has nothing to do with the compressor not running.

On the surface, I think the programmer might me bad causing all these problems. I wrote a thread on how to repair the programmer if defective. It may pertain to this situation. Someone just pulled the thread up yesterday and stated how they repaired their programmer problem.

https://www.corvetteforum.com/forums...-repaired.html

I don’t know if there is any more information or if the problem occurs all the time. WDURHAM has not posted with any updated information.