Sauce06

Hey all. Acquired a 92 f150 xlt a few years back and im now really getting into fixing it up. Well, i sm attempting to get the a/c up and running. 1st i checked to see if the clutch is frozen, but its not. Then i tried to bypass the low pressure switch on the dryer, the compressor still didnt kick on. With the bypass in, i checked the voltage at the compressor plug with no voltage present. Anyone have any other ideas?

Phil63146

With a truck that old anything's possible. Start with the A/C Fuse and make sure you're getting 12 volts in and out. Assuming you're getting the 12 volts out at the fuse it's a matter of checking the voltage between fuse and A/C coil, starting at the fuse. Normally, it's a pain.

Sauce06

Ok, so the low pressure switch connector was very corroded. Cleaned it up and i am now able to jump the switch. The compressor kicks on now. I hooked it back up to the switch and the compressor wont kick on. Went to autozone, picked up a new low pressure switch and still the same thing happens. I can jump it and it works but with it hooked up, it wont. Checked the low side valve for pressure and it shows 35 psi.......any ideas??

Phil63146

You will have to jump out the low pressure switch to charge the system. If you are low on Freon 12, as soon as the compressor starts it lowers the system pressure and the L/P switch kills the voltage. Fortunately the charge on older systems is not critical, but you first have to follow some basic procedures.

If you are not familiar with basic procedures and safety precautions LEARN THEM via books or YouTube.
PRACTICE THEM.
An A/C isn't worth getting hurt over.

1. Empty system of Freon 12. Change the A/C orifice tube. Most likely it has crud clogging the screen which would cause the low side to pull into a vacuum and shut off the L/P switch when the system is turned on. Also change the o-rings and garter springs on all of the vehicles A/C hoses.
2. Pull a vacuum with a vacuum pump. Afterwards watch the gauges for 5 minutes to see if the pressure rises. Make sure the gauge set is in good shape and the vacuum pump has fresh oil in it.
3. Jump the L/P switch and start the truck. Make sure the engine is around 2000 RPM so plenty of air is flowing over the condenser (a must). Have the cab fan on high.
4. Start charging Freon 12 into the system. The factory charge was on a label somewhere around the fan shroud. Most likely it is gone. Add about a pound of refrigerant 12 and let the system stabilize (5-10 minutes). The metal hose at the inlet of the evaporator should be cool to the touch and the outlet should be warm. Be sure to check where the metal pipes enter/exit the evaporator. Add refrigerant a little at a time (allow system to stabilize before adding more) until the inlet and outlet are the same temperature using your "calibrated fingertips". Then add about 1/4 to 1/2 pound more. Remove jumper.

IF you are using R-134a, get an orifice tube made for R-134a. Follow the same procedures as above. However, you will find when the vehicle is idling the R-134a doesn't have the same cooling capacity as R-12. When you are driving and there is adequate air flow over the condenser and R-134a will perform OK. To solve the problem you would have to change the compressor with one that has a higher displacement, + other mods.
Good luck.

Sauce06

Thanks phil. Im just a mild DIY'r so it sounds like i need to take it to a shop. Thanks for all the help!!

Sauce06

Ok, found out compressor was bad. Replaced it, orifice tube and dryer. Got the a/c cold. NEW PROBLEM, when it doesnt blow as much air as it seems it should. Found that the a/c is on, when turned to MAX a/c there is no change. So basically i only have regular a/c without the MAX a/c option. Could it be the blower motor, resistor, or a vaccum leak somewhere?

Phil63146

I'm out of town until the middle of next week. I'll contact you then.

Phil63146

Just sent you a PM.

Sauce06

Ok, it was a combination of a leaky vaccum line and the fresh air door actuator being bad.
Now I'm wondering if these trucks had a heater control valve installed on the heater core lines?? The previous owner installed a plain water shut off valve in the inlet hose. I thought iv read something about a mod for this. Anyone have any ideas?

Phil63146

I could find no mention of a heater control valve in the service manual, nor find a replacement heater control valve listed in any of the big box parts stores web sites.

Heater valves are absent in many vehicles (if not most) because 1) they cost $ and 2) when closed, sediment and other crud in the water tends to settle in the heater core and plug it up.

Adding a shut off valve will definitely keep the cabin cooler in the summer, especially if there is no A/C.

Keeping the valve in the system is your call.