Update: FIXED. A/C compressor always has 12v at clutch

[AtlantaDan]

My 89 GW has a weird / annoying issue with the ac. The clutch is always engaged, even when the ac is off. If the key is on, the clutch is on. I've disconnected it at the pressure switch to get the clutch to disengage, but that's not a viable long-term solution, especially in Atlanta. I've checked darn near everything and looked at the wiring diagrams but I am completely stumped here. I found one relay under the dash that clicks when the ac control is flicked to "AC", so the relay appears to be working.

Big orange wire connects to brown and goes to one side of the pressure switch - that one always has 12v when the key is on. The other side of the pressure switch goes to the (York) clutch.

Disconnected the aforementioned ac relay, both fan harnesses at the HVAC controls, and the blower resistor and the compressor still gets 12v. Is there another relay in the system somewhere? Starting to wonder if the control panel itself is bad...

[tgreese]

Maybe someone will know something about the AC system that you have overlooked. Have you located the AC clutch on the diagrams? Which sheet is it?

What is the purpose of the pressure switch? If power goes to the pressure switch - as it should when the key is on - and always exits the other side to the clutch, it seems reasonable to either condemn the pressure switch, or assume that something in the system is keeping pressure on the switch and keeping it on all the time. I don't see how you can have pressure if the compressor does not run, so how can there ever be pressure at the switch?

It may help to read the TSM and understand about how the switch circuit works with the clutch.

[Srdayflyer]

the pressure switch is in the system to prevent burning up the compressor, when the freon drops below a certain pressure without the compressor running it prevents the compressor clutch from engaging and running the compressor, there are inly 2 wires at the clutch ground and power, a quick check is to jumper the 2 wires with a 12v source (battery) you should see the clutch lock up and hear a click when it does, also you can jump the plug at the low pressure switch with a paperclip and with the cabin switch selected to on the clutch should also lock up, if you dont get that you are not getting power at the cabin selector switch, the a/c power system isnt rocket science these vehicles are as basic as you can get.of course you can spend 90k for a new waggy with 14 computers, multiplex bussing systems and pay the dealer , i choose old school every time LOL

[AtlantaDan]

Thanks for the replies. The root problem I have is that the pressure switch always has 12v, which causes the clutch to engage whenever the key is on even if the ac system has been shut off. I found one relay and disconnected it to test - clutch still has 12v and engages.

[tgreese]

You wrote that you looked at the TSM wiring diagram. It's online and I would look at it, but it's a lot of sheets. Which sheet has the AC system?

[seventynine]

Why did you create another thread on this?
You stated again that it is a York compressor. Are you sure? ‘89 would have been using a Sanden compressor. York compressor systems don’t use pressure switches.

[Srdayflyer]

i have never seen an a/c system in ANY vehicle without a pressure switch somewhere in the system, its there as a safety mechanism to prevent burning up the damn compressor, its like all the new cars having the stop start feature on them where the engine shuts off at a traffic light you take your foot off the brake and it starts up again , just great for corporate cafe ratings bad for your pocket book, my neighbor just replace his starter in his 23 ram 1500 , 3400.00 cha ching $$$ dollars later he was back on the road, starter alone was 1400.00, he found and override plug in module for 57.00 thru a dodge blog , so again i really find it hard to believe there isnt a pressure switch somewhere in the system IMHO

[seventynine]

York compressors are oiled internally. They have their own oil reservoir. You can run them with no Freon. They don’t rely on the freon for lubrication. I have several set up as on board air compressors. Jeep used them up to 1988 in FSJs for air conditioning.

[seventynine]

httpsoljeep.comgw77_tsm18 1977 13A-Air Conditioning.pdf.png

[sonoraed]

86 and newer used a hybrid aftermarket system that incoperates thermistor in the evaporator core and potentiometer on temp lever,they send a signal to module usually bluebox, That cycles the clutch.

Nearly impossible to find but worth a try, alternative would be to add a thermostat in line electrically to the clutch and push the cap in the evap core,assuming the ac shuts down in heat mode.

This system is Rube Goldberg to the max ( the oem controls)and probably could design some thing better, good luck!

Ed Sunday


Sent from my iPad using Tapatalk

[seventynine]

After looking at some of the other TSM’s I stand corrected…sort of. It looks like Jeep started using a low pressure switch on their York A/C systems around 1983. I suppose it was to keep the compressor from running and loading the engine with out reason if the Freon was too low and not going to work anyway. I know there was never one on my ‘79’s or ‘77. You will burn up a Sanden compressor without freon in the system though.

My real question is why you would have a York compressor put in an ‘89 GW. They had gone to the Sanden compressor by then…which is a newer & a better A/C compressor. I’ve heard of people converting from York to Sanden…but not the other way. You have change mounting brackets etc etc.

