Trouble diagnosing ac issues

[guitarfreak235]

Hello all, as you may know im new here and just recently bought an 88 gt. Im having trouble troubleshooting the lack of cool air coming from the ac.

The previous owner said it had been working not long ago, but didnt when i test drove it. The compressor does seem to come on since there is a dip in idle speed when i turn it on. I checked the refrigerant, and it was really low, almost gone really. Charged it up but still not getting cold air. While idling with the ac on now it seems to build pressure to full, then let off with a click  and reduction in pressure, then builds again when pressure reaches low. It cycles like this constantly.

I checked to see if theres debris behind the resistor in the front compartment, per fierofools recomendation. Nothing on the resistor, the heater core, or in the passage from what i can see. Looks exceptionally clean and shiny actually. I did find a single pine needle sticking out of the heater core but nothing else..

Any ideas? I had a problem with my 86 k10 cycling the compressor before, but that was due to low pressure in the system. The fiero seems to have sufficient pressure, and the click from it cycling seems to be audible from the front and not the compressor.

[Fierofool]

With the AC activated, open the engine lid and watch the compressor clutch to see if it cycles off and on.  Jf it's a rapid cycle, it's low on refrigerant.  Long periods of OFF and short periods of ON might indicate an overcharge.  Too much pressure.  I once drained my system very slowly so that I didn't blow the oil out of the accumulator, then added 2 cans of R134a back to the system.  It started cooling much better.  I later picked up one of those cheap refill kits with the gauge and trigger hose and topped it off with about 6 more ounces and it works great.  The final thing I did was to remove the old R12 cycle switch and install a new R134a cycle switch. 

Also, with the AC NORM button activated, move the temperature slider control from cold to hot to cold to see if the temperature changes.  Try the same thing with the AC button on MAX.  If the temp changes, then the mixer door is working and connected to it's cable. 

When converting to R134a, I think it's best to just replace the cycle switch with one that's already set for R134a, rather than try to adjust the old R12.  Someone on Pennocks just pointed out that there isn't a linear on and off pressure difference in the two refrigerants, so adjusting the old switch won't give you the precise ON-OFF set points.  I think he said there was about a 9 psi difference. 

[Roger]

Had the previous owner changed over to R134a or was it still R12?
What did you add?

[Raydar]

It's possible that the temperature door is not moving.
With the A/C running, grab the pipe connected to the accumulator (aka receiver/dryer) in the front compartment. That's the metal canister on the passenger side. It should be cold to the touch, in a working system. If it's cold, the A/C should be blowing cold air from the vents.

If the compressor is working, but the A/C is not cooling, it's possible that the orifice valve is clogged. It's located in the metal tube that runs along the bottom of the front compartment. Unfortunately, the system has to be evacuated in order to check or replace it. If it's clogged, it's possibly because the compressor is failing.