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Pcm Tuning For Efans And A/C Climate Control


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This is very important follow-up information for those working on their cooling systems, and/or replacing their clutch fans with electric fans - in short:

 

YOU MUST MAKE SURE WHEN THE A/C IS "ON", AIR IS MOVING THROUGH THE CONDENSER/RADIATOR - IF NOT, THE A/C COMPRESSOR WILL VENT REFRIGERANT AND THEN SEIZE - THIS WILL REQUIRE MAJOR SERVICE AND EXPENSE TO REPAIR.

 

The A/C compressor in contemporary vehicle climate control systems is active much of the time, more than just when you desire cold air - the A/C is also "on" when defrosting windows in the winter, because A/C pulls the moisture out of the vehicle cab. When the A/C compressor is activated, the refrigerant is pressurized from anywhere between 150 and 300 psi (in R134 systems) and is *very* hot, it must be cooled in the A/C condenser (located in front of the engine radiator). When the A/C refrigerant is not cooled it will over-pressurize until either (1) the built-in safety valve on the compressor vents-off the excess pressure to the atmosphere, and/or (2) one of the seals or hoses of the A/C system bursts. When the A/C system is overheated and pushed to ultimate duress, the A/C compressor can seize or burn-up; if you are fortunate enough to catch the problem as it is happening you might be able to disable the A/C before causing permanent damage the compressor and rest of the system.

 

Depending on how your aftermarket electric fans are controlled, you have two options to make sure they will move air when the A/C is operating.

 

Option #1 - triggering them off the A/C compressor relay

For trucks that use a pre-2003 PCM, you must wire the efans so that, in addition to being triggered by high engine temperatures, they are additionally triggered to turn on when the A/C compressor is active - one way to easily achieve this is to add a diode, relay, and length of wire to join the efan trigger wire to the A/C compressor relay trigger wire.

 

Option #2 - program the PCM to turn on the Efans when the A/C is on

If you have a 2003+ truck, not only does it have a PCM with logic to fully control efan operation but also a pressure sensor on the A/C system - so the PCM is very aware of exactly what the pressure in the A/C system is at all times; this running pressure information is used to trigger the operation of the efans, and you can instruct the PCM exactly what pressures will turn the fans on & off using EFILive or HPTuners tuning software. I HIGHLY recommend programming the A/C pressure set-points in the vehicle's PCM to operate the efans when the PCM supports it.

 

Limitations of Some Tuning Software Packages

Make sure your tuning software allows you to input not only the desired engine coolant temperature 'set points', but ALSO the A/C system pressure set-points as well - this will guarantee that the efans are running when the A/C is on no matter how hot/cold the engine is. I took a phone call from a customer that suffered an expensive setback with his truck, he purchased and properly installed a Torrent E-Fan kit on his 2004 Silverado SS, and then took his truck to a local performance shop to program the PCM to operate the efans; the failure was that the tuner was using an older copy of HPTuners, an earlier version of the software that does not support updating the A/C pressure set-points in the PCM - the result was that the efans would turn on when the engine got hot, but not always when the A/C compressor was running and this required complete A/C system repair when his compressor burned-up.

 

The EFILive software tuning package has always fully supported efan programming; only recent versions of HPTuners (versions released after approximately April 2010?) have this capability. No matter what software package, hand-held programmer, or professional tuner that you employ BE SURE that the software used to enable PCM control of the efans allows you to enter the A/C pressure set-point values (in addition to the engine coolant temperature set-points) - THIS IS CRITICAL, if you do not have the capability or software to do this then you must work around the issue by hard-wiring the fans to run when the A/C compressor is active, or hard-wire them to run all the time, or program an artificially very-low engine temperature set-point (like -20 degrees) so that the PCM runs the fans at all times.

 

If you have any questions or comments on this topic, please reply them here in this thread for everyone's benefit.

 

Thanks -

Steve Poythress (Mr. P. :))

254-413-3733

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YOU MUST MAKE SURE WHEN THE A/C IS "ON", AIR IS MOVING THROUGH THE CONDENSER/RADIATOR - IF NOT, THE A/C COMPRESSOR WILL VENT REFRIGERANT AND THEN SEIZE - THIS WILL REQUIRE MAJOR SERVICE AND EXPENSE TO REPAIR.

 

 

 

Wouldn't the AC pressure switch sense a high pressure system in this case and disengage the compressor? I thought the safety blow-off valve was secondary protection in the event of a pressure switch failure.

 

Regardless, the fans should be on whenever the A/C is on.

Edited by Krambo
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Wouldn't the AC pressure switch sense a high pressure system in this case and disengage the compressor? I thought the safety blow-off valve was secondary protection in the event of a pressure switch failure.

 

Regardless, the fans should be on whenever the A/C is on.

That would be a great idea for Delphi to include in their PCM/ECM OS's - but they didn't, there is no software pressure over-protection logic in the PCM. :( I have had one member call me after installing GM fans, he was freaking because his R134 had vented all over the front of his motor; in that situation no permanent damage happened, he re-wired his relays and added walmart R134 and the truck was fine. In this second case, the person lost his A/C compressor completely.

 

Mr. P.

