I think I lost some of the charge in in the AC over the winter in my 1994Corvette. Last week-end I couldn't get the cooling fans to come on unless the motor was up to 227 degrees. I added one can of 134A which seemed to help. The AC came on and cycled but did not cool as well as it had before. This week-end I can't get the AC unit to even cycle. When I turn on the AC the fans blows but the Compressor will not cycle. Ran the car up to normal temp ... nothing. Kept going to 227 degrees and the cooling fans came on but the AC still wouldn't. Where should I look next? I don't think I overcharged the system.

Pressure?
Relays?
Compressor?
Any opinions or advice will help. Thanks

 

The AC will not cycle if the freon is low...

You can either try to add some freon and it will start to cycle. Or you can jumper the pressure switch on the passenger side (near the firewall near the heater core )wall it will cause the compressor to cycle.

hope this helps...

MO

 

Not only can guessing and parts changing get very expensive real fast on an A/C system, it can also cause more expensive problems than you started with. Without the proper equipment and understanding to properly diagnose and make repairs, this is something that should be left to pros. If it's anything beyond a basic electrical problem causing this, get qualified help. I offer this advice as one trained in automotive HVAC systems repair.

 

if the comp does not cycle on and off its because the cycle switch is bad or the evaporater isnt getting cold enough to warrent the system to cycle. does the ac blow cold?

 

Cycling, in a/c speak, refers to the on/off cycle of the compressor. If it's not turning on at all - that's a different story - so clarify that will you? Otherwise, specs are no more than 8 cyles per minute at 70 degrees. Realistically, a fully charged system at this temperature or higher isn't going to turn on and off or cycle. It will always be engaged because the low side pressure will be above the cutoff threshold (22.5 psi for R134 or 25 psi for R12). So also let us know what the outside air temp is - or better yet - the temp of the air at condensor - use the outside air temp readout if your Vette has electronic air.

 

OK I'll try to answer some of these questons.
It's not blowing cold air, but it's cooler than ambient.
The outside air is below 70 degrees. I am just trying to add 134A to refill the system.
It will not cycle at all.

 

Im assuming you have gauges but if not,you can buy a simple but effective a/c gauge hand tool that you push onto the freon valve and it will read the pressure on a gauge and let you know right away if its low,just right or high.

It reminds me of a tire pressure gauge somewhat,only fatter.Most parts store carry them for under 9 bucks.Very handy to begin diagnosis.

Sorry cant help more ya like Sun and the others could,theyre a wealth of knowledge and helped me fixed my a/c problems before as well!

 

My 94 had low freon at one time and would not cool, but the compressor would come on and it did cycle, but way to much as the low side went to low to fast due to low freon causing excess cycling. Not coming on as said is a whole different situation and working with out gauges is working in the dark. Harbor Freight has a nice set of gauges for $50. Just fine for usage a few times a year.

Even a system that cycles, if low on freon, may have poor cooling. Perhaps one can of freon was not enough for normal cooling but that does not answer the question of why the compressor would not come on. Possible low pressure switch or relay would be some of the easer solutions. Of course if the compressor does not come on, you cannot add freon and if you actually did get in one can of freon, you should at least get compressor start at system turn on.

Do you have the C68 climate control automatic system. If you do, has there ever been any flashing LEDs?

What troubles me in you write up is that your cooling fan does not come on when A/C is requested. This function is a combination of the HVAC programmer, PCM and CCM via the serial data link used for communication and interaction of these systems. The serial data is actually used to turn on cooling fan and compressor. This could take the troubleshooting is a whole other direction so symptom details are real important.

Check it out again, what’s the latest.

 

I am going to take it that you mean that the compressor isn't engaging and that there's no trouble code for anything. That points to a faulty low pressure switch or more common for your Year, a loose connection at that switch. It's on the Evaporator outlet. Remove and reconnect the harness once or twice or hold the connector onto the switch and see if that cranks up the compressor. If not, verify that the switch contacts are closed by checking continuity across it's terminals. If that shows a complete circuit, jumper the harness contacts with a paperclip - if that gets it running, you need a new connector.

