I’ve only had my vette or a couple weeks, ad I love it. but the ac is giving me fits. It is a 92 with the automatic electronic climate control. it’s a long story that’s occurred over a couple days, so bear with any typos, one day the ac just blew hot air, the clutch wont kick in. I checked the Freon pressure and it is in a acceptable range. I made sure there was voltage to the pressure senor on the low side, so my next step was the comp. clutch control relay. I took the old one out to show the guys at the auto parts store. When I put it back in and headed home, the engine got up to about 235*F, so I turned the heater on and kept moving and I got the temp down that way. When I got home, I stole a relay from what I now believe to be the cooling fans and plugged it in and the fans worked, but no ac clutch. So I went ahead and got the relays, and replaced the two I just mentioned. B.T.W: the ac one is on back of the left front fender well, and the one I believe to be the cooling fans is in the "hole" in front of the front left wheel. both are black and sort of oval shaped. anyways, I got the new ones and installed them both, the car started getting hot again, so I unplugged the lower one(fans) and they kicked on and the car cooled, the whole time the ac mp was cycling off and on due to needing more Freon. so I drove home with it off and the windows home. next morning, it wouldn’t cycle at all. I did find that I could jump the two wider terminals in the harness that plugs into the ac relay and could turn the compressor on and it would stay on...what on earth is going on?????

 

Confused, you took a fan relay out and the fans run?
235F is not hot for a C4

Should be 2 fan relays ; one for each fan

If the Freon is low, the pressure switch in the A/c line may not be seeing the right pressure to activate the A/c relay.

 

When you checked the freon pressure, what was you "acceptable range". Both high and low side should have been the same with the compressor not running. A normal pressure is about 90-100 psi. Much under 90 might give you cycling problems.

If you connect the 2 wires to the cycling switch together (one goes to A/C programmer, one is from the ECM) the compressor should run. This test checks out many items that makes the compressor run. ECM, Clutch, relay and related wiring. If the compressor runs and things get cold they way they should, the cycling switch is probably defective. If the compressor does not run, then you are dealing with any number of items. I would tend to think since the A/C ran at some point the clutch is OK. That might leave you with the A/C relay, ECM or the HVAC programmer. The A/C programmer actually grounds the blue wire to the cycling switch when A/C is requested that starts the whole process. You might also have a compound problem of low freon but you really can't tell till the compressor runs and cycles while checking the pressures with the gauges.

There’s actually a bit more but that is the major basics and a starting point.

I’m not even going to advise you on the fans and cooling at this point which is a whole different topic and don’t want to confuse the situation. One thing at a time.

 

I was advised to be at an absolute minimum of 80, but i do have it right at 90 maybe a touch above.
im not sure what the cycling switch is or where it is. The ac relay that i am talking about, i jump between the two bigbladed terminals, cant remember colrs of the top of my head. (would pics help??) yes i believe the clutch to be fine. when i jump those two teminals, the clutch engages, everything blows cold, and also the rad. fans kick on. can you advise as to a location of the cycling switch? or even wire colors would be some start...
thanks for the input. im a corvette nube.

 

Im not certain what the "stolen" relay i honestly. if anyone has a diagram i could point it our, or better yet ill upload a pic of both I am talking about,if i can figure that out lol

also i was told by a mechanic that i deem to be incredibly knowledgable and reliable that the low side pressure sensor is not a switch but just a sensor. however i did jump it and it did not change anything

 

http://s419.photobucket.com/albums/p...t=DSC01506.jpg
http://s419.photobucket.com/albums/p...t=DSC01507.jpg
http://s419.photobucket.com/albums/p...t=DSC01508.jpg

 

It sounds like the lay out of your 92 is somewhat different that my 94. There is a low pressure sensor, however there is also a cycling switch which is a switch, opens and closes. It probably is right by the evaporator case, off the fat pipe, a few inches down from the top, two wires going to it and should look like the switch can screw off the “T” fitting. There is a schrader valve under the switch which will not let the freon out.

