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AC Compressor Only Engages When Relay is Jumped


NAXJA Forum User
I've been having some issues with my AC recently. I have a 1999 Jeep Cherokee Classic. I replaced the AC compressor and condenser in hopes of fixing it. But then i realized my compressor clutch wasn't engaging.

I replaced the relay and checked all the fuses. Nothing changed. I jumped the compressor clutch with a wire in the fuse box. That engaged the clutch with and without the engine on. When the relay was jumped, the AC was blowing pretty cold. I filled the system to about 50 psi with a DIY can.

I even jumped the pressure sensors and nothing changed. I tried the shim trick but that also didn't change anything. I'm wondering if it could be something with the climate control panel? The Ac mode selector sometimes cuts power in between selections.

Another thing too. When the compressor clutch doesn't engage, The temp doesn't seem to change between the coldest and warmest settings. I even tried manually moving the blend door and it only changed the force of the air coming out. I'm losing my mind!!
Time to get out the meter and the wiring schematic and work backwards. Hit cruiser54.com for wiring diagrams. I'd probably start at the switch myself, then move to checking the contacts at the a/c relay. If you have an e-fan, does that turn on when you turn on a/c?
What were your original problems that led you to replace them?

Did you evacuate the system prior to charging?

Was that 50 PSI with or without the compressor engaged?
Sorry for taking so long to reply. Forgot I had posted this. But anyway, I filled the system to 50psi with the engine on, ac at max cool and speed, and I had the relay jumped so the compressor actually engaged to let the Freon flow through. Original issues that led me to change the parts were my ac condenser was cracked and the compressor seized due to water from off-roading. And I did not evacuate the system. I was thinking of taking the jeep down to my mechanic and having him empty and recharge the system.

I do have a Mishimoto 3 fan radiator fan that I had my mechanic wire up. That I'm pretty sure kicks on when the ac does. I jumped the compressor strait to the battery and the clutch engaged. Also tried the shim trick but that didn't do anything either. I hate wiring and don't dab in it a lot although I've almost all my issues be wire related. I replaced my engine and main under hood harnesses for ones in way better condition than mine, so I don't want to believe its a wiring problem.

Once again sorry for the delayed response.
So a few things.

First, you really need to draw a vacuum PRIOR to charging. The vacuum removes air and moisture from the system. You pull a vacuum as low as you can get it, like -26-27 PSI and hold for 30 min or so. This also helps identify any leaks. Any remaining moisture could freeze inside and damage the compressor or other components.

Next, your 99 uses a fixed orifice in the liquid line. Since your compressor seized, you may have debris inside. Everything should be flushed out and possibly the orifice changed.

Last, R134a systems are filled by weight. That's the only way to ensure the correct amount of oil and refrigerant are present in the system, especially since you changed out parts. Those DIY cans *may* be OK for topping off a system, but a total fill really requires a manifold set with a high pressure gauge to verify proper operation.

Anyway, off the soapbox.

By all means, take it to a shop and have them evac/charge/test it. You spent more than a few dollars to replace the parts. Spend a few more to make sure they all work properly before the AZ heat kicks in.
Awesome thank you. Yeah I wasn't sure about taking it to the shop just because I didn't know what was wrong in the first place after replacing everything and I like to try and figure things out as for myself so I get more experience before becoming a mechanic when I graduate in May from high school. But I will take it to my mechanic sometime next week probably and have him do whatever it takes to get it working again. Don't wanna go without it this summer...
I don't know about the 1999 that schematics but my 1996 had the same symptoms as your 1999 have with one exception, everything else worked according to the switch settings. It turned out to be a problem with the ECM/PCM. The PCM was not sending a signal to activate the relay. Probe the two small wires under the AC relay for a ground and a positive with the engine running and the AC set on high.

I assumed you jumped the 2 large wires under the relay... that would rule out defective wires between the positive bar in the relay box and the wire going to the AC compressor.
Yes that's what I did. Pretty sure it was the bottom and top slots (looking at the fuse box from the front of the vehicle). The other two caused the fans to kick on so I assumed it was maybe a relay but I tried 3 different ones and still nothing. Just gonna have my mechanic look it over and then post the fix for those having a similar issue.