[AtlantaDan]

@tgreese The page with the wiring diagram is 8W-326 in the 89 FSM, and it's what I am running off of. Unfortunately, being a Jeep, some stuff matches up and some does not. It's like a hodge-podge of different years. Example - my LP switch and clutch have orange wires, but the FSM shows them as being brown.

@seventynine The compressor is a York and it's stock. I did not change it over, and the old lady I purchased the car from bought it new and while possible that someone switched it out, it's unlikely as the Sanden's are more efficient and less expensive than the York units. The system does have a low pressure switch inline. I had the entire dash and bulkhead out this weekend to try and chase down the phantom 12v and came up empty handed. The B5 connector at the bulkhead is the one that goes to the clutch, but on the inside the wire bypasses the fuse box and appears to go up inside the harness to a location I've not sussed out yet other than seeing continuity between that and the two larger white wires at the HVAC controls. There's a small black box that - I think - is part of the voodoo AC system, but all of those wires are very small and the ones at the compressor and switch are much larger. I find it hard to believe that the wires from the box would be spliced to the larger wires, but who knows. I have to cut open the harness and start tracing the wires.

I even dismantled the HVAC control panel to see if something was bridging contacts incorrectly inside the system and everything is functioning as it should as far as the controls go.

As for why I started two threads, the forum appears to double post threads that I start for some weird reason. Apologies, and thanks for everyone that's chimed in with assistance. Bodywork and mechanical is my jam - electronics are far outside of my comfort zone....

[runuover86gw]

From the description of all your symptoms and what you have done to try and troubleshoot. I would say more than likely if you follow out that wire from the low pressure switch that heads back towards the cab. There will be a connector inside the plastic wire loom if it is still there near where the carb is and the wires run along the passenger side intake. I bet someone accidentally or was an idiot that shouldn't be working on it and they plugged it into one of the orange carb wires instead of to the connector that goes to the bulkhead and then back to the AC control module. If you pull the AC control module (little black box) and you still have power going to the low pressure switch and over to the clutch, then this is more than likely the issue. Then you will just need to trace back the correct wire and hopefully it hasn't shorted out your AC control module by hanging loose and possibly grounding itself to something.

[seventynine]

@seventy-nine The compressor is a York and it's stock. I did not change it over, and the old lady I purchased the car from bought it new and while possible that someone switched it out, it's unlikely as the Sanden's are more efficient and less expensive than the York units.

Right ok. Well this kind of thing existed with Jeep in those days...you never know what you would get exactly. Maybe it was an early model or who knows what.

[AtlantaDan]

Update - Today, there wasn't 12v at the AC clutch and I've not done anything differently. Broke out the DVM and started testing. Everything tested out ok up to the small ac control box under the dash. It has 12 on the green lead when the AC slider is moved to AC. The thermistor resistances are all correct according to the table in the FSM. The blower does kick on, so everything is working ok to that point. On the AC box, the orange wire next to the green 12v has been confirmed to be the one that goes to the clutch and it has .04 ohms of resistance. When 12 is seen on the green lead, it should energize the orange based on the resistance from the thermistor to engage the clutch and it does not.

This leads me to think this little box has gone bad but I am not able to find anything close to it as a replacement unit online.

Does anyone have any leads on one of those things?

[Srdayflyer]

its, one of the un-obtanium parts, you rarely see any fsj's in bone yards anymore and the ones that are are generally like a carcass in the desert, picked clean, i have a friend who has 6-7 fsj's stashed on his property not sure what years ill see if he may have it, but dont expect low balling on the part tho, i recently picked the impossible to find pie plates, and an a j-truck accelerator pedal , i muscle grill and the rear windows non-vent for a cherokee, and a vented window set , but its switches and controls i look for the most, ill let you know if he has one

[Srdayflyer]

i spoke to my friend , no can do, however he said beginning around 85-86 the fsj panel was redone after chrysler took over amc and that the 86 he has in his yard looks exactly the same as 2 dodge dakota's he's got you may be able to track down the part in some dodge chrysler product in the boneyards perhaps even new or NOS (new old stock), do you have a part ##

[Srdayflyer]

hope this helps you a bit

[AtlantaDan]

Thank you @Srdayflyer - I'll keep an eye out.

Update: FIXED. I took apart the control box and started testing caps and resistors. Everything tested out a-ok. What didn't test out correctly was the white plastic box on the board, aka the relay. I cut and snipped my way around the plastic cover and exposed the relay mechanism itself. The relay wasn't switching, and I'm guessing that it had welded itself closed so that's why I was getting a constant 12v at the clutch right up until I wasn't. I cleaned the contacts off on the relay, double checked the solder joints and now everything is working as it should, and the car has functional A/C.

The pick is pointing to the relay contact that was cleaned.


The high side pressure on the compressor was bouncing so I am going to start looking for a Sanden mount and compressor to swap over.

Thanks again to everyone that's chimed in with assistance. I hope this thread helps someone out.

[devildog80]

Nice work reviving the old relay