I went to drive my truck yesterday and no longer had cold blowing AC, which is just awesome when you live in Phoenix! The fans come on inside, but it just blows hot air. I'm pretty sure it was working fine Sunday, but I only drove it short distances up the street and may not have used it. It was definitely working and blowing cold Saturday, but when I went to drive it yesterday, no cold air at all.

The truck is a 02 Ranger 4.0L, here's my troubleshooting from last night:
-Popped the hood and found the clutch isn't engaging on the compressor, but I can still turn the compressor clutch by hand.
-No popped fuses (panel on left side of dash), and the relay (box under the hood, drivers side near windshield) seems to be okay. I swapped it with another, didn't fix the problem.

So with the car running and AC on max...
-I hooked up a refrigerant gauge to the low pressure side and it read ~140psi. I read this high of a reading is normal if the compressor is not running and shows there's enough pressure to active the switch and kick on the compressor.
-I put a jumper wire across the lower pressure plug, but the compressor still didn't kick on
-High pressure sensor switch is reading ~13.5v
-Clutch wires on the compressor read no voltage (even with low pressure switch jumpered)

I was starting to think the clutch coil on the compressor failed, but if that was the case, shouldn't I still be seeing voltage across the plug for the clutch wires? I feel like the lack of voltage here is my problem, and the signal to engage the clutch just isn't being sent. but I tried jumping the plug on the low pressure side (where the signal comes from, right?) and the clutch on the compressor still didn't engage. Any ideas? When I get home today, I'll start looking more at the electrical side (ugh). Just wanted to bounce some ideas off you guys in the mean time.

Thanks for any help guys!

 

If the A/C clutch relay is working then I would check out the A/C pressure cycling switch. They can go bad and are not expensive. It should be on same line that connects to the accumulator.

 

Isn't the "A/C pressure cycling switch" the same as the low pressure switch? It's what sends the signal to tell the clutch on the compressor to engage, correct? If it was bad, shouldn't the clutch still have engaged on the compressor when I jumpered the two wires at the plug?

 

Update:

I hit the compressor coil with 12v from the battery and the clutch engaged fine, but jumpering across the LPS connector still didn't engage it.

I hooked up the gauge and rechecked the low side pressure with the coil still jumpered with 12v so the compressor was running, and the gauge read about 20psi. According to the gauge, with an outside temp of 95* (@12:30am, got to love phoenix!) it should be in the 50-55psi range. I adding three 12oz bottles, and it only got up to about 45psi. I let it run for about 10 minutes and it at least seemed to hold at 45 psi. Figuring 45psi was still enough to engage the clutch, I removed the jumper wires and reconnected everything but still the clutch would not engage the compressor. So far, only when it's jumped with 12v directly from the battery will it engage.

Tomorrow I guess I'll replace both the pressure cut-off switch and cycling switch just to rule them out since they're not that expensive.

 

I tried jumpering the cut-off switch and the compressor kicked on. So I replaced it and everything seems to be working fine now. A failed cut-off switch (high side) explains why jumpering the pressure cut-off switch (low side) didn't kick the compressor on. I didn't realize they were in series.

Smarter every day. Hopefully this will save someone time in the future.