Air Conditioning Cycling On/Off Issue
 

I have a 1995 VX with 138K miles and I'm having an air conditioning problem. I recently got the AC recharged because it was cycling on and off. I assumed it as low on juice. It turns out that it wasn't very low and now that it has been topped off the system still cycles. The system was completely evacuated and re-filled.

I get about ten minutes of cold air followed by about 2 or 3 minutes of warm over and over. Anyone have an idea or a thread that might be insightful? It is slated to go back to the AC shop soon, but I?m hoping to now as much as possible going in and perhaps even fix it myself before having to pay the chop shop. :D

Thanks in advance!

Have you checked to make sure the system is not freezing up.

I went on a drive yesterday through the hills and when the engine was under load the AC was going on and off every 30 seconds. Really irritating. It seems like it depends on engine load to some degree. Any ideas?

Maybe someone with more A/C knowledge can help you. I do have one suggestion and it is something I have done in the past when I ran into a problem I couldn't figure out. I have called dealer service departments and spoke with the service manager about the problem, they are usually willing to give a few suggestions of things to check. It may be something as simple as a loose connection or something that's just not making contact the way it should and road vibration is effecting it.

There are 3 switches (aside from those on the HVAC control panel) that have to be satisfied for the A/C system to run. First, the thermo-switch on the evaporator... If the evaporator gets cold enough, it switches off the A/C system. Second, is the dual-pressure switch. If the refrigerant pressure on the high pressure side of the system gets low enough (not enough refrigerant in the system) or high enough (an obstruction in the system), it will shut the system down. Finally is the thermal protector in the compressor housing. If the compressor overheats, it shuts off.
You can rule out or confirm the thermal protector by checking if the condenser fan is still running when the system cycles off. If the fan is on, your compressor is overheating. If not, look at the other switches.
The evaporator thermo-switch is probably the next easiest to reach. I would put a volt meter on the connector terminals at the switch and drive until the system cuts out. If you see voltage, the switch is what's breaking your circuit. The same can be done for the pressure switch, but it's up in the front of the engine bay somewhere, so significantly less convenient to test.

Normal AC Behavior
 

I did a little more testing with the AC and it appears to be staying on during hill climbs, but it wasn't that hot during the testing. When I've taken a number of road trips and it was around 100F I seem to have the problem more than ever. The combination of really hot weather and hills climbs seems to make my AC turn off every couple minutes.

What is normal VX AC behavior? Does anyone else have these issues or is this specific to a problem to my car? In general does a healthy VX compressor ever turn off during normal opperation? Is there any sort of fuel saving algorithm on the VX or does the AC behave like a normal car?

I want to make sure I understand the baseline performance of the AC system.

Thanks!

Baseline for a 134a system is generally 20 degrees colder out the vents then ambient(either outside air or recirculated air) while normally driving. r12 is generally 25-30 degrees colder and a hydrocarbon system like I used to run in my Toyota can be close to 40 degrees colder.

On an 90 degree day I could have 35 degree air out the vents no problem in-town, the expansion valve usually regulated the system so it didn't freeze over but it happened occasionally. But, hydrocarbon refrigerants(while more efficient and safer for the environment) are illegal for use in automobiles in the United States.

Yeah, 'coz if you have an engine fire, the relatively small amount contained therein, might burn cleanly, instead of making flourine or phosgene gas and taking out half a city block. :rolleyes:

As an Example
 

To give you an idea of the situation. Around town for the most part the system seems to work well.

I recently took a 350 mile roadtrip to the Bay Area from Southern California. I was driving along the 5 freeway (almost no hills, but temp of 100F) at 75mph (I typically drive slower, but was in a rush...A sin I know) and the AC would turn off about every five to ten minutes. It would blow hot air for a few minutes and then you would feel the available power decrease as the AC compressor came back online. The air would then started to get cool again.

As long as the compressor stays engaged I can stay comfortable. The issue is that at time the compressor was only working roughly 50% duty cycle and things started to warm up making for a long hot drive! :thumbdown:

In the instructions I have seen for adding aftermarket refrigerants, they say you should add until the duty cycle is about 50%. So I'd say it's working as designed, it's just that "designed" wasn't all that good.