Hey all.

I have a 95 base coupe, and I'm working to diagnose some AC issues. I've searched around here and elsewhere, but I can't seem to find anything applicable to C4s. Maybe my Google Fu's off?

Basically, when the A/C is turned on, the compressor will rapidly (every two seconds or so) cycle on and off. My first assumption was that the R134 was low.

I've used the cheap cans from the parts store before and had success, so that was what I tried first. Problem is, the compressor won't stay on long enough to give me an accurate pressure reading. I added a little to see if I could get the compressor to stay on, but no luck. I don't want to overcharge the system, so what should my next step be?
Thanks in advance for the help!

 

I should clarify that while I have used the cans before, I have never done AC work on this exact vehicle.

Thanks!

 

add some more. I add to mine with it on max cool with the windows open till it stops cycling then add a bit more.

 

My concern is that I don't want to overcharge it. What pressures should the low side show? With the car not running, low side is showing around 60 PSI. With the car running, the compressor kicks in at around 45 PSI, and the pressure quickly drops to around 12, which is when the compressor shuts back off until the pressure ramps back up to 45.

Thanks!

 

I do not know that never use gauges just feel the hose if it cold flashes it enough. you can jump the low pressure switch with a wire to keep the compressure running to test.

 

For your '95, the Low Pressure Switch, mounted on the outlet from the Evaporator (not the Condensor) opens at 22.5 psi and closes at 50 psi. With the system static (off), allmost any amount of gas will provide a system pressure of 50 psi and the contacts should be closed (which is what your Manual is telling you). Once running, the refrigerant is compressed into a high pressure gas, cooled at the Condensor into a high pressure liquid, and then metered into the Evaporator as a low pressure liquid where it boils back into a gas and in the process, absorbs heat. Design is such, that a system, when fully charged and with an average outside temp of about 70 degrees to 90 degrees, that this heat absorption cycle takes place right around the freezing temperature for the refrigerant, which for R134 is about 25 psi. So at these outside air temps, the compressor should be running continously - unless: there's insufficient refrigerant in the Evaporator because the charge is too low or refrigerant flow is backed up. In either situation, whatever gas gets into the Evaporator flashes immediately and with the oomph gone (below 22.5 psi) the Low Pressure Switch contacts open and the clutch disengages. As the stopped system equalizes, the low side pressure rises and when it reaches 50 psi, the contacts close restoring the electrical circuit to the clutch coil and the cycle repeats, fairly rapidly if the volume of refrigerant reaching the Evaporator is too low.*The '95 ECM not only drives the Compressor Relay but monitors how frequently it is being turned on and off by the Low Pressure Switch. A rapid opening and closing is recognized as a problem. When that happens, a trouble code is set and the system is shutdown until the Code is manually cleared.*The '95 ECM also monitors the high side pressure through a Pressure Sensor mounted on the smaller line from the Condensor to the Evaporator inlet. This sensor returns a voltage signal to the ECM that is proportional to pressure (more volts more pressure). When the compressor engages, the liquid refrigerant flowing towards the Evaporator should rise and the ECM expects to see a voltage increase at this Sensor. If it doesn't, ground is removed from the Compressor Relay and a troublecode is stored for the Sensor.Most systems (by design) work best when the liquid that meets the Restriction (orifice) at the Evaporator inlet doesn't exceed an average pressure of 200 psi. (AND they also work best when high side pressures are no greater than 2.2 to 2.5 times the air temp at the Condensor). That allows for precise metering so that the boil off is right around the freezing temp - but not cold enough to make ice. The ECM uses the Pressure Sensor signal to turn on and off the radiator coolant fans to maintain this pressure at idle and low vehicle speeds.Systems that have a plugged up orifice or poor air flow over the Condensor Coils can build operating pressures that can blow a hole in something if the compressor isn't turned off. The ECM uses the Pressure Sensor to make sure that doesn't happen and if the signal from this Sensor indicates a pressure of 400 psi, power is removed from the compressor relay. Your Year may or may not set a troublecode for this condition.An intermittent connection at the Low Pressure Switch would explain what's going on with your '95 - if only because the system is so good about shutting things off when things go wrong and letting you know it via a trouble code. You should monitor pressures with a Manifold Gage Set (though the high side can also be read with a Scanner) and if it's cutting out intermittently above the cutoff threshold (ambients above 70 degrees), wriggle around the Low Pressure Switch Connector - try holding it firmly onto the Switch and if that gets it running - it needs a new connector - possibly a new switch. Otherwise, post your operating pressures and outside air temp for further help.


found this for you hope it helps

 

if you have a digital climate control look on youtube to see how it self tests.

 

ChrisFix did a (somewhat contrived) test session that shows how to use the HVAC diagnostics:

 

Not a bad vid! Nice.


That is too low. Pressure shouldn't drop below 20 PSI w/the compressor running. It also depends on the ambient temps, but 12 PSI is way too low. Add more.

 

Mine is doing the same exact thing as OP. I'm afraid to keep adding more as I can't tell how much is in there due to the constant cycling of the compressor assuming what I see when the A/C is turned off is not an accurate measurement?

Looks like the OP is likely no longer active on the forum. Anyone else deal with this before?

No codes on the HVAC, well at least my OFF light is not flashing so I didn't bother checking codes.

 

I just thought of something. Am I only supposed to measure the reading when the compressor is engaged? If so, then yeah I need to add more.

 

Yes, it appears the compressor should be engaged before reading the gauge. Mine quit cycling on/off after it got into the green on the gauge. It never went higher than barely into the green but I think my can might be empy, I'll buy another one and see if anything else will go in.

Be interesting to see how long it lasts, hopefully it's not a bad leak.

 

If you have the cycling on off rapid issue while charging freon it is most likely a bad pressure switch. Ac systems are quite simple. I replaced the ac pump, dryer and oriface on my 01 Suburban. I had the cycling issue. Replaced the pressure switch and it charged right away.
There's one on the pump and one on the dryer on the Subs. I replaced the one on the pump with the new pump. I had to also replace the one on the dryer.