Hi everyone,

I have a '91 vert that has been giving me problems with the air conditioning. I know, I know . . . put the top down and forget about it, but anyway . . .

I have found out much using the search function and I think it is down to a low pressure switch problem. I have jumped it and put in 18 oz. of R134, but when I go back to the switch, the compressor still refuses to cycle on. I have read that the switch itself can be a simple fix. I am wondering how to go about replacing that switch, or is it even possible without the right tools.

Just wondering if I could give this a try (if it is inexpensivve) before just taking it in for potentially a bigger fix (and more money). Thanks.

 

You say you put 18 ounces in? If the A/C system is still to low it still may not kick in. Do you have the gauge type fill kit? Your A/C clutch will usally start to cycle at about 25 psi low side. Max is 44psi. When you bypassed the switch did the compressor come on then start to cycle after adding 134a?

 

I do have the gauge type kit . . .

The compressor did come on with the paper clip jumper but it didn't cycle; just stayed on continuously. After putting in a can and a half, I stopped thinking that it must be something else. I will check the pressure reading tonight . . . could it possible need even more freon?

 

anyone know where the low pressure switch is located on a 96?

 

The Low Pressure switch is a freeze and low charge protection device. Its contacts open at 22.5 psi (R134) to 25 psi (R12) cutting power to the compressor clutch circuit. Those contacts reclose at 50 psi and power is restored.

Regardless of refrigerant, a lb of it has the same static (not running) pressure as a full charge (2.25 lbs of R12 for the '91). Therefore, if you got 16 ozs of gas in it, the switch contacts should be closed (assuming ambient is above 32) and the compressor should engage - at least once.

These later years ('90 and up) monitor how often the low pressure switch contacts open and close and if they do so rapidly, the ECM takes that to mean that the charge is low (because that's how a system acts when there's some gas, but not enough of it). That sets a trouble code and the ECM won't re-drive or ground the compressor relay until the code is gone. On a rebuild or if you're adding gas, disconnecting and reconnecting the battery gets the code cleared.

The best way to troubleshoot the switch is with a manifold gage set, but if its got any gas in it, the switch contacts should be closed. And if there isn't a low gas code and those contacts are closed, it's either got a bad connection or a bad relay and more often than not, it's simply a bad connection, particularly if you're getting it to run by jumpering the harness contacts (with a bad relay, it wouldn't crank at all). Try wriggling the connector around. Fashion something to bridge the female connectors in the harness to the switch (I just cut some solid core out of 12 gage Romex and stick a piece in each connector and then touch the switch).

As to putting R134 into a '91 - hope it was converted properly and that you're not dumping that in on top of R12. That won't cause the low pressure switch to fail - but two, unlike gases, can raise system pressure to dangerous levels (so ignore whatever miracle in a can advertising you might see at Walmart). In fact if you've got them mixed up, the low side might never get low enough to open the switch contacts and if you want to make cold air, the system needs to operate just above the threshold for those contacts to open.

Low pressure switch on the '96 is either on the Evaporator outlet - big pipe back to the Accumulator or tin can - or it's on that Accumulator. Absent a troublecode, I doubt there's anything wrong with it. More often than not, the connection is bad.

 

Thanks SUNCR.. I sold my 96 LT4 last year to a friend of mine and he has been having problems with the A/C.( a/c will blow cold air if we send 12v to the clutch) He brought it over to the dealer to fix the problem, they found the computer to be throwing "low side pressure switch" code, however the tech tells him that the 96 Vette doesn't have a low side pressure switch according to the service manual, so they can't fix the problem

 

Low pressure switch wasn't eliminated until '97 or the C5 when GM switched to a variable stroke compressor. That unit doesn't need to cut power to stop a freeze up, the compressor simply senses demand through a valve and varies it's stroke to reduce or increase pressure. GM was using that unit on all most everything but trucks and Vettes and I guess they finally figured they got the bugs out of it (or it was just cheaper to use it in everything in spite of warranty problems). I have to admit that it's nice to sit in traffic on a marginal day and never feel the compressor cycling on or off as you can in a Vette (when the ambient is low enough to ice up the evaporator).

