My car's air condition has been working intermittenly. At first I thought I was low on refrigerent. My theory was that the compressor was unable to build enough pressure to constantly to have cold a/c. Luckily I have UV dye in my lines. I checked for leaks using a black light and nothing.

Next I hook up the manifold gauge set. The low side pressure is high and the high side pressure is low... Like really low, they are almost matched. So that would point to two things (based off my research), 1) a bad compressor, 2) a bad expansion valve.

Set up: My car is a LHD 95, it had R134, MANA from the factory. I have an HKS v-mount and was one of the first (if not the first) to adapt the JP3 Rx-8 condensor, compressor kit to an HKS V-mount. While I was in there I used their kit to replace the expansion valve. It comes with the cork tape, valve, and seals. All of this was done about three years ago.

I really have a hard time believing any component failed given that they are all three years old or less (I replaced the expansion valve after the other parts were installed), but I am leaning towards the expansion valve, given that the compressor is not making any kind of weird noises, the clutch is engaging properly, and the AC works some of the time. Back me up, am I on to something or am I out to lunch?

 

I have had issue with the blower speed rotary switch. it would disengage the clutch at different speeds. i would have A/C on positions 1 and 2 and sometimes on position 3 and never on position 4. its worth checking it out

 

Couple of possibilities here...

R-R-Rx7's point about the fan blower speed switch is a strong possibility, especially if you're NOT running a stock ECU. To rule that out get a helper to sit in the car with the A/C switched on, and cycle thru all the blower fan speeds while you watch the A/C compressor clutch closely to verify that it stays engaged when the blower switch is set in each speed and doesn't cycle off.

The next possible electrical issue could be a bad A/C pressure switch OR the thermal switch on the evaporator core. These are both normally closed switches, wired in series with the A/C clutch circuit, so if either one is bad, it could cause the compressor clutch to short cycle or not engage at all. You didn't say what pressures you were reading on the low/high sides, but if they both fell in the range of between 15 ~ 300psi, the pressure switch is probably fine. The thermal switch should be closed at temperatures slightly above freezing (0C/32F), and open below freezing. It's purpose is to shut off the compressor if/when condensation on the evaporator core ices over.

The last thing I would consider before throwing parts at it is the possibility that some moisture got into the system when it was last charged with R134A. This can happen if you or the shop that charged it forgot to properly bleed & flush their charge hoses/manifold of all air before charging the system. Way to check for that is to evacuate the system again, pull a deep vacuum on it, verify the vacuum holds for a few hours, bleed the charging hoses & manifold of air and then recharge it.

 

What are the pressures exactly? We also need to define what you are saying is "low" or "high", R134a has a specific range it needs to operate in, so what pressures are you seeing?

I assume you are seeing the compressor running right? Not just the outside pulley spinning but the clutch engaging and spinning constantly? If its cycling on and off constantly it would show this type of behavior because you keep getting small bursts of pressure, just enough to raise it higher on the hot side but not enough to actually build the pressure needed for the system to operate.

If its not running at all you will have almost static pressure in the system, you would typically see slightly higher on the hot side because all of those components are in the engine bay where its hot.

If the compressor is running constantly and the pressures are actually low (need to define low) then its probably the compressor (or low on freon.) But if the pressures are reasonably high (again need to define this based on what 134a pressures are supposed to be) then its most likely the expansion valve stuck open.

 

I should have payed more attention to or cycled the blower speed settings when I was doing my testing yesterday. I have already evacuated the system. Yesterday when I was testing, it was 80 F out and I have the fan on "4". The Compressor was running and the air was kinda cold. The pressure on both gauges, high and low, was 90.

As far as testing the pressure switch or thermal switch, what is the best way to do that? Also for this fan speed problem how would I go about fixing that, is it an ECU software issue or hardware issue? I am running haltech.

I am about positive there is no moisture in the system. I always running a vacuum for several hours prior to refilliing the system and purge the line.

I pulled the refrigerant too soon, but as I mentioned earlier, when I was testing it yesterday, the compressor seemed to be running constantly and the low side and high side were both at about 90.

 

It does sound like the expansion valve then, that high of pressure should have resulted in a larger difference on the low side.