94 Jeep Cherokee troubles

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  #1  
Old 06-24-10, 05:30 PM
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94 Jeep Cherokee troubles

1994 Jeep Grand Cherokee Laredo.
318 V-8, auto trans, air conditioning, 99,000 miles.

Problem:
Erratic idling, mainly when cold. Seems to idle better when warmed up. I checked the codes and it shows code 25 which is the idle air control valve. (I had this same problem with a Ford Ranger and when I changed the valve the problem was gone.) My question, can this be tested easily to make sure it is the problem before I change it?

The air conditioning also does not work. Code 33 comes up which says open or shorted connection. There is only one wire that goes to the air pump. Should this wire have current in it? I checked the pressure in the system and it was right in the middle of where it should be. Can it still be low on R-134 and have the required pressure in the system? I thought maybe if I added more to it it may make it work?

Next and last is code 12. "Direct battery input to PCM was disconnected within the last 50 key-on cycles." What does this mean? I haven't unhooked the battery at all. I assume the pcm is the computer that runs the vehicle? Should that not be direct wired? Could this be what is making the air conditioner not work? Could this be why the idling is erratic? Could the erratic idling be making the code 12 show up?

If there was just one problem I don't think it would confuse me as much as this is right now, but I realize that a lot of these electrical problems can be tied together.
Thanks in advance for any info.
 
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  #2  
Old 06-25-10, 05:29 AM
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Location: Lady Lake Florida
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I checked the codes and it shows code 25 which is the idle air control valve.

Was the code tc25 A, B, C, D or E ?

(Direct battery input to PCM was disconnected within the last 50 key-on cycles." What does this mean?)

Either the PCM (power control module) has lost battery input within the last 50 key-on cycles, OR it thinks it has.

(There is only one wire that goes to the air pump. Should this wire have current in it?)

Assuming that what you describe as the "air pump" is actually the air conditioner compressor, Tho only time the wire should have power is when tha a/c is turned on.

( I checked the pressure in the system and it was right in the middle of where it should be.)
Two questions here,
1, Was the a/c turned on at the time you "checked the pressure" ?
2, Was the a/c clutch engaged "center of the pully turning"
When the a/c is on and the clutch is engaged, you will see the the CENTER of the clutch/pully spinning.

( Can it still be low on R-134 and have the required pressure in the system?)
No, if the freon (r134A) is low, then the pressure will be low.

When you retrieve the answers to my questions hopefully we can further assist you...

Good luck and have a great day
 
  #3  
Old 06-25-10, 08:15 AM
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Thanks for the reply.

Replaced the idle air control valve and that took care of the idle trouble.

The air conditioner:
The clutch on the compressor wasn't engaging. The pressure in the system read about 46 lbs. I unhooked the pressure switch at the dryer and jumped a wire across the terminals. The clutch then engaged. The pressure when running and with the clutch engaged read right at 45 lbs. I let it run like this for about 5 minutes or so. The clutch remained engaged but there was no cold air coming out of the vents, however, the small aluminum tube that goes into the firewall did get very cold. I removed the jumper wire and the clutch dis-engaged. I plugged the pressure switch back on and the clutch wouldn't engage. I unplugged it and put the jumper back in and the pump again engaged. So, I assume the pressure switch is bad? The pressure in the system remained at 45 lbs, whether running or not. The pressure switch just unscrews from the dryer, but if I take it out will it release the freon, or is there a check valve under the switch? Could the pressure switch be stuck or dirty, could it be cleaned?

I don't care so much whether the air works or not, but the check engine light comes on after I drive for a 1/2 hour or so and I don't like to see the check engine light showing when I drive.

The code 12, about the PCM, this is the one that I worry about. Is there a relay inline that could be bad? A fuseable link? There is a new battery in it, and the connections are clean and tight. Any idea of where to start looking for this trouble? Anything that I can test?
 
  #4  
Old 06-26-10, 06:11 AM
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Join Date: May 2008
Location: Lady Lake Florida
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The air conditioner:
Based on your info, the freon IS low, the pressure switch USUALLY won't engage the compressor until there is 50lbs. pressure.
Connect a can of 134A to the suction line, use your jumper on the pressure switch.
with the compressor running, turn the freon can upside down for aprox. 1-2 sec. just enough to allow a small amount of liquid freon in the system. turn the system off , re-connect the pressure switch.
If the compressor engages, then finish topping off the system.
If the compressor DOES NOT engage, repeat the above step,

Powertrain Control Module (PCM)

The PCM, is mounted in the engine compartment and contains circuits necessary to energize the ignition coil, fuel injectors and alternator field in order to minimize electrical noise in the passenger compartment. The PCM contains a voltage converter which converts battery voltage into a regulated eight volt output to energize the ignition pick up coil. The externally mounted Automatic Shutdown (ASD) relay, which interrupts power to the fuel pump, ignition coil and injector when necessary, is controlled by the PCM.
The ASD relay is turned on and off by the PCM in response to distributor reference pulses. When distributor reference pulses are transmitted to the PCM, the PCM activates the ASD relay. However, when no reference pulse is transmitted to the PCM, the ASD relay is deactivated and power is shutoff to the fuel control system and ignition coil.

Good luck and have a great day
 

Last edited by liljman; 06-26-10 at 06:45 AM.
  #5  
Old 06-27-10, 02:51 PM
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Got the air conditioner working. The info that I had said that 45 lbs was the max a system should have.? I did what you suggested and the compressor engaged, so I dumped two cans into the system and the air works great now.

Thanks for the time you took to answer my request.

Someone else had mentioned a bad ground for these other problems. I checked the ground on the engine and it looked kind of white powdery on it, so I cleaned it up and put anti-seize on it. It wasn't terrible tight either, so I put it on good and tight. so far no more engine light, but then again I haven't driven it too much either.

Again, thanks to all who took the time to read this and especially those who took the time to answer.
 
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