A/C Compressor Not Engaging After Recharge

[GraniteCow]

I disconnected the A/C lines on my 1996 4.0L XJ and replaced the dryer while I did the heater core and had the system evacuated/recharged at a local shop. I picked it up about an hour after they charged it, drove home (10 minutes), popped the hood once I was out of the snow, and saw that the compressor doesn’t kick on. The caps on the charge ports are also missing :/

I checked the relay and 2 fuses (engine bay and cab), they’re fine. Compressor turns freely by hand. When I unplug the A/C relay and plug it back in, the clutch (sometimes) kicks on for a split second and turns back off. It’s acting like it has no pressure, but it was just at a shop for a full recharge. I called the shop and they said they do a vacuum test before filling to check for leaks. They also said they tested it afterwards to ensure it works though, so...

My thoughts are:
a) bad pressure switch
b) a fuse/circuit somewhere I don’t know about
c) shop had a new guy work on it and me messed up

How often do these switches go bad? Are they safe to replace while the system is charged? Is there another circuit to check? I’m scheduled to bring it back in Monday morning, but I’d like to figure this out since I might not be getting a straight story from the shop. A new switch is $40 at O’Reilly, I don’t have any gauges to measure pressure.

 

[GraniteCow]

I disconnected the A/C lines on my 1996 4.0L XJ and replaced the dryer while I did the heater core and had the system evacuated/recharged at a local shop. I picked it up about an hour after they charged it, drove home (10 minutes), popped the hood once I was out of the snow, and saw that the compressor doesn’t kick on. The caps on the charge ports are also missing :/

I checked the relay and 2 fuses (engine bay and cab), they’re fine. Compressor turns freely by hand. When I unplug the A/C relay and plug it back in, the clutch (sometimes) kicks on for a split second and turns back off. It’s acting like it has no pressure, but it was just at a shop for a full recharge. I called the shop and they said they do a vacuum test before filling to check for leaks. They also said they tested it afterwards to ensure it works though, so...

My thoughts are:
a) bad pressure switch
b) a fuse/circuit somewhere I don’t know about
c) shop had a new guy work on it and he messed up

How often do these switches go bad? Are they safe to replace while the system is charged? Is there another circuit to check? I’m scheduled to bring it back in Monday morning, but I’d like to figure this out since I might not be getting a straight story from the shop. A new switch is $40 at O’Reilly, I don’t have any gauges to measure pressure.

 

[Agreen]

To test:
Unplug the compressor and start the engine, turn the AC on. Use a voltmeter to see if you're getting 12v to the clutch coil.

If you are (and I suspect you are) carefully plug it back in and tap the clutch with the handle of a hammer. If the compressor picks up, your clutch coil is going out. It's generally best to replace the whole compressor if that's the case.

 

[GraniteCow]

To test:
Unplug the compressor and start the engine, turn the AC on. Use a voltmeter to see if you're getting 12v to the clutch coil.

If you are (and I suspect you are) carefully plug it back in and tap the clutch with the handle of a hammer. If the compressor picks up, your clutch coil is going out. It's generally best to replace the whole compressor if that's the case.

There is 0V at the wire. If I jumper the compressor to pos, the clutch works fine without hiccups. It’s not getting shy power, but the circuit appears intact. All I can think of at this point are a) low pressure or b) bad pressure switch. The shop obviously didn’t check very well to see if it’s working if the compressor doesn’t turn on, and whoever worked on it forgot the caps for the fill valves. I’m trying to give them the benefit of the doubt, but it’s hard to atm.

Is the pressure switch s variable resistor? Any idea what the resistance should be at full charge but not running?

 

[Agreen]

It's a switch. It comes on above a set pressure, probably somewhere close to 15-20 psi. You can jump the low pressure switch to see if it comes on, but that would mean it's either a) a bad switch or b) low pressure. If it's low refrigerant pressure, you obviously don't want to run it long like that.

 

[Agreen]

Forgot to add:

To check to see if the shop added enough refrigerant, get one of these simple gauges.

https://www.amazon.com/DASBET-Refri...ocphy=9010718&hvtargid=pla-955612689323&psc=1

It's just the low pressure side gauge, but it's sufficient. A quick rule of thumb for R134 is that the low pressure side should be pretty much the same pressure as the temperature of the evaporator (so probably in the low 30's right now). High pressure should be about 100# above ambient temperature (add some for humidity). You're only concerned with the low pressure for now though.

 

[techno1154]

Remember, if it is very cold outside, the AC may not run, i.e. the compressor may not engage. Try running in the defrost mode. It should cycle on and off then.

 

[lawsoncl]

The shop should have pulled a vacuum on it and let it sit for awhile. Sounds like they didn't if you got it back in an hour, and you have a leak somewhere. They also should have run it to see if the pressures looked good and it was getting cold.

 

[GraniteCow]

This is the receipt. I asked about the “recharge with reclaimed r134a” part, and the guy sounded like nervous nelly while he gave me some explanation of how that’s legal mumbo jumbo so the epa doesn’t sue him. Is that normal wording? I told them the lines had been disconnected for a few days and the dryer replaced.

 

[GraniteCow]

Picture of receipt since it won’t upload: https://imgur.com/a/4Q2bsmv

 

[Messy Jesse]

How are you checking the relay? I've had the relay go bad 2x on my 96.

 

[GraniteCow]

How are you checking the relay? I've had the relay go bad 2x on my 96.

I swapped it with another relay that works

 

[GraniteCow]

Update: the shop looked at it, said there was a “glitch” on their end, put more charge in it, and now it’s working.

The funny thing is, I had swapped in a cheap autozone sensor, got it to turn on but not blow cold, then put the old one back in worried the cheap one was garbage and I’d burn up the compressor. Looks like I was right and it was a low charge!