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That would be a great idea for Delphi to include in their PCM/ECM OS's - but they didn't, there is no software pressure over-protection logic in the PCM.

 

 

Yea, I am aware; however my comment is not PCM / software related. The high pressure switch, when activated at a certain psi, simply cuts power to the compressor preventing an overpressure situation and the compressor damage / loss of refrigerant you described. It is independent of the PCM and even the A/C controls in the cab. I have a hard time believing GM would design a system that doesn't have this kind of a very simple safety mechanism built in. I'll admit, I have very limited experience with AC however do have a working knowledge of the science behind it and what I described just seems too simple not to be engineered into the system.

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Yea, I am aware; however my comment is not PCM / software related. The high pressure switch, when activated at a certain psi, simply cuts power to the compressor preventing an overpressure situation and the compressor damage / loss of refrigerant you described. It is independent of the PCM and even the A/C controls in the cab. I have a hard time believing GM would design a system that doesn't have this kind of a very simple safety mechanism built in. I'll admit, I have very limited experience with AC however do have a working knowledge of the science behind it and what I described just seems too simple not to be engineered into the system.

 

 

I have no idea why GM didn't as you are 100% correct it would be too easy... Yet I have seen this happen in my own driveway with another members truck........

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so a wiring question: can I use my nelson harness, and run my AC wire to pin 33 of the PCM?or do I need to change the harness?

 

 

here guy is saying to remove it from the AC switch and connect it to the Pin and then thru your tuning software change the settings?

http://www.performancetrucks.net/forums/showthread.php?t=432250&highlight=2+pin+fan+harness&page=4

 

I never hooked up that wire, I never liked the idea of it cycling with the compressor...just might have to connect the pin 33 if it is doable since I can change the fan settings.

 

thanks

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I cannot comment, because I have never seen a Nelson harness - I know they are designed very different from the OEM 'series/parallel' setup.

 

- Mr. P.

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IMO connecting it to pin 33 is the way to go. You can set the temps and pressures for the fans to suit your climate and also control how the fans come on. My setup doesn't cycle with the compressor with Ac on or defrost on. It cycles when the pressure or temp reaches the set point that I tuned into the PCM. I also set delays between the two fans to take the initial load off the relays and charging system. This helped with voltage drop with stereo and HIDS on at night at idle alot.

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so a wiring question: can I use my nelson harness, and run my AC wire to pin 33 of the PCM?or do I need to change the harness?

 

 

here guy is saying to remove it from the AC switch and connect it to the Pin and then thru your tuning software change the settings?

http://www.performan...+harness&page=4

 

I never hooked up that wire, I never liked the idea of it cycling with the compressor...just might have to connect the pin 33 if it is doable since I can change the fan settings.

 

thanks

 

 

You have to rewire the nelson harness to work with the PCM. The A/C trigger wire is a 12v reference and if you pin it to the PCM you might end up frying the Efan circuits in the PCM. So, might have to rewire the trigger relay so that it turns into a ground trigger wire, then you can run the wire to the PCM

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Steve, it has three relays, one triggered by the PCM for low speed (6v) and the other is triggered by a 12v source, giving 12v to the fans running them at 'high.' I think both fans run all the time but would need to double check.

 

So on the relay that triggers the high speed I would change it to trigger negative. I wonder if it would change any of the other wires, I'll print both diagrams and look at them next to each other.

 

(when) I pull it off i'll change the wires and bench test it so I dont fry my PCM

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From The Manual - The Nelson Performance harness does use 3 relays, but IS NOT designed this way:

 

During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low

speed relay trigger wire (green wire); this closes the low speed fan relay contacts, and positive battery voltage is

supplied from the low fan fuse to the left cooling fan - the return path for the left cooling fan is through the

single-pole relay and thusly the right-hand cooling fan, the result of which is a series circuit of doubled

resistance and both fans sharing 12-volt power and running at low speed.

 

During high speed operation, the PCM also supplies a ground path for the high speed fan relay via the blue

relay trigger wire (in addition to the low-speed green trigger wire). When all three relays are energized a

ground path is provided for the left cooling fan, as well as battery positive voltage to the right cooling fan – thus

during high speed operation both engine cooling fans have their own ground path, resulting in a parallel 12-volt

circuit with both fans running at maximum speed.

 

 

How the PCMs pins 33 and 42 work, keep this in mind when designing your own efan control harness:

 

...turning on the efans is very, very simple – just ground the green wire and the efans

operate at low speed; ground both the green and blue wires together and the efans run at high speed. Keep in

mind that the efans operate completely independent of any other electrical system in the vehicle since they are

on their own circuit; even if the ignition key is off and removed from the steering column the efans will

continue to run if the green and/or blue wires are grounded.

 

The PCM does not supply 12v + to the relays; exactly the opposite, it grounds relays. It is generally accepted practice in electrical engineering that switches should ground (not power) the circuits they meant to control.

 

A good starting-point is the GM schematic for the f-body fan control relays.

 

Mr. P.

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I have no idea why GM didn't as you are 100% correct it would be too easy... Yet I have seen this happen in my own driveway with another members truck........

mine.that definetly made us all say wtf, which is why i have a switch to turn on my fan on high on the passenger side

Edited by ss wrecker
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