I have a 1992 Explorer XLT, 4.0 V6 and the A/C is not blowing cold. I bought a kit to test the refrigerant and top it off if necessary, but I'm not sure if that's the problem. When I turn on the AC to max, the compressor clutch begins engaging on and off, on for 2 seconds and off for 6 or 7 seconds.

Sounds fine when it comes on. During the intervals, however, while I'm trying to measure pressure, while the compressor clutch is engaged the pressure goes way down, then when not engaged begins rising to caution levels on the guage.

My question is, If I begin topping off the refrigerant, will the clutch engage correctly and stay on? If it goes off will the pressure be too high for the system?
Does it sound like the compressor and clutch are functioning correctly?
Hopefully someone can help me and discuss this. Thanks.

 

First things first. What kind of kit did you buy? 92 was originally an R-12 refrigerant system and unless you are licensed, I doubt your kit is for R-12. Most likely it is an R-134a charging kit and unless your system has been converted, will not connect up since the service fittings are different sizes/shapes.
Assuming you have a converted system, you really should be looking at pressures on both the low and high sides to determine if charge is low or a problem is present. Depending on the outside air temperature, a good working charge should be somewhere between 30-45 PSI on the low side with the compressor running and somewhere in the 250 PSI range on the high side. Static system pressure should be around 70-80 PSI range (system off) both high and low sides. If your system shows unusually high side readings and very low side readings, or pressure does not rapidly equalize after compressor shut-off, suspect a stopped up orifice tube.
Assuming the high side readings are not out of whack, system probably needs a top off charge. There is a low pressure compressor cut-off switch on the dryer tank that opens compressor clutch power if low side pressure falls too low. Not sure of the exact setting, but it is somewhere around 25 PSI. You can disconnect the wiring harness to the switch and short the 2 leads together to keep compressor running while you measure the pressures.
Add freon till low side stays above the cut-off setting and compressor will stay engaged. Run engine up to 2000 RPS's and watch the pressures, adding small amounts of freon as necessary.
Do not add too much, more is not always better. High side pressures can rise rapidly to point of system failure/ compressor lock-up. If problem re-appears in a few weeks/days, you probably have a leak in the system that needs to be fixed.
Hope this helps
Dialtone

 

Thanks very much, that is helpful. The system must have been retro fit to R-134A because all of the fittings are correct and fit. There is no sticker which would normally indicate the refit.
I will try the switch to engage the clutch and keep that running while I take some readings, and I will test the pressure on the high side as well.

Thanks for all your advice.