Remove the connector to the switch and connect to two wires together. If the switch is bad the compressor will start. Make sure you turn on the A/C in the car. You can also check the resistance of the switch to see if it is open or closed. With the compressor not running and sufficient freon in the system it should show closed.

 

Ok thanks, i know exactly what youre talking about. ill check that out and post back.

 

ok so i tried jumping thosepressure sensor wires on the low side. no luck. i also saw a similar fitting/sensor on the high side with 3 wires, i tried jumping some of them, ad twoof them actually killed the engine, so i put it back not to mess with it anymore. ive acually been driving it with the two wide terminals on the ac relay jumped across, it gives me a/c and the coolig fans still work.but i would like to fix this problem corectly. in drivig like this i notice that whatever it is that controls the direction of the air flow i.e. defroster, vent, floor etc, will close after about 15-20mins. i can still hear the blower motor running fast as ever down under the dash but none of the air is allowed out. any ideas? thanks again.

 

Oh crap - the 3 wire pressure sensor on the high side is a thermistor with a 5 volt reference from the ECM. As the pressure rises, the return voltage increases (2nd wire with each volt representing approximately 100 psi) and with this, the ECM calculates the high side number. The third wire is simply ground. Depending on what you jumpered, it may have shutdown the ECM or in the worse case it fried the ECM. Check fuses, reset memory by disconnecting and reconnecting the battery and then see if it'll start.

The Low Pressure switch simply opens and closes the Compressor Relay ground based on low side pressure (25 psi is the threshold for an R12 charge). It's monitored by the ECM and rapid on/off's (cycling) is used along with the Pressure Sensor Signal to determine whether or not the charge is sufficient for safe operation of the compressor. Jumpering this switch maintains the clutch ground and if you did this and got decent pressures, it either has a defective switch or a poor connector (the latter being more common - so try holding the connector on the switch, wriggling it around, disconnecting and reconnecting it a couple of times and see what it does).

Cooling Fans - a/c operation - are controlled by the ECM based on the Pressure Sensor signal. The target is to maintain a high side that averages 200 psi so when it sees about 2 volts from the Pressure Sensor signal, one of more fans should be cranking (and if it's 70 degrees or better, both probably should be if the charge is right). Non a/c operation turns on the Fans based on Coolant Temperature from the Coolant Temperature Sensor. Best to diagnose either with a scanner which will give the Pressure Sensor signal in volts/psi along with the temp of the CTS while you look at the fans.

 

well it started right back up so i assume it justshut it down. jumping the low pressure sensor does nothing to remedy the situation and i do not see any other sensors on the lines besides these two...this is a baffing problem. im trying my best to keep it out of a shop oreven worse a dealer, i dont want to spend the money right now.

 

If a low gas code is set, the PCM won't respond to the a/c request; ie, it isn't going to drive the Relay so jumpering the Low Pressure Switch gets you nowhere. Disconnect and reconnect the battery to clear it (or any glitches). Retry.

 

no luck with that fix...i left it disconnected for about 10mins...should have been plenty long enough

 

If the a/c does not blow cold after disconnecting the battery, then there is not enough freon in the system to cool the air down.
You probably need to have it checked for leaks.

 

plenty of freon. i emptied the system, pulled a vacuum and put 134 fittings, oil and o ings in then recharged. so freon and pressure problems are ruled out. i can force the compresor on and it blows ice cold.

 

Did you jumper the Low Pressure Switch after clearing Codes? Are the Low Pressure Switch contacts closed (should be a completed circuit with a DVM across the switch terminals)? Can you scan it? Might want to see if the PCM is getting an a/c request and what the return voltage pressure is from the a/c pressure sensor (with a full charge on a 70 or better day should be at least .8 volts or 80 psi). Basically, a/c is requested from the Dash Panel over serial data lines (assuming electronic air) to the processor (Programmer) then onto the PCM. If there's a Pressure Sensor signal and the Low Pressure Contacts are closed, the PCM drives (grounds) the Relay and the clutch engages. PCM then looks for a voltage (pressure) increase from the Pressure Sensor and begins to monitor the Low Pressure Switch.