Air conditioner question

[fritzimax]

I have an AC question, in one of my previous post I had mentioned my AC has not been working properly now for two years. I have brought the truck to my mechanic and he and I have tried several different solutions but nothing has worked. AC pressure is fine. I have changed the pressure switch 2x, changed the thermistor on the evaporator, changed the AC clutch relay, changed the power distribution center.

As it turns out the AC clutch relay is not getting a ground signal. If I temporarily ground the relay the AC will work, sometimes it will stay working for several minutes. Then other times it won't.

My question, if I were to put in a switch and manually ground the AC relay when I need AC will I do damage to my system? I don't want another summer with no AC. Since no one can fix my system I don't know what else I can do. I am just afraid since when I manually ground the relay the pressure switch and all other system controls will be bypassed and then the compressor will run constantly until I turn the switch off. Will the system over pressurize, will anything burst, will the compressor burn out? I know my fuel economy will suffer but I could deal with that just to stay cool.

I thank all who reply.

 

[BDaugherty]

if you really want to bypass the factory controls, swap in a trinary switch in place of the pressure transducer and add a thermostat to switch the compressor on and off. power can come from a relay and ground goes through the tstat and psi switch in series.
or find someone who can fix your a/c the way it is.

 

[Bob4x4]

As long as it is not an intermitant problem the fix should not be that difficult. A scanner is key to see what is going on in almost all systems these days without spending way too much time chasing simple issues. The common feed terminal (30) receives a battery current input from a fuse in the IPM through a fused B(+) circuit at all times.
The coil ground terminal (86) receives a ground input from the PCM/ECM through the A/C compressor clutch relay control circuit only when the PCM/ECM electronically pulls the control circuit to ground.
The coil battery terminal (85) receives a battery current input from PTC 1 in the IPM through a fused ignition switch output (run) circuit only when the ignition switch is in the On position.
The normally open terminal (87) provides a battery current output to the compressor clutch coil through the A/C compressor clutch relay output circuit only when the A/C compressor clutch relay coil is energized.
The normally closed terminal (87A) is not connected to any circuit in this application, but provides a battery current output only when the A/C compressor clutch relay coil is de-energized.

 

[RVTRKN]

I have never worked on either of my 3rd Gen AC systems, but I replaced my evaporator on my 94 2nd Gen gasser, and the low-pressure switch is the cycle control on that system. The reason for my evaporator failure was due to the LP switch not opening at the specific pressure and froze up my coil which expanded inside the evaporator and split it open. Bob on the 3rd and 4th Gen trucks is it not the LP switch that controls the compressor, by opening the circuit to the clutch? My theory (guess ) is that the grounded circuit tells the system its OK to cool and the LP switch opens and closes at the specific pressure's maintaining the evaporator temperature.

 

[Bob4x4]

The pressure switches are in series but rarely fail. Not all 3rd and 4th gen ckts are the same and the factory wiring diagrams are not always correct.

 

[RVTRKN]

So does the LP control the clutch for pressure/temperature in the evaporator or is it a sensor that relays the temperature to the ECM which cycles the compressor?

 

[RVTRKN]

The reason I ask is that the OP can ground the relay that allows the system to maintain temperature through the LP switch.

 

[RVTRKN]

Or to put it in other terms, what is the sequence of operation?

 

[mweiman]

Fritz are you saying that when you ground the relay it may or may not work and maybe only for a short time? If the compressor clutch is a direct feed from the relay then it should come on unless a wiring problem or clutch problem. Is the compressor original? Bob would an overcurrent on the clutch coil cause the pcm to open the relay ground? I had that on an 06 but not sure if the wiring/controls are different.
Matt

 

[Bob4x4]

Fritz are you saying that when you ground the relay it may or may not work and maybe only for a short time? If the compressor clutch is a direct feed from the relay then it should come on unless a wiring problem or clutch problem. Is the compressor original? Bob would an overcurrent on the clutch coil cause the pcm to open the relay ground? I had that on an 06 but not sure if the wiring/controls are different.
Matt

On the 06 the tipm is the protection from an overcurrent situation of the clutch

 

[Bob4x4]

The pressure switches are inputs to the pcm

 

[RVTRKN]

What I'm getting at, is what regulates the temperature, are the LP & HP safeties only?