 

If your A/C system had low freon the computer will not allow the compressor to cut on. This is built into the CCM to prevent the compressor from cycleing continuously due to the fact that the low side will pull down too quickly. If the CCM does not intervene you will quickly overheat the clutch and burn it out.
There is a very simple solution whenever this happens and is true for ALL C4 Corvetts.
SOLUTION
once you have put some R12 or R134 into the system to create some static preassure, Remove the negative battery terminal for 3 minutes then reconnect it. This will reset the low preassure fault in the CCM and the compressor will cycle as normal again. This has to be done as this fault will not reset in the CCM just because to low preassure signal goes away. The only way to reset it is to disconnect the battery.
Once the compressor is enguageing, you will need to follow the manual and completely fill the system with gas.

Hope this hepls anyone else that encounters this problem in the future.

Jerris

 

Other than diagnostics - CCM has nothing to do with compressor engagement. It's entirely controlled by the PCM which relies upon info from the Pressure Sensor and Low Pressure Switch. Granted, if there's an HVAC trouble code, the PCM won't ground the compressor relay, but in the absence of a code, the clutch should be engaging as long as the Low Pressure Switch contacts are closed (and a half pound of gas is enough to close them). The PCM then looks for a rise in voltage (pressure) from the Pressure Sensor and monitors the Low Pressure Switch for how often it's turning on and off. Too many on and off's (and there will be a lot of them if there's only a half pound of gas), the low gas code is set shutting things off until you clear memory (and better yet fix it).

Absent a code and a set of gages, you can do a couple of things to check it out: Scan it - that will give you the Pressure Sensor signal. The PCM likes to see about .8 volts (approximately 80 psi static pressure) but it's going to vary with temperature and the PCM is looking for that voltage to rise once the clutch engages. Exceed 4 volts (about 400 psi), and the PCM will shut it off to keep it from blowing up. That, on some GM's (and definitely by '96 or with OB2), will set a code too - usually in the 70's if it's OBD1 and it's a PCM code, not something you're going to get out of the Control Panel.

See if the Low Pressure Contacts are closed. Again, it only takes about a half pound of gas to give you a static pressure of 50 PSI and with that much AND NO Trouble Code, the Relay should be energizing the compressor. That's why it makes sense, in the absence of a code, to go to this switch first. If it's closed and the compressor isn't engaging, something in the harness is keeping the juice from the Relay. Usually it's the connector.

Running the compressor with a little to no gas isn't going to fry the compressor clutch - well not until the compressor seizes - and the probability of that happening (compressor seizing) rises proportionately with the amount of gas and oil that's leaked out of it. That's why the system was designed to shut it down and most of these later Years do a good job of letting you know that by displaying either a Blinking Light or a Check Gages or Check Engine Light. A lot of clutches though survive the Big Bang because the engine simply stalls when the compressor seizes. The clutch assembly doesn't rely or need the compressor oil or the refrigerant that carries it to keep from falling apart.

The PCM also uses the Pressure Sensor for fan control and the Low Pressure Switch (closed contacts) for idle control. Not having the fans running suggests that the compressor isn't engaging. It takes very little for the system to build pressure (heat) and it does so a zillion times faster than the coolant in the radiator. That's why the fans come on allmost immediately after the compressor engages - even when the Temp Gage has yet to register. And, you can diagnose HVAC fan operation while you're scanning it - though looking at the compressor should tell you whether or not the clutch is engaged and if it is and the fans aren't running, keep your fingers crossed that the pressure sensor signal shuts it down before it blows up. Otherwise, stand back.

Finally, there's always the possibility it's simply got a bum relay - no code for that. Get out your meter and check it's inputs - though on some of these later Years where it's buried in a Fuse Box somewhere, it's allmost easier to just replace it (which is the first thing I did when my truck had this problem - wouldn't you know it? It was the low pressure switch connector - damn!).

 

Thanks to all of you. Your input will undoubtably help me fix this problem. I just got back from work and will hit the garage in a few minutes. I will post my findings as soon as I am finished. Thanks again.