I don't mean not blowing cold, I mean seemingly MIA. It was in the hi70s-low80s yesterday so I pushed power on the HVAC and dropped the temp to Lo. It blasted hot air in my face. Check to ensure Econ wasn't pressed, it wasn't, so I popped the hood and sure enough the compressor clutch wasn't engaging. The aux fan wasn't spinning either. I've checked out a bunch of things but haven't been able to nail it down.

1) Fuses and Relays verified good by testing fuses and swapping relays with known good siblings.
2) Verified Hi/Lo pressure switch is closed with a meter, jumped it just to try. Assuming this rules out low charge (AC worked fine last year).
3) Ran the ATC self-test, came up with 00.
4) Confirmed ground on the Black wire at the compressor connector, NOT getting 12V on the Blk/Gn wire with the AC "on". Haven't checked for voltage at the aux fan but I'm pretty sure I know what I'll find. The fan does spin freely however.

Today is only 73° so I'm kind of on the ragged edge of it being hot enough to come on, but it should have worked yesterday. Anything else I should be testing?

 

You are on the right path. You can jumper the compressor relay and get it to come on to see if the clutch works, or maybe check the compressor clutch coil resistance. You said the pressure switch is closed, did you check for voltage at the relay trigger terminal? I would put a set of gauges on it and check the pressure, Oreilly has a set you can rent.

 

Thanks for the reply, Extinct! I replaced the clutch/coil last year so I have faith they're good. I did check compressor relay voltage, it has 12V at Pin 5 with the key off, and and also at Pin 2 once the Main Relay is energized, so it's just waiting for the BCU to supply ground. I'll jump 12V to what I think should be Pin 3 and see if the compressor coil pulls in, it doesn't help that the Rave goes by the old fashioned 30 and 85. I have a set of gauges and can put them on tomorrow but jumping the pressure switch is a known good way to eliminate a low (or high) charge situation from preventing the compressor from starting. The part that has me most confused is that the condenser fan isn't coming on either. That circuit is independent of the refrigerant charge circuit. According to the Rave it should be coming on when the BCU tells it to. This leads me to believe there's a bigger system-wide issue, although apart from a screwy BCU I can't put my finger on what it could be. I'll try energizing the condenser fan at circuit tomorrow too and see what happens. I swapped in a different ATC panel tonight, ...and no surprise, no change, but I have piece of mind.

Edit: Well, re-reading one of my own threads I've re-discovered that the aux fan doesn't come on unless ambient is over 78° or the engine is over 212°. So there's that. Still, I wasn't getting 12V to the compressor coil connector so I guess I'll start by applying 12V manually to see if the compressor kicks in, or if maybe there's an issue with the wiring from the fusebox to the compressor.

 

New **** has come to light, man. The compressor clutch is not pulling in when I jump Pin 5 to Pin 3 on the compressor relay. 12V is observed at the connector but no click. However, when turning on the AC 12V is NOT observed at the connector so there are two problems occurring. The condenser fan does run when jumping the relay so for now I will assume that noting is wrong with that circuit.

Problem 1: As for the clutch, I replaced it about a year ago and all was fine as far as cold air. This winter the bearing was squealing a bit so I removed the pulley to replace the bearing. Not wanting to do this job over and over I ordered it from a bearing shop instead of Amazon. While waiting for the bearing I used a pulley from different compressor just so I could put the serp belt back on. I was careful not to use the HVAC at all for those 3 or 4 days, I drove the truck very little (if at all) and the HVAC stayed off. I put the new bearing in the original pulley and put it all back together. I didn't pay attention to the amount of shims I used, but when spinning the clutch by hand the friction feels the same as one of my other trucks so I assumed all was good. Could using the wrong number of shims cause the clutch not to pull in?

Problem 2: I'll put gauges on the truck and see what the pressures are just in case the pressure switch being closed isn't enough for the ECU to send ground to the compressor relay.

 

Yes, the shims could cause a problem, but you should still hear the relay click. Just jumper the relay terminal at the relay and it should pull in. Or ohm the coil and see if it is open or shorted.

 

I ohm’d the coil and got 3.9 ohms. No clue if that’s in spec, but it was better than open. Yes, jumping the relay does NOT cause the clutch to click. Nada, silent, nothing. I did it on another truck and it clicks so I know what I’m looking for. Left town for a few days but hope to dig back in this weekend. Thanks for the suggestions and dialog!

 

4 ohms at 12 volts gives about 3 amps. That sounds reasonable to me for a big coil, but no click is no bueno. Maybe the mechanical parts are worn or stuck.