My ac is not working on my 2000 mustang 3.8L. I'm getting power from the compressor and I had the clutch replaced. We jumped the low pressure switches and then replaced the fuses. The clutch will still not engage. Is there a relay somewhere other than the junction box? Maybe on the firewall?

Could have a bad low pressure cutout switch, if you can jump across it , I would replace it. Also if you have access to a scanner , you might want to make sure you are getting "compressor on" command from computer

Looks like the low pressure switch gets power from a 10amp ac fuse. Power then goes to the ac relay which is powered on by the ac heat controller on the dash. Power then goes thru the high pressure switch to the compressor. The ac relay is grounded to the body. Do you have power on the lite and dark green wires going into the relay and is it grounded ?

it's either low (in spite of your readings) or your a/c clutch is not working correctly or you have too big an air gap in the clutch.
First, just disconnect the 2 pin connector from the a/c pressure cycling swithc and jumper the 2 pins with a wire to force the clutch to stay engaged. See if it blows cold then, and stays engaged. You can run this way for about 15 minutes before you have to pull the plug.
report reulsts.

please don't forget to rate.

Put a jump wire in the high pressure switch plug. If the compressor works. replace the high pressure switch. Let me know and we can go from there.

"Most Fords have a CCRM (Continuous Control Relay Module) thats on the passenger side fenderwell. It has 4 relays: one for the AC compressor cluth, one for the fuel pump, and two for the electric fan (high- and low-speed.) The AC is the one that fails the most. You can replace the CCRM by removing the intake air filter box and working through the fenderwell openings. If it gets to difficult you must remove the wheel and splash cover to replace it from the wheel opening." - H-andy Mann

with the engine idling, tap on the ccrm and listen for the clutch to come on. if it comes on, you need to replace the entire ccrm.

This could be a few things. The freon could be low, one of the switches (high or low pressure) could be bad, or you could have a bad compressor clutch. You can test the clutch by checking for battery voltage and ground at the two pin connector with the A/C on if you have both power and ground, then the clutch is bad. You really should start by hooking up pressure gauges to the A/C ports to check the pressures, and these vary depending on ambient temperature and whether the A/C compressor is engaged or not. I am willing to bet that the freon is low though and may be just as simple as recharging the system.
Hope this helps. Good luck!

If freon is low, AC will not engage. There's a low pressure safety switch incorporated in compressor so it doesn't burn up under low pressure condition. Have system evacuated, refill w/oil & freon

FIRST I CHECK ALL FUSES AND RELAY.TO BLOWER MOTOR IF IT STOP BLOWING.AND CHECK FUSE TO HVAC SWITCH CONTROL.AND CHECK FUSE / RELAY TO AC COMPRESSOR.IF FUSE / RELAY GOOD POWER GETTING TO AC COMPRESOR CLUTCH COIL AND LOW PRESSURE SIDE 28 PSI AND HIGH SIDE 200 PSI. CLUTCH COIL IS OPEN.IF REFRIGERANT LEVEL LOW YOU MAY NEED MORE REFRIGERANT.YOU NEED AT LEAST 28 PSI ON LOW SIDE AND 200 - 220 PSI ON HIGH SIDE.IF ALL IS GOOD STILL NO AC PROBLEM COULD BE A FAULTY AC CLUTCH CYCLING PRESSURE SWITCH.

The power to run the compressor clutch coil goes from the battery, to the AC Clutch Relay, somewere in the engine compartment fuse box, then it goes to the positive side of the AC compressor clutch coil; goes thru the coil, activating the compressor, and then goes to complete the circuit through another switch (high pressure protection switch) in the back of the AC compressor and finally reaching negative or Ground.

Disconnect the AC compressor clutch connector and using a 12 V bulb with 2 wires (test lamp) check the following:

1.- connect 1 lamp wire to battery neg; with the other check both pins at connector with the AC in the RUN (AC and ignition switch on ) you should have power in one side.

2.- connect 1 lamp wire to battery positive and with the other check both pins at connector, lamp should light getting the ground from the clutch connector thru the high pressure compressor switch to ground.

On the other side what makes the relay close and let the power flow to the clutch works with the low pressure switch, (if the freon pressure is too low it opens and kills the ground to the relay coil and that kills the compressor), that low pressure switch is in the low pressure side of the freon tubing, the fat pipe that comes from inside the firewall into the accumulator/dryer, a big aluminum cylinder sitting on the right side of the engine compartment in GM cars.

To know if the low press switch is the cause you can unplug it and connect both legs with a paper clip for a minute or two, no more, you don't want to damage the compressor. anyway if jumping the switch makes the compressor come alive and kick in, you either have low freon pressure or a bad switch; it's time for the pressure gauges and check how the compressor is pumping and that is better left to a pro.

By the way you said that your system is a R-12 system, have you considered switching to R-134a the new "ozone friendly" freon? to do that you need to evacuate the system with a recycling machine and pull vacuum in your AC, if it holds the vacuum for a while, then you can be sure that there are not leaks and then dump 1.5 or 2 pound (check the labels in the engine compartment) of R-134a into your system. it will be a good idea to put some fluorescent leak detecting dye in the system to check for leaks, just in case..

check the low psi switch and the high psi switch both will be on the lines you will have to look for them not sure exactly where low side may be on the accumulator try jumping them with a paper clip and see what happenes watch the pressures on the system because if pressures are to high or low you could damage the compressor or blow lines if either switch is bad it will cause computer to cut out the a/c clutch let me know how you make out