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AC Issue

1430 Views 18 Replies 4 Participants Last post by  Fourspeedman
Hello all,

I have run into an issue with the AC system in my 1999 Cherokee 4.0 Sport. I think I know what the problem is but I am not sure how to proceed.

The AC does not blow cold air at all, regardless of how long the engine has been running, outside temp, etc. I have confirmed that the system has refrigerant and pressure and that the compressor clutch is engaging (sporadically but I see it spinning). I ran into the same issue some years ago and I recall my mechanic saying there is a gate in the vent system that switches between cold and hot air. I am certain there used to be a "ka-thunk" sound coming from the vent shortly after switching the climate control from heating to air conditioning, followed shortly thereafter by blessed cool air. :)

Does this sound familiar to anyone? If so how do I diagnose and repair the problem? I am currently unemployed and trying to spend as little as possible but I need to get this resolved.

Thanks!
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thinking this is a 1999 Cherokee XJ a common issue is blend door -

This is a link to info - will describe quick fix and improved parts. A picture is worth 1000 words; let us know. There is another source for these fixes but these people (kits) have worked for me.

http://heatertreater.net/jeep-cherokee
 

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Discussion Starter · #5 ·
UPDATE: I replaced the blend door and confirmed that is now working. Still no AC!

It looks like the compressor clutch is no longer engaging. I used a paperclip to bypass the low-pressure switch but the clutch still does not engage.

I checked the fuse for the climate controls (#25 if anyone is interested) and that is still intact.

I re-checked the low pressure valve and that still registers decent pressure.

I'm hoping I don't need to replace the compressor. Any thoughts?
 

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OK this is a 1999 cherokee XJ with a 4.0L eng?

ideally you should check the AC pressures HIGH and LOW with gauges.

AC compressor (load) is fuse #20 a 20amp fuse in the PDC.
AC compressor (control) is fuse #10 a 15amp in the junction box- in cab fuse box.
 

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Discussion Starter · #7 ·
Hey JTEC. Thanks for the info. I checked the #20 fuse in the PDC and the #10 fuse in the junction box. Both are intact.

I noticed that there is an "AC Compressor Clutch Relay" in the PDC. Could that go bad and cause this problem? Is there any way to check that?
 

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the relay is a great place to ck circuits for AC. A test light and we can ck - relay control circuit and comp 12v supply

ANSWER - swap with a similar relay, hopefully horn, wipers, are the same for testing.
 

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Check to see if the compressor is getting 12v. If it is, smack the center of the A/C pulley with the handle end of a hammer if you can safety get to it while the engine is running. See if that makes the clutch engage when you have the A/C turned on and the low pressure switch is bypassed.
 

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Do you have a gauge set? If not get one. The static pressure can be as high as 60psi, yet when the compressor engages it can drop below 20psi. That causes the low pressure switch to cut the compressor off. As static pressure builds back up the compressor engages again, cuts off again, etc, and you have no cold air. If the charge is very low the compressor will never come on. The correct procedure is to purge the system completely, vacuum it down with a vacuum pump to get any moisture out, then recharge with the correct amount. If all else is good you should get proper AC. But without doing that you are guessing about everything else. .
 

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Discussion Starter · #11 ·
the relay is a great place to ck circuits for AC. A test light and we can ck - relay control circuit and comp 12v supply

ANSWER - swap with a similar relay, hopefully horn, wipers, are the same for testing.
I tested the relay: connected terminals 1 and 2 to the battery and heard the click of the electromagnet engaging. Checked continuity across terminals 3 and 5 on the relay: no resistance when the relay is engaged. I didn't see any similar relays I could test with.
 

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Discussion Starter · #12 ·
Check to see if the compressor is getting 12v. If it is, smack the center of the A/C pulley with the handle end of a hammer if you can safety get to it while the engine is running. See if that makes the clutch engage when you have the A/C turned on and the low pressure switch is bypassed.
Smack it with a hammer! I can get behind that! I thought of that too but the front of the compressor is immediately behind the cowling for the fan blade. Hard to get to safely.

Good idea. I have not checked to see if the compressor is getting 12 v. I'll need to dig around a bit to find where I can check that.
 

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Discussion Starter · #13 ·
Do you have a gauge set? If not get one. The static pressure can be as high as 60psi, yet when the compressor engages it can drop below 20psi. That causes the low pressure switch to cut the compressor off. As static pressure builds back up the compressor engages again, cuts off again, etc, and you have no cold air. If the charge is very low the compressor will never come on. The correct procedure is to purge the system completely, vacuum it down with a vacuum pump to get any moisture out, then recharge with the correct amount. If all else is good you should get proper AC. But without doing that you are guessing about everything else. .
Good suggestion but I'll need to find someone I can borrow that from. I am currently unemployed so I need to get this AC situation resolved on the cheap.

I did try to bypass the low pressure switch by sticking a paper clip into the connector and running the AC. I have seen folks on several videos do that. Maybe I can try the same procedure on the high pressure switch?
 

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No, the high pressure switch is normally closed so it can cut the compressor off if the pressure opens it. The low pressure switch is normally open but closes with sufficient pressure. By shorting it you are fooling the system into thinking there is pressure to operate. Find out if the wire to the compressor is getting 12 volts then try shorting the low pressure switch. If there is any freon at all you should get some cooling. But it is better to borrow some gauges and know for sure.
 

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Smack it with a hammer! I can get behind that! I thought of that too but the front of the compressor is immediately behind the cowling for the fan blade. Hard to get to safely.

Good idea. I have not checked to see if the compressor is getting 12 v. I'll need to dig around a bit to find where I can check that.
As the compressor wears, the clutch may not engage when power is applied. I've seen them where the gap is just a bit too big for the magnet to pull the clutch in. A slight tap makes it pop right in. Just something to check that can throw you for a loop.
 

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Or check for 12v at relay then procede step 1 then 2 and on to 3....
B1 and B5 should have 12v,
B3 is a ground (PCM)
B2 goes to compressor


Need help with that? Just a test lamp is needed and the above diagrams

1999 XJ 4.0L.
 

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Discussion Starter · #17 ·
Hi JTEC,

Thanks for the guidance. I'm not following what you are suggesting:

Or check for 12v at relay then procede step 1 then 2 and on to 3....
B1 and B5 should have 12v,
B3 is a ground (PCM)
B2 goes to compressor

Need help with that? Just a test lamp is needed and the above diagrams

1999 XJ 4.0L.
I have a multimeter. What's the procedure for checking the relay for 12 volts?
 

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Discussion Starter · #18 ·
UPDATE: Bypassing the low-pressure switch (I used a paperclip to jump the connector) causes the compressor clutch to consistently engage. I'm thinking that means I have a bad low-pressure switch. Is it possible to replace the switch without venting all my coolant into the atmosphere? Also, running the AC with the low-pressure switch jumped is not producing cool air. I didn't let it run for more than a minute or two - I was worried about damaging something - but should I be getting cold air?
 

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If the freon charge is very low the low pressure switch will never close. Test the system pressure first before condemning parts. A minute or so is not enough time to get cold air. IIRC the low pressure switch can be changed without losing the charge. I think there is another schraeder valve under the switch. Without a gauge set you are flying blind so I would concentrate on getting one. Even a used set on Craigslist or Fleabay is better than nothing. Just be sure it is for r134a, not r12.
 
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