Diagnosing AC Issues

[Salvagedcircuit]

My Mj did not have functioning AC on purchase. The AC compressor wires were spliced 5 times which I replaced. I believe it's wired correctly now, but not 100% sure. There is no inline diode as shown in the electrical diagram in the 88 electrical manual. I spent time this weekend troubleshooting and it turned out my ac compressor clutch coil was completely shorted. I have replaced the ac compressor clutch and coil and recharged the low side of the system with a walmart ez-chill recharge kit. Surprisingly, the small gauge on the bottle shows the system is holding pressure, which is wild considering the front condenser looks very old and toasty.
Unfortunately, the A/C compressor clutch never engages and the engine rpm never falls indicating there's a load from the A/C compressor. I tested the ac clutch separated from the comanche with a 12v variable power supply. The new clutch engages fine.

I have an REM Renix module. It states "off A/C" which means the clutch signal is not active and the ECU is not allowing the A/C Compressor to activate. I have "max ac" on and the dial all the way to cold. It looks like there is an additional "REQ" setting for A/C diagnosis that I did not check yet.

 

Tested:

-A/C compressor clutch shorted (checked with multimeter)

-no 12v signal to the compressor when max AC on (checked with multimeter)

-A/c compressor never comes on, even after recharge and new clutch.

-new A/C compressor clutch freewheels smoothly

What I did:
-new AC clutch. spacing within service manual tolerance.
-new AC compressor wiring
-recharged system

Possible Culprit:
-Failed Pressure switch
-I accidentally removed the air temp sensor from the driver side engine bay. I think A/C system needs this?
-ac expansion valve?
-compressor has failed?

 

Let me know if you guys have any input. I'm very new to A/C systems.

Thanks!

 

[eaglescout526]

There should be a relay as well that could have failed. I once had an intermittent relay in my system that would go on like normal and fail when really hot. 

[Salvagedcircuit]

2 hours ago, eaglescout526 said:

There should be a relay as well that could have failed. I once had an intermittent relay in my system that would go on like normal and fail when really hot. 

Yeah, I'm going to replace the relays. When tested externally, one of them seems to be slightly slower than the others, but they seem fine otherwise. They had a good run. Stamped 1987, made in England. Quality Checked by the Queen herself

[gogmorgo]

If you jump the pressure switch and the clutch kicks on, that tells you everything else in the system is working but either the system pressure is too low or the switch has failed. Or I guess just unplug it if it’s a normally open switch, I don’t remember off the top of my head which way it goes. 
 

You can’t really get a good low-side pressure reading if the compressor isn’t running. But assuming everything else is working if you have a “good enough” charge with the compressor off it should run the compressor for a few seconds, pull a vacuum on the low side, and kick the compressor back off until the pressure bleeds back around and keep cycling like that. 

 

If the service valve is all the way open it blocks off the Schroeder valve and you won’t be filling anything more than just the fill port. That square bit here:

Should be a 1/4” square, you might get it with the open end of a wrench, or a square socket, or like I did a couple weeks ago, a screwdriver bit holder for a 1/4” ratchet with an Allen bit and a matching 1/4” drive hex socket sitting backward on it in some hectic docking action 👉👈. Give it a turn or two in the clockwise direction to start filling the system. Leaky Schroeder valves are VERY common and while you’re not “supposed” to backseat the valve in operation, not doing it almost guarantees you’ll lose your charge in short order (even if you change the Schroeder valve every time you touch it like you’re “supposed” to do) , so you’ll typically need to crack the valve to fill the system. Make sure you spin it back closed (counter-clockwise) before pulling your can off the Schroeder valve. 

 

[Salvagedcircuit]

5 hours ago, gogmorgo said:

If you jump the pressure switch and the clutch kicks on, that tells you everything else in the system is working but either the system pressure is too low or the switch has failed. Or I guess just unplug it if it’s a normally open switch, I don’t remember off the top of my head which way it goes. 
 

You can’t really get a good low-side pressure reading if the compressor isn’t running. But assuming everything else is working if you have a “good enough” charge with the compressor off it should run the compressor for a few seconds, pull a vacuum on the low side, and kick the compressor back off until the pressure bleeds back around and keep cycling like that. 

 

If the service valve is all the way open it blocks off the Schroeder valve and you won’t be filling anything more than just the fill port. That square bit here:

Should be a 1/4” square, you might get it with the open end of a wrench, or a square socket, or like I did a couple weeks ago, a screwdriver bit holder for a 1/4” ratchet with an Allen bit and a matching 1/4” drive hex socket sitting backward on it in some hectic docking action 👉👈. Give it a turn or two in the clockwise direction to start filling the system. Leaky Schroeder valves are VERY common and while you’re not “supposed” to backseat the valve in operation, not doing it almost guarantees you’ll lose your charge in short order (even if you change the Schroeder valve every time you touch it like you’re “supposed” to do) , so you’ll typically need to crack the valve to fill the system. Make sure you spin it back closed (counter-clockwise) before pulling your can off the Schroeder valve. 

