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Discussion Starter · #1 ·
Yesterday my A/C dropped out on me suddenly. Blower works but the air is hot. Basically, the compressor clutch refuses to pull in. I ran the codes using the key dance and came up with nothing. When I ran the diagonostics on the ATC head it came up with a code 36, communications error. Naturally, I blamed the ATC head.

Today I made it to the junkyard and found a single ATC equipped Intrepid. I swapped this unit into my Concorde and still had no A/C. Got a code 26 (bad thermistor) so I passed on it.

When I got home I decided to dig in a little bit and found the following:

1) When I apply 12V directly to the compressor clutch it pulls in fine and makes cold air. Compressor and clutch appear to be fine. There is no voltage at the clutch connector under any condition.

2) Clutch relay audibly pulls in when the engine is running and the A/C is on. Swapped it with the wiper relay in case the relay itself is bad. No dice. Relay appears to be working properly.

3) Pulled the radiator cover and pulled the connector from the A/C pressure trandsucer. Jupered between the ground and transducer signal terminals and did not see any difference. Not sure if the BCM is looking for a simple closure or a 5V signal from the transducer. Don't want to apply 5V without knowing for sure.

At this point, it doesn't sound like a bad ATC head anymore. It seems to me that if the ATC head was the culprit, the clutch relay would not pull in. Am I wrong?

I don't think the refrigerant level is too low, since the air cools fine once the clutch is forced in. Also, the air worked fine before yesterday. My experience in the past is that the effectiveness of the A/C begins to drop gradually over time before the pressure drops low enough to cut the compressor out.

I'm thinking it's either a bad pressure transucer or a broken wire, but I'm leaning towards the transducer. I don't see any procedure for testing it in the shop manual and don't want to waste the money if it is still good.

My questions are:

1) Anything I missed in the diagnosis? I'm no expert in A/C systems so any suggestions would be helpful.

2) Can the pressure transducer be tested either on or off the car? If so, how?

3) Is there a way to safely jump out the transducer to see if the clutch engages without it?

4) Any chance this is the PCM? I'm hoping not.

Thanks in advance.
 

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The transducer seems like a likely culprit. It is possible though that the ATC system is not communicating properly and therefore isn't sending a signal to the compressor to turn on. I know there isn't very much info in the FSM about the transducer because I suspected mine was shorting out so I did some research and pretty much came up with nothing. I'll see if I can find something though.
 

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Discussion Starter · #5 ·
Thanks, Steve. Did you actually replace your transducer, or did you find something else wrong?

I'm betting against the ATC head, since the relay seems to be working correctly. It seems that the problem is somewhere in between the relay and the clutch. I'm hoping I don't have to ring the wires out.
 

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Chief Brody said:
Thanks, Steve. Did you actually replace your transducer, or did you find something else wrong?

I'm betting against the ATC head, since the relay seems to be working correctly. It seems that the problem is somewhere in between the relay and the clutch. I'm hoping I don't have to ring the wires out.
I never did replace the transducer. Its a pain to do and its $80 so I didn't want to do it unless I was sure it was the problem. My car would idle high and then stall due to a shorted 5v sensor feed and another member said his transducer was the culprit. Because it only occurs instantaneously and then is fine for months, its nearly impossible to test it so I didn't bother with it. The FSM doesn't show much on how everything is wired so its not much help. Your best bet is to find someone with the wiring manuals [cough]Ron[/cough] that can see exactly how the transducer connects to the circuit so you can figure out how to bypass it. Sorry I can't help you.
 

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Chief Brody said:
Thanks, Steve. Did you actually replace your transducer, or did you find something else wrong?

I'm betting against the ATC head, since the relay seems to be working correctly. It seems that the problem is somewhere in between the relay and the clutch. I'm hoping I don't have to ring the wires out.
If the A/C clutch relay coil is pulling in and the clutch works when you apply 12v directly to it, then it sounds like a wiring, connector or ground problem. Sometimes there is a diode accross the connector terminals at the compressor but it should rarely burn up but you might check it also.. The basic A/c clutch power circuit should only consist of : 12v (+) from battery--to 30a fuse in power distribution center--from pdc fuse to pdc compressor relay-- from relay to compressor.., The other side is ground connection, which it shares with the washer pump motor.
The ATC controls and pressure transducer should be on the control side of the relay, If they were not working then the Compressor relay would not pull in.
Also might pull the clutch relay out make sure you have 12v from the fused feed; and if it does, apply +12v to the terminal in the PDC that feeds the clutch to see if the wiring from PDC to clutch is good.
 