The '96 still uses a low pressure switch or cycling orifice system, but it also monitors the high side through a Pressure Sensor. That has a 5 volt reference and returns about 1 volt for every 100 psi of pressure. PCM generally looks for .8 volts to ground the relay and nothing over 4 volts. More importantly, it needs to see a pressure or voltage rise from the Sensor when the compressor engages. There still needs to be a completed circuit through the Low Pressure Switch and it's also still monitored by PCM for how often it opens and closes. The Pressure Sensor is easily diagnosed with a scanner and there are OBD2 codes for this part. However, if there are no codes at all, that sounds like a bad connection at the Low Pressure Switch. Had same on my '97 S10 and there seems to be a lot of posts around here with it happening on Vettes starting with the '94 up to '96.

 

If you have climate control, the computer may have sensed multiple on-off cycles due to a low charge and locked out the compressor "on" command. Check for a code "9" on the AC screen for this. If so, then disconnect the battery for a couple of minutes to clear the trouble code.

If the problem is the low pressure cycle switch, it can be easily changed out with a common open end wrench. Replace it with an AC Delco switch; parts store switches made in India leak like a sieve.

 

OK, so I am getting close and thanks for the help.

When I wiggle the switch and hold it just right, the compressor starts to cycle. so I think that I found the problem.

But a stupid question, am I replacing the harness or the switch (2-prong) screwed into the AC line? Also, could someone help me out with exactly which part I should be buying . . . it is located about the entire way back to the oil dipstick on the narrow aC line . . . is this the low pressure switch or the cycle switch?

Sorry for being so naive. Thanks.

 

Oh, I forgot to mention, the wires running into the switch are green (with a black stripe) and blue . . .

 

That's the high pressure switch - not the low pressure switch. The Low Pressure Switch is on the Big or larger line at the Top of the Evaporator. The high pressure switch opens at 400 psi and kills power to the compressor before the system blows up and puts a hole in your hood. It should be closed and since you can wriggle the connector around and get it to work - it probably is. Sounds like you need a new connector and you're going to have to scrounge around to find one. You could try NAPA. You might also see if there's a part # at www.acdelco.com and if there is, order it from a dealer. Otherwise used from Contemporary Corvette - www.contemporarycorvette.com - might be your only choice.

 

Sorry, I got that wrong.

It IS the switch on the bigger line -- the higher one, not the one further down in the engine. So, it is the low pressure switch.

I have heard people reference a AC cycling clutch switch gthat sounds like the same thing . . . so the low pressure switch has two prongs . . .

-K

 

Here is the Napa item #TEM207883 . . . looks just like this . . . there is a harness available online (TEM207908) but it says "for R12". Could there really be a difference in the harness for R12 as compared to R134 . . . the switch connection looks the same . . .

 

There is no difference in the connector. An R12 switch will cut power at 25 psi. An R134 switch has the threshold set to 22.5 psi and seems to be more common (without realizing it, the last one I bought - and I'm pretty sure I picked it up at NAPA - had the lower threshold and I didn't think about it until I noticed ice. I'm still using R12). R134 freezes a few degrees lower than R12 - that's why there's a difference. Either switch may be adjustable. If so, there is a screw between the terminals. To adjust, put the system on max and while monitoring the low pressure gage, disconnect the blower motor. The lack of airflow will drop the low side and the compressor should shut off at whatever is dialed into the switch. Note the pressure on your gage. Disconnect the switch and turn the screw 1/2 turn counterclockwise to reduce the threshhold, or clockwise 1/2 turn to raise it. Plug it back in and repeat the test. You may have to do it a couple of times to get it right.

 

I know I'm dredging up an old one here but the issue I'm having is my '93 was converted to R134. Occasionally the AC will quit working and I'll get a flashing light on my climate control. Sometimes I can just pull the fuse to reset it and other times have to disconnect the battery. I believe it has the original low pressure switch for R12. Could this be the problem? Should I replace the switch with a '94-96? Thanks

 

I have the same situation. Did you find a fix?