Atomic

iTrader: (42)

My truck is a bit of a hybrid in the AC system. Its a 99 body with 05 fans and an 02 computer (DBC) with a something year AC hoses since I put it back in piecing from parts from rockauto. The AC lines do not have a pressure sensor on them, but my PCM has AC pressure fan settings and high/low control. I use a harness I made that runs the fans in series on low (half speed) and in parallel on high (full speed), which is the same as the factory harness.

I like to be able to turn the fans on high even with the truck off with an LED switch mounted in the cab. I like the LED because it is an easy indicator while driving when I cant hear the fans, but most switches like these have diodes so you have to wire it a certain way which is troublesome because the fans are ground controlled not power controlled. The first relay reverses this polarity, and the second relay will turn on the low speed circuit if the high speed circuit is on. It will also not backfeed (low circuit turn on high circuit) when the PCM commands the low speed fans on. I figured it would be cleaner and better to do this with relays instead of diodes. Basically the switch will force both fans on high no matter what the PCM is telling it to do, and the fans will do still what the PCM commands when the switch is off. I could make it fancier by having manual high and low speed control with a DPDT switch, but I don't really need that.

My question is really with the AC circuit. I want both fans to come on high with the AC, but since by truck doesn't have a pressure sensor I figured the clutch activation would be the easiest way to do that. Question is, is the clutch ground activated or power activated?

The other way I thought to do this was tap into the AC pressure pin on the PCM with one of the relay outputs and set the enable pressure to 0 (or whatever it needed to be if grounded), but I'm not sure if that is ground activated or power activated. This may get rid of the last relay if its the right polarity. Anyone have a diagram for the pressure sensor circuit?

tim wellington

my 01 has 05 fans .was done that way when i got it but my fans go to high when ac is on has 3 relays and 2 signal wires tap into computer

Atomic

iTrader: (42)

Does one of your signal wires tap into the AC clutch?

That's how they are typically done on the earlier trucks; 1 wire turns on both fans on low, and the second wire turns on by the AC and turns both on high.

George C....

99 truck is a bastard...its wiring is unlike any other year..all manuals will tell ya this

Atomic

iTrader: (42)

And to make it even more fun I am using a 2002 PCM, but the digrams I found for the truck it came from say it is a DBW truck, but mine is a DBC, so...

tim wellington

yes ,break the ground on relay that control low .then engages relay for high speed i have toggle switch on another relay so i run my fan s on high at track

Atomic

iTrader: (42)

Well I found what I was looking for if anyone finds this thread and needs it,



However it looks like hptuners added some tables I overlooked, and this one looks like what I need to get it to work without me dicking with a relay for the AC.



Doh.

Atomic

iTrader: (42)

Well that setting didnt work. The PCM sees the AC Clutch enabled, and AC Requested, but never says the AC Active is always no so not sure what that means.

I can get the fans to come on with the AC by setting the enable pressure to 0, but then they will never turn off once they are enabled until I turn the truck off.

So options are to either add the pressure sensor, or use a relay above. I think Ill go the relay route.

Wolftrk99

iTrader: (5)

If you dont have a pressure sensor in the line its not going to come on or go off as the pcm is seeing 0 psi, too bad there isnt a way to run it off the pressure sensor on the compressor or the low side switch.. Instead of the condensor pressure switch

Atomic

iTrader: (42)

It will come on, but it will never go off, so not really that useful.

The way I understand this system, once its enabled and it stays above the low switch on the accumulator and below the high pressure switch on the compressor the compressor clutch stays engaged.

Well looking at the wiring diagrams, on the later trucks that use the AC Pressure sensor the pin that it uses on the PCMs is not used on the earlier PCMs (Pin #14 on the Red/Green connector). The tables are there in the tune, so it looks like it could be added, but this is just fan settings not compressor cycling. There looks to be a test port on the high side line near the condensor, but not sure how the sensor is supposed to attach.