 

 

From the REM manual, it states

 

Quote

REQ = Air Conditioning Request Signal. Indicates if the A/C Selector is on an AC mode, if the
Evaporator Temperature is at an acceptable range for A/C use, and if the Low-Pressure switch
is closed. If any of these are open then there will be no request signal.
- (YES) All conditions met for A/C operations
- (NO) One or more conditions are not met for A/C operations

So I'd have to short the pressure switch worst case if nothing else works after all my tests. If I do this with the vehicle running and AC set on, it should kick on on the clutch and start the compressor temporarily. If it does not, the switch can be toast.

 

I think you're right on the money about the fill port. The gauge on the A/C fill can showed "filled" within 3 seconds of plugging it in and pressing the trigger. Refilling A/c is not that quick. I'll try opening the valve first then filling it again. If it fills, then I'll try the A/C again. If it does not blow any cold air, I'll try jumping the pressure switch. I have a spare pressure switch and some O-rings on order.

 

The A/C parts I ordered:

- Four Seasons 38901 Expansion Valve

- Four Seasons 35758 Low Cut-Out Pressure Switch

 

Thanks

[eaglescout526]

13 minutes ago, Salvagedcircuit said:

So I'd have to short the pressure switch worst case if nothing else works after all my tests. If I do this with the vehicle running and AC set on, it should kick on on the clutch and start the compressor temporarily. If it does not, the switch can be toast.

Yes. It should jump the system regardless. If it does not, something else is preventing the system from working but it sounds like you have everything else but the pressure switch figured.

[Salvagedcircuit]

Does anyone have a photo of how their AC clutch it wired?

Previous owner clipped off the entire area. I have no ac clutch diode or connector. The clutch will not engage with the AC

 

So far:

-ac system officially charged and held when I opened the valve.

-A/C fan kicked in

-A/C req signal shows on the REM module

 

I have tested my clutch externally with a 12v psu and the clutch does indeed work. for some reason it's not being tripped.

 

If anyone has a photo that would be awesome.

[eaglescout526]

I dont have a 4.0 L so I can not help much, and mine is the dealer add on as well. But how did you get the clutch off?

[Ωhm]

Doesn't REM have two AC readings, both Request and Select? ECU needs both for AC Clutch Control.

[Salvagedcircuit]

Looking through the REM manual it shows A/C request and A/C on/off.

Mine shows A/C request but the clutch never engages and obviously no cold air :/

[Ωhm]

I thought REM showed both readings. Manual does say no REQ without SEL. Could be reason for only one display reading. ECU still needs to see both signals.

 

Try this:

Remove the AC Clutch Relay. Jumper (fused) 12vdc to D2_6. Both AC Clutch and Cooling Fan should energize. Use CAUTION.

 

 

[Salvagedcircuit]

Edit: the AC clutch was indeed wired correct. I manually checked the wiring again.

The fun part is going to be finding a diode locally that I can put in parallel to the A/C clutch.

 

Here's a diagram that shows the inline diode for the ac clutch:

https://www.cherokeeforum.com/f2/ac-help-i-am-electrically-beyond-stupid-229318/

[gogmorgo]

A standard relay is just a switch between terminals 30 and 87 that closes when you put power through pins 85 and 86. Polarity going through it generally doesn’t matter on either load or switch circuits.
But if having the load on the “wrong” side of the relay means the rest of the relay is also wired incorrectly, it’s likely enough the switch and load circuits have been crossed around as well I suppose.

[Ωhm]

12 hours ago, Salvagedcircuit said:

The orange wire is connected to "30" on the A/C relay. This is incorrect.

 

 Pin30(COM), I believe is correct. Pin87(NO) should show as LT GRN and Pin87a(NC) as BLK.

[Salvagedcircuit]

Update 6/16 11:45pm:

-Relay works

-A/C relay wiring is correct, harness is not damaged other than cur out diode

-Diode added from failed ionization fire alarm.

-AC compressor has a rebuild tag on it with a "converted to 134a" in ~2003.

-My low pressure valve was closed after I filled it on friday. I just added more refrigerant and the fan kicked on and the clutch engaged properly, but sparked near the clutch, so I shut the AC off. I believe I need some more spacing on the clutch plate. I measured it within service manual tolerance, but it's a complicated area to measure with a feeler gauge since there is a shoulder of a pulley involved. Going to increase spacing and try again.

 

Previous tested:

-A/C fan kicked in

-A/C req signal shows on the REM module

-A/C low pressure switch works

 

Backup plan: Bring it to a service place to look for leak sources or use my Flir meter.

Worst case:  Replace the condenser, AC lines, Expansion Valve, O-rings, fill system. Don't plan to replace the evaporator core and rip apart the entire dash.

 

Thanks!

[eaglescout526]

Could try to add the oil you can see with a UV light and see where your leak is if you can force the compressor to run for a little bit to cycle the fluid through. Or have it vac tested one of the two.