creilly

Yesterday I was out driving, and with the increasingly warm weather and the encroaching pollen menace I decided to keep the windows up and put on the A/C for the first time this year- I think the last time I used it was probably November for some particularly aggressive defrosting, and it was VERY cold at that time. In any case it currently behaves the same regardless of whether or not the A/C button is on or off. I have no check engine light, everything in the cabin functions perfectly, so I don't think it's related to the switchgear itself. A friend helped me check the operation of the compressor yesterday- it appeared to engage when the switch was pressed, engine idle increased from 650 as expected, and there was a faint noise from the vents when the A/C switch is on. Today when I checked again the compressor was not engaging when the A/C switch was pressed. Pertinent information here is that my car was converted to R134a at some point during its life in Japan. I tried a spare can of refrigerant I had sitting around from a previous recharging effort, and the low side pressure increased per the gauge on the can, but no effect on the system's ability to cool. Interestingly the compressor engaged as soon as I started adding refrigerant though. I have also checked the fuses under the cover on the right side of the engine bay, though I have not dug out the other one behind the headlight or the one on the interior. In reading through the relevant sections in the service manual I have uncovered further ways of narrowing down the problem by use of the sight glass in the front of the engine bay- I always wondered what that thing was! Tomorrow I plan to check the system's operation again making note of that sight glass.

In searching around I have found references to A/C problems being caused by bad ECU capacitors, though seemingly none that matched my symptoms. I have not laid eyes on my ECU as the car has run perfectly until now, so I am unsure of its condition. With my limited experience and the knowledge at hand I suspect the compressor of being the culprit but of course I'd love to hear there's a good chance of it being something as relatively inexpensive as ECU capacitors.

I would appreciate any insight anyone might be able to offer, or similar experiences that may be able to help me pin this down. Thanks!

creilly

I have now checked again using the tests laid out in the service manual. Sight glass next to the right headlight does not appear to have bubbles or foam in it at any time, and clears fairly quickly when the A/C is turned off. The clutch on the compressor appears to be functioning properly and it spins with no problems. Idle still increases from 650 to 900 when A/C is turned on. To the touch the inlet and outlet pipes on the compressor are different temperatures- the pipe closest to the cabin (I believe this is the outlet) is much warmer than the other, though not hot. Per the gauge on the can of refrigerant, the low side now reads about 45 psi (pretty sure I overcharged it yesterday) when A/C is turned off, and immediately vacuum when A/C is turned on. From inside the cabin a quiet metallic hiss is audible when the A/C is turned on. I'm not sure if that's normal because it's been so long since I last had the A/C on. Page AC-27 of the service manual says "If a metallic sound is heard, replace the compressor assembly" - not what I want to hear! A/C diagnostic shows flashing "00" indicating normal.

Doing some more googling and reviewing the service manual I now suspect the drier or evaporator pressure regulator may be the source of a blockage, though without the high side pressure gauge I have limited ability to diagnose myself. It seems likely I will need to take it to a shop to solve this problem, especially where refrigerant is concerned.

oldskewel

iTrader: (1)

I'm not an expert in this, but I have maintained my R12 system in my 1991 and it is still working. So some comments that I hope help ...

the metallic hiss is not normal

45 psi when it is off is not necessarily low. When the AC is not running, pressure high and low should equalize. They also are highly temperature dependent - and the temperature is the temp of the AC system, which may be in a 150*F engine bay. Once the pressure is equalized, assuming your system contains some fluid (i.e., not all gas, as you would have when you're almost bone dry) you should be able to look up the temperature vs. pressure. You can also watch the temp drop as the hot engine bay gradually cools down; or vice versa as it warms up with the AC turned off and engine on.

Cold-side pressure should not go to vacuum ever. If there were no Freon, you might get a vacuum, but before that the compressor should turn itself off.

The thinner of the two pipes should be hot, with compressed refrigerant in there. The fatter should be colder, with low-pressure refrigerant, sometimes insulated to keep it from taking in engine-bay heat.

The most common problem on AC systems, and the problem I had on mine was simply leaking pressure port valves, which are very similar to those on your car or bicycle tires. Checking for leaks there may be easier than you think.

creilly

Thanks! I'll go over the inlet and outlets again with an eye for thickness to verify what's going on in there. Sounds like a set of those AC pressure gauges might be a decent purchase too, so I can see what the high side pressure is. I did notice a little bit of bubbling in the low side port when I was replacing the cap this morning, too.

And when you talk about the temperature of the AC system, where should I be looking to measure that?

oldskewel

iTrader: (1)

"temperature of the AC system" - my basic point is to realize that the pressure varies with temperature. So if you walk out there now, with a cold engine and measure it (engine and AC off), and then measure it once the engine has been warmed up, or even if the weather is warmer, you'll get different numbers. As things warm up, a little more of the liquid will turn to gas, and that raises pressure.

If you look at the gauge faces on here:
http://aircondition.com/tech/questio...ith-Gauges-FAQ

You'll see that the needle points to the pressure (because it's a pressure gauge, of course), but also there are inner rings of markings for temperature. Different numbers for different refrigerants (R12 vs. R134a being the most common here) indicating the relationship between system temperature and pressure.

So you don't need to worry about any particular temperature, but you probably do need to understand that things vary, so you don't start drawing conclusions based on pressure readings that will vary with temperature due to the most basic physics.

And yes, on buying the real gauges, it might pay for itself the first time you avoid needing to pay a pro to fix it. But there's some learning you'll need to do to make sure you do things properly. If you're interested in that, and like to learn something so you can take car of your car by yourself, it may be a good idea. But if you just want this problem fixed now and don't care about thermodynamics, maybe not.