lescaster

I have a 1999 cherokee and the AC doesn't work. I can pull the relay from the power distribution center and put in a jumper wire and the A/C then works.
I'm thinking I'm not getting any power to the relay - where should I look for power? There is no power at either the low pressure or high pressure switches.

Thoughts?

bobvalli

If you can jumper the relay base and make it work, what makes you think it's not getting power??? Try swapping a similar relay from that box (I think there's at least 2 others). If you're not sure they're the same look on the case of the relay for the little schematic of the internals and compare them. If the AC fires normally, you have your answer. When mine crapped, I ran the relay # on e-bay motors and ended up picking up 3 for $10.00. If your freon is low, maybe the low pressure switch is keeping it from coming on. Jumper a wire from the battery to the compressor connector and see if the compressor starts (and stays running, but don't do it for too long). If that kick starts the compressor let it run and put a pressure gauge on the low pressure side fitting. If it's low, add R134a until the pressure comes up and then re-connect the condenser connector and see if the compressor cycles normally.

hope that helps

-bob-

lescaster

Thanks Bob

I tested the relay per the Jeep manual and the relay appears to be good.

What is a "Powertrain Control Module" and where is it?

I'm wondering if this is caused by a faulty control on the dash?

bobvalli

That's cool that you checked it as per the manual, but.....a relay under no load conditions (meter test) is different than a relay under full load conditions. You're relying on little internal metal wipes with even smaller contact points on the end to make the circuit and carry the amps the device draws (in this case, the compressor which is a big ticket drawer of amps). My point being that even though it tests good with a meter doesn't necessarily guarantee that it's conducting properly when under heavy load.

Also did you check the system pressure? There is a low pressure cutoff switch to protect the compressor. If it's not satisfied the compressor wont start. You can pick up the gauges @ most parts stores for not too much

With that said, if you tried the swap test and the pressure checks out, I'll have to put myself back on the sidelines and hope someone with more experience can help you further.

-bob-

lescaster

Bobvalli - thanks - I did check it with pressure gauges and everything there is as it should be - I took your advice and bought new relays off of eBay(2 for 9 dollars including shipping) both the low and high pressure switches appear to work - and now I'm at a loss.

again - Bobvalli thanks - I didn't want you to think you weren't being listened to

freegdr

is there freon in it if its low it aint gonna go

freegdr

jump thepressure switch but just long enough to see the comp kick in

bobvalli

Actually I think you'll have to keep it going in order to charge it or check it. I hate to sound like a broken record, but pressure at the inlet without the compressor running is much higher than pressure at the inlet with the compressor running. What I did was separate the wire connector at the compressor. From my manual it says it's a dark blue/black wire. Put a jumper lead on the 1/2 going to the compressor, start the vehicle and touch the jumper to the + of the battery. That should pull in the compressor. Have your gauge on when the compressor starts. If it goes really low, than add freon with the compressor jumped. Once it get's up to pressure shut the truck off, remove the jumper, plug everything back in and re-start it to see if the compressor pulls in by itself now. If anything I said about the wires doesn't jive with your truck, please write back before you do anything.

Hope that helps

-bob-

Megan Rollins

Having the same issues. 2001 Jeep Cherokee
Tried two new relays. Yet a paperclip works fine.

Refrigerant pressure is fine. Sensor is fine. No codes in dash. Mechanic pal is as confused as I am.
Any insight appreciated!

BlueRidgeMark

I'm betting a 10 year old post by someone who hasn't been on the forum in 4 years is not going to get much of a response.


If a new relay doesn't solve the problem, but a jumper does, then we can assume the relay is not getting the correct signal. It needs ground on one side, and voltage on the other. Since it seems you know which two terminals to jumper to make it work, then by process of elimination, the other two must be the power and voltage to trigger (signal) the relay.

So, let's probe those terminals to see what they are doing. Here's the tricky part - Jeeps like to provide constant power to things, and then the PCM (computer) provides ground when it wants to turn something on. Most vehicles do it the other way. (There are pros and cons to both.) For example, the coil gets constant 12 volts, and the ground is switched when the PCM wants to fire the coil. Same with your coil packs. Similarly, the injectors get constant 12v, and the PCM grounds them to fire the injectors.

All of that to say, I'm not sure which way the AC relay works. Does it have constant 12v, with the ground being switched? Or constant ground, with the 12v being switched??

You can find out by simply measuring those two terminals. If one is always hot (12v), then the other one is almost certainly a switched ground. And vice versa, if neither is always hot, then one of them will show constant continuity (connection) to ground.

Once you have that figured out, you can start testing the signal terminal to see when, or if, it does what it's supposed to do.

If neither of them shows either condition, well, that's another kettle of fish!

Was that all clear as mud? If you think you've got it, try those tests and let us know what's up, and we can go from there.