MisterRisky

Like a number of folks on here I am diagnosing AC issues. My '91 964 has been converted to 134a. I was getting only a very little bit of cool air from the vents so I decided to debug. I have not done a lot of AC diagnosis, but I get the general idea of how they work. For those who want to refresh on how AC works, this is a good resource .

When debugging my AC, the first thing I noticed was that my compressor kicks on for, literally, one second then is off for about 30 seconds then on for one... I suspected low refrigerant pressure. I have a low pressure el-cheapo 134a pressure gauge, so I hooked it to the low pressure side of my compressor and expected the needle not to move. To my chagrin, holy cow, 75 PSI.

So I am concluding that I don't have a leak. But somehow I am getting high pressure in my low pressure side of the system.... or my system is WAY overcharged. In my simplistic mind, if the pressure is that high on the low side, that means that the expansion valve must be messed up. Is this right thinking? What else should I be checking?

I took a quick video of the pressure checking process. You can see the clutch engage twice in the video, each time for only a second. When the compressor kicks on, the pressure drops a little (naturally). The gauge is hard to read, but 12:00 is 100 PSI and 9:00 is 25 PSI. I thought this might be useful given the previous posts asking what it looks like when the compressor clutch is on.

jimq

Are you reveing the engine to 2K RPM or so when checking the pressure?

MisterRisky

Nope. Didn't do that. Will it really make that big of a difference in pressure, compressor switching on, etc?

jimq

It does on mine.

MisterRisky

Hmmm... I'll redo the test tonight and hold the engine speed at 2000. Thanks for the tip!

911URGE

When I purchased my 89 964 it did not blow cold air. It was a 134a conversion. I took it to a local p-car shop and they determined the system had no leaks and the a/c pump was sound. I had one relay replaced as well. They speculated the 134a conversion may not have been done correctly. After evacuating the 134a and replacing with R12 the vent temps were much better but not within specs.

I suggested the expansion valve be replaced (based on an article in Adrians 964 companion book) as it was a $75 part and the vent temps fell right into spec after that. That was March 2006 and its still working fine. YMMV.

MisterRisky

Thanks Stephen, that's good insight. The even better news is that expansion valves are only $27 at Pelican Parts. Do you recall on yours if the compressor was cycling a lot, or was it running more normal?

I wish, however, that I had a better diagnosis decision tree. The part is not expensive but replacing it is a PITA.

dfinnegan

What about the labor? Doesn't the gas tank need to be removed to get at the expansion valve?

Does anyone have any more information on the expansion valve and how it "goes bad"? I thought these were simple throttle valves with no moving parts. Is that not right?

911URGE

All systems running normal - A/C pump, condensor fans etc... just not blowing cold air. There was a relay replaced as the tech's felt it was old (dated 8/89) and that helped get things running better but ultimately replacing the 134a with R12 and expansion valve did the trick.