Hey guys I've got a 99 f150 with a 4.6 I cannot get the ac compressor to come on. I've checked all the relays they all seem good. I have no power at the plug for the compressor or the plugs for the high and low pressure switches. I believe the switch in the cab for turning on the ac works too I can hear it pull a vacuum when I go to max ac. I'm stumped. Any help is greatly appreciated.

 

With the key on engine off and AC controls on max AC you should have power on one side of the pressure cut out switch located on the high side line. If no power there check for power on both sides of the fuse.

 

"Believing" is not a valid diagnostic test. Go get your meter or test light out and TEST it. The vacuum switching is in a separate part of the switch from the electrical function.

 

Haha fair enough. There's so many connections back there I'm not sure which would be the wire that would trigger the ac. I'm guessing one of the middle wires on the red plug? Anyway I'll try rechecking fuses and the wiring behind the climate control tonight after work. Thx

 

Ok I went and picked up a light tester and the fuse in the cab #5 has power on both sides. 1 side of the high side switch has power as does 1 side of the low side switch (when the key is on engine off on max ac setting). I won't be able to test the wires behind the ac controls until tomorrow night. I worked late and take a welding class Wednesday nights.

 

With the cycling ( low side ) switch plugged in you should have power on both sides. With the conditions explained above and the cycling switch unplugged place a jumper wire in place of the switch and the compressor should come on. If so the system is either low or that switch is bad.

 

I put a paper clip between the two connections in the low side switch. It doesn't kick on the compressor.

 

So if I jump the low side switch connection and it sends no power to the plug at the compressor what would be next on the list to check?

 

Then check for power at the compressor with that jumper in using your test light. The coil in the clutch may be bad

 

Actually you should try the jumper with the engine running and listen for the compressor. If it doesn't come on then check it with a test light at the compressor with the jumper in place.

 

How it works:

The clutch is controlled by the PCM on a 99. If the series circuit of the mode select switch, the HPCO switch, and the low pressure cycling switch is all complete, a 12 volt signal is input into the PCM indicating that AC operation is requested.. If the rest of the PCM's programming then permits it (no faults that would disable the function, engine at proper operating speed), the PCM will apply a ground to the coil of the ac clutch (WOT) relay. The relay will be energized, applying power from fuse F5 via the relay to the coil.

Condensed testing instructions

Testing must be done when the temp is above 50F, otherwise, compressor operation cannot be expected.

1. Verify there are no PCM DTCs that would indicate a fault that would disable the AC function. If your MIL is on, FIX THAT FIRST!
2. You've now already checked the series switch circuit up to the last accessible test point (cycling switch).
3. You've verified that the power to the relay's load terminal is available by checking at fuse F5.
4. (Advanced) Using a scan tool look at the ACCS PID and see if the signal is recognized.
5. (Advanced) Using a scan tool, verify the PCM has set the relay to on (WAC).
6. (Shortcut) Check and see if the AC clutch relay "clicks" when the system is turned on or not. Swap it with a similar, non-critical relay, if possible and check operation. Or, remove the relay, identify the pins in the socket for each function (the relay's body should be marked, match up the pins with the socket) using your test light or meter connected between the battery POSITIVE terminal and pin 5 of the socket, verify that the pin goes to ground potential when the clutch is supposed to be energized.
7. Verify the the circuit from relay pin 5 through the coil as follows: All power off. Check resistance from pin 5 to chassis ground, should be around 3-6 ohms (you're measuring through the coil to ground). If you measure an open, either the wiring or the clutch coil is open. Test each circuit segment individually. The coil usually measures a few (3-5) ohms.

 

ok so there are no dtc codes on my edge programmer. As far as I can tell the relay does not click when the ac is turned on. I tried another relay as well. With the light tester lead clamped to the battery positive it lights up in slot #5 where the ac relay would plug into. As far as testing the resistance in ohms I'm not that great with all this. How would I test to find the ohm resistance?

 

With an ohmmeter, a function of a multimeter.

If the relay did not energize (verify condition!), checking the resistances is a moot point (assuming your reported observation is correct).

 

Yeah I tested for continuity through the relay and it works. I also purchased a replacement relay and it didn't help. I had the multimeter set on ohms and I was testing the different pins where the relay goes and got different readings. I think it's just gonna go to a shop and hopefully they fix it. It's about to be hot in Arizona and luckily this is a 3rd vehicle for me.
Thx for the advice but I think it's a little more complicated than jumping a pressure switch that seems to solve everyone else's ac problem. Lol

 

I had someone look at the truck and he was able to get the ac compressor to work by using a tool that supplied direct power to the center prong where the ac clutch relay goes under the hood. His best guess was it was the switch on the dash. I replaced the ac switch (far right switch) with a new one from ford but no dice. When the relay got power to that pin the ac was blowing ice cold. I don't want to have to ghetto rig it to force the ac on but I live in AZ and no ac is not really an option out here. Hopefully someone can point me in the right direction. Thx