2nd Gen Non-Engine/Transmission Bypass pcm in order to get a/c to work???

[Bunyan]

Several years ago I read a thread that someone post, (due to a malfunction in the pcm) they bypassed the pcm for the a/c to work. Does anyone know what wires need to be connected for this procedure.

 

[Missouri Mule]

Seems to me all you would have to do is unplug the low pressure switch connector annd jump it. Of course it could be the high pressure switch keeping it from working or a number of other things.



The PCM supplies the signal to the low pressure switch which then turns on the a/c clutch as I recall.

 

[Bob4x4]

Irrc the pcm just grounds the ac clutch relay. The hi and low pressure switches are in series and a pcm input

 

[Bob4x4]

I can't recall ever coming across a pcm failure to not switch the relay but it could happen

 

[Bunyan]

Thanks guys. I replaced everything in the A/c system. Sometimes the ac will cool and sometime it wont. Some on here said they their pcm was not allowing the signal to swith the ac on. So they bypassed the pcm and this is what I'm trying to do. I have the service manuals and I don't now which wire to jump and connect together. The member said when he did this and you turned the ac on, the ac worked even when heavy acceleration was needed. Under normal operation when you pressed the accelerator for passing or speeding up the ac would shut off and once the heavy acceleration was done the ac would engage again.

 

[Missouri Mule]

From 96 service manual.

Low pressure cycling clutch swtch. The switch is connected in series with the A/C request signal circuit, between the heater A/C mode control switch and PCM. The switch contacts open and close causing the PCM to turn the compressor on and off.



The PCM grounds the coil side of the compressor clutch relay.



The compressor clutch is controlled by several components: heater- A/C mode control switch,the low/high pressure cut off switch,the fin sensing cycling clutch switch, and the compressor clutch relay.



You have newer truck but should be about the same.

 

[Bob4x4]

It depends on what is not functioning on what you need to fix. Bypassing control ckts is never a gooD idea

 

[fest3er]

Years ago, the A/C shop ran a wire from the A/C relay in the PDC to the low pressure switch. The only purpose for running it through the PCM is to allow the PCM to disengage A/C under full throttle. The Cummins diesel doesn't notice the slight power consumption of the compressor. I've never had a similar problem with the A/C since. The evap has leaked since '98 or so, but that's not an electrical issue.

But first be sure that's the problem. Next time it doesn't work, jumper B+ to the PCM side of the low pressure cutoff switch. If the compressor kicks on and then shuts off when you disconnect B+, then bypassing the PCM is the solution.

 

[Bunyan]

Years ago, the A/C shop ran a wire from the A/C relay in the PDC to the low pressure switch. The only purpose for running it through the PCM is to allow the PCM to disengage A/C under full throttle. The Cummins diesel doesn't notice the slight power consumption of the compressor. I've never had a similar problem with the A/C since. The evap has leaked since '98 or so, but that's not an electrical issue.



But first be sure that's the problem. Next time it doesn't work, jumper B+ to the PCM side of the low pressure cutoff switch. If the compressor kicks on and then shuts off when you disconnect B+, then bypassing the PCM is the solution.

Thanks fest3er. This is what I'm looking for. Every component has been replaced. So I think it's a the pcm malfunction. No leaks and still intermitten cooling. I'll give this a try and see what happens. Thanks to all. Bunyan