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lextrep said:
The ATC controls and pressure transducer should be on the control side of the relay, If they were not working then the Compressor relay would not pull in.
Thats great info. Thats what I was wondering after I gave it some thought. So what you're saying is, the transducer signal is fed to the BCM and when the BCM wants the compressor to come on it activates the relay and power goes directly to the cluch, right? So that means there is nothing in between the clutch and realy so if the relay is pulling in and the compressor turns on if 12v is applied, there is a problem with the 12v from the relay getting to the clutch. That narrows down the problem quite a bit.
 

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Hey Chief...don't know if this has been looked at already but I'm stepping into this late. Your first post up there says you can hear the A/C clutch relay activating. You also indicate when the relay is swapped that it works fine in the other circuit. That indicates that fuse 21(10a) in the junction block and the circuitry for the relay coil is good. Have you looked at fuse "B" (30a)in the PDC(power distribution center)? That supplies power for the contacts of the relay and then to the compressor clutch. I apologize if I missed that you checked it and I didn't see that info...otherwise something to look at. Hope it's an easy one. By the way it's the PCM that tells that relay to turn on after it's get's it's communications from the BCM via the PCI bus. The A/C pressure transducer is also connected directly to the PCM as a sensor input.
 

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Ronbo said:
Hey Chief...don't know if this has been looked at already but I'm stepping into this late. You're first post up there says you can hear the A/C clutch relay activating. You also indicate when the relay is swapped that it works fine in the other circuit. That indicates that fuse 21(10a) in the junction block and the circuitry for the relay coil is good. Have you looked at fuse "B" in the PDC(power distribution center)? That supplies power for the contacts of the relay and then to the compressor clutch. I apologize if I missed that you checked it and I didn't see that info...otherwise something to look at.
Thats a really good point as well Ron. That would defintiely explain the relay activating but no power going to the clutch.
 

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00r/t.mopar.muscle said:
Thats great info. Thats what I was wondering after I gave it some thought. So what you're saying is, the transducer signal is fed to the BCM and when the BCM wants the compressor to come on it activates the relay and power goes directly to the cluch, right? So that means there is nothing in between the clutch and realy so if the relay is pulling in and the compressor turns on if 12v is applied, there is a problem with the 12v from the relay getting to the clutch. That narrows down the problem quite a bit.
Or could have a problem with power from the fuse in the PDC to the hot side of the relay (or just a blown 30a fuse) or no feed to the fuse from the battery( but useually then other stuff will quit working too).... The relay coil and the contacts of the relay are 2 different , separatelyy fused circuits. One for control the other for power.. Thats why you check all fuses first (I think this fuse was labelled brake lights or something else on my 2000 Intrepid)

And the transducer sends a pressure signal to the PCM, which then energizes the clutch control relay based on signals recieved from the BCM. This allows the pressure transducer to serve as one of the engine control sensors while the ac is being used as well as a low pressure cut out switch for the A/C system. It gets 5volts and ground from the PCM and sends a varying milliamp signal back to the PCM depending on the line pressure. This also lets the PCM monitor compressor load and adjust engine speed accordingly
 

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2) Clutch relay audibly pulls in when the engine is running and the A/C is on. Swapped it with the wiper relay in case the relay itself is bad. No dice. Relay appears to be working properly.
If you hear the relay it just means the coil inside of it is energizing (and possibly de-energizing due to low refrigerant). It does not mean that the internal contacts are making the connection between terminals "B" and "D".The pcm, ac head, and transducer control the coil side of the relay via terminals "A" and "C". The other two terminals "B" recieves power from the fuse at all times. Terminal "D" goes directly to the clutch. Check for 12v at terminal "B". Then Check for continuity from "D" to clutch. You should be able to engage the clutch by jumping relay cavity "B" to "D". If not then the problem is fuse,or relay or wiring batt to fuse,fuse to relay,or relay to clutch. If the clutch engages with "B" and "D" jumped then it's most likely low refrigerant or the transducer.
 
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Discussion Starter · #14 ·
Well, I am an idiot. I only checked the fuses for cicuits labeled A/C. I didn't realize that the clutch gets its power form the high speed fan circuit. Although I glanced at all the fuses, the 30A "B" fuse looked fine. On close inspection, I can see that it is in fact blown, along with the 40A low speed fan fuse. I'm headed out now to get some replacements.

