AC troubleshoot

Alrighty... got a 93 with a 4.0 and no A/C.

So it's warming up and I'm going to want A/C.

I'm going tomorrow to get freon to make sure it's all charged up.

I was more interested in getting the AC compressor to kick on by itself first. It may not need to be charged.

With the engine running and the A/C on 'MAX A/C' and the blower on high the compressor clutch never kicks in.

I jumped the wire over to the batter and it comes on and works fine. I let it run for about 5 minutes and the air never got cold. I assumed that means it needs freon...

Next I found the low pressure switch and unplugged it. When I unplugged it the engine idle fluctuated a little. I assumed this meant that the low pressure switch is engaged (Tells me that the system should be charged enough to work...right?) I jumped across the two contacts on the plug. Compressor still didn't kick on...

I replaced the circuit breaker under the dash. I also replaced the A/C relay under the hood.

With the low pressure switch jumped, ac switch on MAX AC, good fuse and good relay the AC Compressor clutch SHOULD kick in, right? Regardless, of how much freon is in the system...

By looking at the Haynes manual I see that I still have to check out the electronic thermostat and electronic control unit...

Ideas?

Thanks, Andy.

mite try defrost, it should run when your defrost is on (it removes moisture so the windows will clear)

you can check for pressure by blipping the schrader valve to fill the system.

The fact that your clutch doesn't engage when the low pressure switch is bypassed is kinda troubling.

Redsand said:

(it removes moisture so the windows will clear)

Click to expand...

I had no clue! Thanks!

zluster said:

The fact that your clutch doesn't engage when the low pressure switch is bypassed is kinda troubling.

Click to expand...

That's what I'm thinking...

Anyone else?

Am I correct in thinking that the compressor should come on when I bypass the low pressure switch regardless of the actual freon level?

Ok, so several things... Maybe someone can chime in.

With the engine running and the AC on Max AC I bypass the low pressure switch and it still doesn't come on.

I checked for voltage at the relay which it has. I checked for voltage at one side of the low pressure switch. It was there so that means the switches in the dash are working.

If you refer to a Haynes Manual on page 12-30 you'll see a "thermostat switch" and the "ECU" are the only 2 items left that could cause the compressor to not kick on.

If I jump across the switch terminals of the replay the compressor comes on. That tell me that what ever tells the relay to switch isn't doing what it's supposed to do...

What tells the relay to come on? The ECU or the Thermostat switch.

Would the thermostat switch stay in the off position if it had 0 freon in it?

Most of all, where is the thermostat switch located? I can't seem to find it... The low pressure switch is in the top of the drier, correct?

I may be incorrect, but if there is a sensor in the T Stat housing, then the ECU makes the decision. The sensor in the TStat is only a temperature sensor not a switch.

I have been chasing AC issues for a while. They are almost worse than electrical problems, until I got a full set of gauges at HB and read up on how to diagnose AC issues. My suspicion is that you are low on/out of refrigerant. Get some gauges and test for refrigerant levels. Since you can get the compressor to engage, you will be able to tell if you are low. Do you have R12? If not it will be much easier to address.

Have you seen the Haynes Diagram?

The diagram definitely shows a switch rather than a sensor. I don't understand what a engine temperature switch would have to do with the ac system but several people have told me that.

Either way, the system is full of freon and the low pressure switch no longer needs to be bypassed.

To make the compressor come on now I have to jump across the relay outlet. Then I can sit in the Jeep and turn the compressor on and off with the ac knob. The problem is that isn't being controled by anything. It's straight wired and that just won't do. I need to figure out WHAT tells the compressor to turn on and back off.

If I sit in the Jeep and run the compressor for about 5 minutes it gets cold now but I don't want to overcharge the system so I turn it back off.

I swapped the ECU out with a spare one I have laying around to see if that fixes the problem and it did the same thing. The ONLY thing left is the "thermostat switch" that doesn't seem to exist that Haynes shows in their diagram.

I really wish I could post the diagram... May i'll draw it and take a picture of it. Does that break the rules?

I'm actually rather surprised there's not more people that can help me. I guess everyone hates working on AC Systems as much as I do. lol I never had trouble finding a expert on every issue I have on .

I would trace the wires from the compressor to wherever they end and see if there is a break or a short...since the clutch works when you jump it that means its never getting its signal to kick on. And yes, when you jump the LP switch the clutch should kick on regardless to freon level.

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All is well between the relay and the compressor...

The problem lies somewhere between the LP switch and the switch side of the relay. The relay is good.

If you look at the diagram which I just drew and am in the process of uploading if my phone would cooperate.

So, here's the diagram...

I have power to the LP switch. and I have power to the 3 oclock position on the relay. I just don't have power to the 12 oclock or 6 oclock position on the relay. If I jump from 3 to 9 on the relay outlet the compressor comes one...

Like I said a few ago, I replaced the ECU to see if that was the problem and it still didn't work. The only thing I see is the t-stat switch.

Someone has to know where this switch is....

I dont think the hayns is telling the whole truth, according to shopkey the low presure switch is downstream of the t sat switch, so if there is power at the low pressure switch the t stat switch is working fine.

I dont fully understand your drawing and where you have power at the relay, what ones are 3 and 9 as to 12 and 6 oclock.

What ones on the relay are you jumping for the compresor to come on as they are listed on the relay?

Im basically eliminating the relay. I jump from the lp switch wire to the compressor clutch wire. According to haynes that eliminates the tstat switch and the computer. I show no voltage on the other 2 terminals. The little jumper diagram I drew is the actual relay in the engine bay. Its not really related to the haynes diagram. I copied that diagram straight from haynes minus all the heater and blower motor crap.

Well according to the diagram I have you should have constant power(run/start) at Terminal #2, Dark blue/White wire, power coming from the low pressure switch terminal #1 light green wire, and it should receive a ground from the ECU on terminal #5 the dark blue/orange wire.

So I would do the computers job and ground terminal #5 and see if it kicks on.

Now, without yanking the fuse block I gotta figure which on it terminal 5. Lol

If Im missing the constand 12 on terminal 2 could cause the problem. With the ac on im only getting 12 volts to one terminal on the relay. If that were the problem tho I feeel like the compress would have kicked off on its own when I jumped the relay.

Well if your only getting voltage on one terminal its either the one coming from the low pressure switch or the hot/run constant. And if you jumped from the low pressure switch then its either the wiring from the low pressure switch or the constant voltage. Did you check fuse #5 in the power distrbution center, its a 15 amp, its the hot/run constant feed.

I'm pretty sure I replaced it. I'll check it in a minute.

I know I have voltage AT the LP switch. That was the first thing I checked.

I was fair confident that the voltage I was reading on the relay was from the LP switch. Ill check for continuity when I get back home.

Fuse 5 is good.

Do you have the actualy pin out for the relay socket? 87 30... so I don't have pull the fuse box to see what color the wires are.

Unfortantly not, just wire color and 1,2,5,4 But the relays typically have them listed on them, then see what termianl the power is at.