A/C High Pressure Switch

The a/c high pressure switch and or wiring is most likely the cause of the issue.you should do a pull test on all of the wires going to the switch and ecm and do a continuity test from the ecm to the switch.

When this circuit goes OPEN it signals the ecm to turn on the engine fan.

It is best to monitor the system with CAT ET and it will let you know exactly what is signalling the ecm to turn on the fan, be it the a/c switch or a sensor going out of range. The intake air temp,fuel temp or coolant temp sensor can all make the fan come on.....

There could also be an issue with the Driver in the ECM.. i have seen a # of issues where only the replacement of the ecm corrected the issue.

I would highly discourage bypassing the system and repair it properly...

 

Thanks for posting that. I have had an issue with my engine fan for over a year now and tossed various parts at it in attempts to fix it as no-one seemed to be able to figure it out. Like you said, and as I have been told, and seen for myself it could be a myriad of things wrong. You just have to check everything. But tossing parts at it gets expensive even if they are only $50 or $75 fan switches, etc. As it is I have the wire harness to the high press switch unhooked and have the end of that harness jumpered with a paperclip and taped up. My engine fan will only come on in manual. So obviously I keep a very close eye on my water temps using it as needed, which isnt a problem because water temp is something I constantly monitor anyways, but it's really importanat for me to turn the fan on manual if I am parked idling with the defrost or a/c on so I don;t destroy the ac. Now that is a little annoying but like I say I have been running it like this for over a year now. Now I know I need to get the laptop hooked to it at some point so we can see what is going on, thanks!

 

We run 8 of these trucks and have run into the ECM problem also. We have found that nothing was wrong and put a spare ECM and it solved the issue.

Have you checked the head pressure on your A/C?? Is it getting to the point of flipping the high psi switch??

 

One thing i forgot to mention is that the A/C system can also cause the fan to act up. the TXV ( expansion valve on the heater box) can stick closed and keep the high side pressure up signalling the switch to open and keep the fan on. the a/c will run on defrost as well as a/c. a quick test of this is to insert a jumper in the binary switch and it can take up to 2 mins for the fan to kick off.. if the jumper solves the problem then you know its the switch itself or a pressure issue..... hope this helps

 

I replaced everything on the ac system before this problem popped up. The compressor was shot. The lines were blown out and new expansion valves were installed at that time. Maybe one of them is sticking, I dunno. So apparently you're thinking it's a pressure issue due to the jumper solving it, well, question is how many high pressure switches do I need in order to fix this problem, I'm on number two. Perhaps it is an expansion valve. It just started coming on one day for no reason at all while driving down the highway. Water temps were fine, I never let those get much over 200* anyways. I do know that the ECM parameters are such that the engine fan will automatically come on if the water temp gets to 217* but mine never gets that high.

 

I do hope it is not a bad ECM situation as my ECM flash is definitely not something CAT or a dealer would ever put in there and I'd hate to have to buy another one new or used then have to spend to get it made right again.

 

If it is a bad ECM...Heaven forbid...be sure to try to have your files downloaded to a hard source. Does whoever did you program save your files? I would do some checking to just have a plan B if you do need an ECM. Just be sure you have it backed up somewhere so if you have to get an ECM if will not be such a choir to get back to where you are.

 

Yes, Mr Haney did my ecm and has it saved. I almost forgot to mention. I did replace a solinoid on the firewall as recommended by some guys in this forum as a possible cause to my engine fan running all the time problem. That did fix it for a short time but the problem came back. That was just like the high pressure switch I have swapped out twice, it was a $100 toss a part at it kind of deal. These little switches and solinoids will eat your wallet alive just changing parts willy nilly that's for sure. I just want to get this thing properly diagnosed and this thread looks like the best advice I have seen so far.