Thanks to all, especially Ron, Lex and Jed. Rep left for all that I am able to.

This leads me to the next obvious question. What would cause both of these fuses to blow? Should I suspect that both fans are shorted? The air quit working (indicating the blown 30A fuse) and yet I had no overheat problems in traffic on Tuesday. On Wednesday, she started to get hot when I was almost home, and I parked the car the last 2 days. Is it possible that the high speed fan being dead caused the low speed fan to run constantly causing the second fuse to blow? What do you guys think?
 

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Chief Brody said:
Well, I am an idiot. I only checked the fuses for cicuits labeled A/C. I didn't realize that the clutch gets its power form the high speed fan circuit. Although I glanced at all the fuses, the 30A "B" fuse looked fine. On close inspection, I can see that it is in fact blown, along with the 40A low speed fan fuse. I'm headed out now to get some replacements.

Thanks to all, especially Ron, Lex and Jed. Rep left for all that I am able to.

This leads me to the next obvious question. What would cause both of these fuses to blow? Should I suspect that both fans are shorted? The air quit working (indicating the blown 30A fuse) and yet I had no overheat problems in traffic on Tuesday. On Wednesday, she started to get hot when I was almost home, and I parked the car the last 2 days. Is it possible that the high speed fan being dead caused the low speed fan to run constantly causing the second fuse to blow? What do you guys think?
I think you're gonna need a new fan. Been there, done that.

They're a lot cheaper now - you should be able to find one for under $200 bucks.
 

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Chief Brody said:
Well, I am an idiot. I only checked the fuses for cicuits labeled A/C. I didn't realize that the clutch gets its power form the high speed fan circuit. Although I glanced at all the fuses, the 30A "B" fuse looked fine. On close inspection, I can see that it is in fact blown, along with the 40A low speed fan fuse. I'm headed out now to get some replacements.

Thanks to all, especially Ron, Lex and Jed. Rep left for all that I am able to.

This leads me to the next obvious question. What would cause both of these fuses to blow? Should I suspect that both fans are shorted? The air quit working (indicating the blown 30A fuse) and yet I had no overheat problems in traffic on Tuesday. On Wednesday, she started to get hot when I was almost home, and I parked the car the last 2 days. Is it possible that the high speed fan being dead caused the low speed fan to run constantly causing the second fuse to blow? What do you guys think?
When the A/C is on, it is quite normal for both fans to run at high speed, and the fuse is able to handle that current draw. If a new fuse blows out right away, then I would check that circuit for a short to ground.
 

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Discussion Starter · #17 ·
I replaced both fuses ($4 each :shocked: ) and the A/C is back. The passenger side fan runs, but not the drivers side. I smell a distinct burned odor near the fans. I have not road tested the car yet so I'm not sure how the fans will behave when the engine is fully up to operating temperature.

Can someone educate me on how the fans work? Do they both run together or will one drop out when the radiator temp is low enough? I'm not really sure if each fan has a high and low speed or if one fan is high and the other is low. I didn't have the time to test each fan individually, but might be able to tomorrow.

Thanks again for the help.
 

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Both fans have a High and Low speed circuit and those are connected in parallel. So both fans should run at the same speed. For example when they're on, they should both run at low speed or both run at high speed. One fan can't be on while the other is off. Sounds like you've got one burned out fan motor or the wiring has a problem. There are new ones on Ebay with both fans and the shroud and wiring for around $99. Or even rockauto.com has the same assembly for around $160.
 

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Discussion Starter · #19 ·
Pulled the fans out today and found the drivers side unit to be pretty well frozen. So ends the saga.

I think I replace the A/C and serpentine belt as long as I have the fans out. Any other suggestions? With 59K, I don't think it makes too much sense to do the timing belt right now, but I'm willing to listen.
 

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Chief Brody said:
Pulled the fans out today and found the drivers side unit to be pretty well frozen. So ends the saga.

I think I replace the A/C and serpentine belt as long as I have the fans out. Any other suggestions? With 59K, I don't think it makes too much sense to do the timing belt right now, but I'm willing to listen.
I thinj you can definitely hold off on the timing belt. The manual says 110k, so its probalby good for 120k. I personally wouldn't wait any longer than 100k though. I plan on doing mine next spring when I should have about 88k on it jsut to be on the safe side. I would do the A/C & serp belts though. I did mine around 65k and while I was at it I flushed the coolant as well. I know its not really related to having the fans out, but have you changed the diff fluid because if not, now would be a good time for that too.
 
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