Strange A/C problem

[RodeDawg]

2003 Ram 3500: Of course this happens the day before a trip. The A/C compressor comes on and the air gets cold if the switch is set to Defrost or Defrost/Feet. But if I turn it to Middle Vent or Middle Vent/Feet the A/C compressor goes off. The light says the A/C is still on but the compressor turns off. So, the compressor is working. Something is telling the compressor to turn off. But it works in defrost mode. Any idea what it could be?

 

[bighammer]

Wonder if it's electrical... The A/C is supposed to function in defrost modes automatically, so I'm wondering if your A/C switch is bad and not kicking things on like it should.

 

[RodeDawg]

Wonder if it's electrical... The A/C is supposed to function in defrost modes automatically, so I'm wondering if your A/C switch is bad and not kicking things on like it should.

That's what I was wondering as well. But electrical has always confused me. Someone mentioned the low pressure switch. Using a freon can with a gauge, the pressure is in the green at about 30 PSI (I know, those cans are notoriously inaccurate). So, maybe it's a bad low pressure switch - wherever that is.

 

[BigPapa]

From the looks of the wiring diagram, compressor control runs through every module on the truck. The one that would scare me is the TIPM. (Tuesdak corrected me, it's an IPM, not a TIPM) Those things go through a process to possess them with demons at the factory....

It starts in the A/C Heater Control. There's an "HVAC Micro Controller" inside of it. Then it goes to the IPM which has a Condenser Fan Relay and an A/C Compressor Clutch Relay. The Clutch Relay is signaled by the ECM which gets inputs from the A/C Pressure Transducer.

 

[Tuesdak]

Pull batteries and see if it resets. 2003 was before TIPM…

 

[BigPapa]

Pull batteries and see if it resets. 2003 was before TIPM…

Gotcha. Wiring diagram says "Integrated Power Module" and I guess I put "Totally" in front of it on my own. I better stick to my old 2nd Gen 12-valves....

 

[RodeDawg]

Pull batteries and see if it resets. 2003 was before TIPM…

I'll give that a try. Thanks.

 

[Tuesdak]

If that doesn’t work it acts like the A/C button isn’t working but the other A/C switches for defrost are. Maybe put it in normal vent, pull the mode door wire off, then use defrost ro run the A/C till your trip is done.

 

[Ozymandias]

Check the evaporator inside the cab for debris, pluging it.
We have seen very bad ones in the past.

It's relatively simple to do, just take out the fan, 3 screws, and then use your mobile phones camera to reach in and do a video. So you can see what's going on in there.

 

[RodeDawg]

Thanks everyone. Figured it out. A new double din head unit was installed in the winter. Apparently all that messing around in there knocked a wire loose on the AC controls (I'm assuming). I went into the dash and simply inspected, checked and tightened connections on the AC controls and put it back together. I'm 1200 miles into a trip and the AC has been working fine. Thanks again.

 

[Christopher McKenna]

Having diagnosed the controllers in the past, there were defects involving P2, the smaller input control side connector - not the high current blower motor mains connector. Remove controller from vehicle and open the injection molded snap shell. I'd do this on an ESD mat tied to a 1M Ohm resistor to ground or, simply buy a mat with an ESD strap already configured and wired ready to go. This will avoid inducing any ESD latent field failures later on at the chip level.

Remove the knobs on the front, remove the CCAs / PCBA ASSY and inspect at P2 for missing solder at the pins. My 2005 controller was missing 100% of the solder across the entire connector pinout thus, it was doing exactly as yours was doing plus going into A/C mode vs heat mode in dead of winter. These were notorious for defects in the workmanship. Check the interconnects as well at the wire harness level and use a chemical cleaner specifically called out for the connector type that will not damage plastic molex type connectors. Some are so aggressive that they will damage plastics and molded components.

Also check for high-resistance on the controller side breakout of the wire harness feeding the control side of controller. Clean those connections as well. They tend to be cheaply made and oxide formation builds up thus, high resistance / impedance is seen thus, the controller will malfunction as a result.

I used to reset mine all the time and then it would crap out repeatedly over time.

Regards,
CM

 

[Christopher McKenna]

In supporting another thread on here regarding HVAC / Thermistor Assy related issues, I generated a very simple rework document for that thread. I am uploading here for convenience for those who come across this particular thread so that they'll have access to it here as well.

Regards,
CM