"No power at compressor" troubleshooting guide

[scrivyscriv]

I’ve seen quite a few “no power at compressor” threads and thought a definitive troubleshooting & analysis thread would help a few folks out. This writeup is being done on a 1993 Explorer 2WD Sport with factory air and a 134a conversion, but may be applicable to 2nd gen as well.

Tools required:
Low-side pressure gauge
Volt-Ohm meter; digital preferred but analog will work
Circuit tester
Jumper wire
Feeler gauge set

Time to complete all steps:
30 to 60 minutes

First of all, if you’re not getting cold air, check the air conditioner circuit fuse. It’s in the driver’s side interior, just next to the parking brake pedal. I don’t have the amps rating or exact fuse location handy but if all of your fuses are not blown or melted inside and are the correct amperage size, you can move on. The fuse panel cover has amperage size/location molded into itself but no labels of what system each fuse is for.

Second, make sure you have refrigerant in your system. If it’s R12 original freon, you most likely won’t be able to find gauges anywhere to check it and will need to go to a shop – which may or may not have R12 equipment. If it’s been retrofitted to 134a, you can pick up a low side gauge & filler hose from Wal*Mart for under $20. Check your low side and make sure it’s 25 – 35ish PSI (in the green). The low side port is under the hood on the passenger side, adjacent to the air filter and electrical relay boxes, on the filter-drier. If you have the correct amount of refrigerant (freon), you can move to the next step.

Third, check to see if your compressor is engaging. This is where this thread picks up – most threads leave off full troubleshooting.

The clutch is a three part assembly on the face of the compressor.



Part #1 is called the field coil and is affixed to the compressor – It does not move. This is the part that has your electrical connector on it. Part #2 is the pulley and bearing assembly. The serpentine belt rides on this, and the pulley freewheels on the compressor shaft when the clutch is not energized. Part #3 is the clutch face. It comes off without removing the serpentine belt. It’s ferrous metal with a friction surface that mates with the friction surface of part #2.



When Part #1 gets 12 volts DC, it instantly becomes magnetic (it’s an electromagnet). Part #3 is attracted to Part #1 magnetically when 12 volts is present. Via friction surfaces, it connects the compressor shaft to part #2 which immediately begins to rotate the compressor shaft. When voltage stops flowing to the field coil, the clutch face is no longer attracted to the field coil, and moves back to its original position, thereby releasing the pulley assembly from contact with the compressor shaft.

The clutch face needs to be a certain distance from the pulley assembly - .025" or more is too much. Check this with a feeler gauge. Washer kits are available from parts stores to shim it to the correct distance.

Turn the key to Run but leave the engine off. Check the harness at the compressor with a multimeter (Wal*mart - $15) to see if it’s getting 12 volts. If it’s getting anything less than 12, like 1 or 2 or less, you’re not getting any power at all. The chance of a significant voltage drop in the harness is almost nonexistent; so if no voltage present, move on.

Move to the filter-drier. It has a hi/low pressure switch on it.




The switch is normally open at 25 PSI or less, and normally closed at 45 PSI or so. Unplug the connector from it and short the terminals out with a wire. Paperclips work but you will blow a fuse if a paperclip grounds out while you’re testing. With the terminals shorted, re-check your compressor harness end for 12 volts. If you have voltage now, your pressure switch is bad, or the system is not properly pressurized. If you still get no voltage at the compressor, proceed to the next step.

Next, check the Air Conditioner Wide Open Throttle Cutoff (A/C WOT) relay. It’s in the relay box by the battery and air filter, under the hood.



If you’re having ignition module or engine computer problems, this relay could be receiving an errant signal and holding the circuit open. I gather that this relay opens the compressor power circuit when the computer detects a Wide-Open Throttle (hard acceleration!), and if your Powertrain/ Engine Control Module (P/ECM) or Ignition Control Module (ICM) are faulting, this relay could be staying open. Or the relay might just be bad and need replacing. If the relay is hot, it’s probably getting an errant signal and you should check your EEC-IV codes (excellent threads all over the forum) to see what’s going on. You can pick up a replacement relay from - for example - O’Reilly, for about ten bucks. If you are still not getting voltage at the compressor, move on to next step.

Next, check the compressor harness. It’s not a big deal or complicated to get to.



Unplug the field coil connector at the compressor. Unplug the switch connector at the filter-drier. Unplug the round, four-pin harness master connector underneath the filter-drier. The harness is clipped to the engine in three or four places with plastic clips. Pull these out. Pull the entire harness, conduit and all, out. Remove the plastic conduit and check the wires and connectors for damage. You’ll see a black heatshrink sleeve towards the compressor end – it’s marked with a diode symbol, and has a diode inline there.



Check your wires for continuity with a multimeter. If you have a diode check function on your multimeter, check the diode. I don’t have specs on the diode but I’m pretty sure you can pick one up from Radio Shack for a few dollars if it’s bad. If your harness is bad you can rebuild it or go to the dealership and get one.



The only item left is the relay-to-compressor-harness harness, feeding from the WOT A/C relay. It's most likely not bad, but worth checking out as a last resort under the hood, short of tracing the relay hot wire back through the firewall. You should have power to your compressor now if you’ve gone through all these steps. There is some troubleshooting involving the A/C on/off control dial in the dash but I haven’t dealt any with that, and there are some other threads here concerning it.

If you’re still struggling with it, post a reply with details and I’ll see what I can do to help. Amplifications & corrections also cheerfully accepted