 

[Bob4x4]

The pcm calculates all the inputs... ... ... ... The A/C system on models so equipped is designed for the use of non-CFC, R-134a refrigerant and uses an A/C fixed orifice tube located in the liquid line to meter the flow of refrigerant to the A/C evaporator. The A/C evaporator cools and dehumidifies the incoming air prior to blending it with the heated air. To maintain minimum evaporator temperature and prevent evaporator freezing, an evaporator temperature sensor is used to supply evaporator temperature input to the A/C-heater control. The powertrain control module (PCM) cycles the A/C compressor clutch off and on as necessary to protect the A/C system from evaporator freezing and optimize A/C system performance

 

[mweiman]

Bob are all the pressure switches and dash control inputs to the PCM and the relay is essentially direct control for the compressor clutch based on the PCM ground signal(Never mind I was typing as you were answering this question)? If so can dealer software view the inputs to the PCM? Is it possible to see what evap temp sensor is reading, as in it could be telling the pcm it is -10 degrees or something.
Matt

 

[B.G. Smith]

What I'm getting at, is what regulates the temperature, are the LP & HP safeties only?

I believe when the system is operating correctly the compressor/evaporator will operate to the maximum that it is capable of and temperature controll will be by the blend door allowing air to pass through the heater core.

 

[fritzimax]

When I manually ground the AC compressor relay the compressor will come on, once I let go of the ground that's when it sometimes will continue to run and sometimes when I remove the ground the compressor stops. Today when I went out in the truck I turned the AC on normally and it worked but only for a few minutes.
My mechanic thought it may be an over voltage problem , I think some of the 06 and later have had this issue. To address this the stock compressor relay is now only used to trigger a second relay for the compressor clutch thus the PCM only sees the voltage to trip the second relay and no over voltage. Probably not the right way to go but it was worth a try and I have read that it worked for others. It has not worked for me.
Another question, is there an ambient temperature sensor that may be telling the AC not to come on and if so does it also send the temperature reading to the overhead display? My overhead temp display is working fine though.

 

[fritzimax]

Well it has been a long time coming but finally my AC is fixed.
After three summers of no air conditioning on my truck (once the engine warms up) on advice of my mechanic I sent my ECM to a place in Massachusetts to have it tested. I received my ECM back and no faults were found. My mechanic still insisted the ECM was at fault so I purchased a refurbished ECM from a place in Florida and guess what my air conditioning works once again. The ECM sent to me was preprogramed and was plug and play. I contacted the company who tested my ECM in Massachusetts and they issued me a refund for my test fee.
As it turns out there is a relay in the ECM that sends the ground signal to the AC clutch relay, once the engine/ECM warmed up the relay would fail. I am happy to have a functioning AC again (though I don't need it this time of year) but it is not without a small issue, with the refurbished ECM installed I get error code P0602. I hade the code cleared but it comes right back. I have driven the truck for over a week now and everything runs fine, actually I think the truck runs better now than before the swap. I have done a search and many people tend to say not to worry about the P0602 code it is common with an ECM swap or a reflash, I hope they are correct.
Happy Holidays to all.

 

[sag2]

From Star case.

Case Number K13104246
Vehicle Issue P0602 Typically there is a driveability concern such as low power, hard start or no start concern, etc...;The ECM may have been recently flash updated and / or replaced.
System or Component DTC / Error Message;ECM
Recommendation P0602 is generally an erroneous fault code, disregard the P0602 fault. Perform fuel system flow tests, reference TSB 14-03-06 revision A to diagnose the original engine performance problem.