Tuning fans to AC pressure on 99-02 trucks.

[CashDude]

Hey guys, I've seen many posts explaining how to change pin 33 from recirculation door to fan stage 2 control.

But, no one explains how to get the PCM to see the AC pressure. Is it possible to add an AC pressure sensor into the 99-02 trucks? My 2000 only has the basic on/off high side and low side switches.

Apparently, the pressure sensor on the 03-06 trucks looks like this.

http://www2.partstrain.com/store/?N=0&Nr=AND(make:Chevrolet,model:Silverado+1500,year:20 08,brand:OES+Genuine,sku:W0133-1800126,part:A%2FC+Pressure+Switch)&ptc=S1171391678W45d204bec356b&ptid=pt16128639474a062fe13a27e

Has anyone set up a 99-02 to run the fans based on AC pressure? Or does everyone just use pin 33 to activate fan stage 2 when the coolant reaches a higher temp than stage 1?

When I scan my truck, the AC pressure always shows 0, so I don't think my truck has an AC pressure sensor. Thanks for any help. This should be great info for any 99-02 truck owners. Let's hope we can make this work. :rockon:

[nonnieselman]

ive wondered this myself.
i have manual fan switchs in my cab incase i turn my AC on when the ECT is cool so i know i get flow over the Condenser

[Mr. P.]

My buddy and I ran across this very problem with our 5.3L swap in his '02 S-10. The AC system in the 03+ trucks (we were using truck wiring harness) uses a pressure sensor and reports this voltage to the PCM; the pre-03 trucks use a simple range switch.

In other words, pre-03 trucks when you turn the AC switch on inside the cab, a light blue wire goes hot underhood (AC Request Signal) and this wire has to pass through the high-side safety switch; if the A/C pressure is too high or too low then the AC Request signal is interrupted, and the relay engaging the compressor clutch is turned off. The PCM is reading the AC Request Signal too, so it knows that the compressor is on or off, but only AFTER this voltage has passed through the high-side switch and turned the compressor clutch on.

In 03+ trucks Delphi changed-up the climate control strategy - turning on a switch in the cab turns on the AC Request Signal (like before) but this goes straight to the PCM, and the PCM has direct control of the AC Compressor Relay; the PCM also gets a real-time pressure reading from the system high-side (via a sensor) so it knows how hard the compressor is working, the heat-load on the cab, if the freon is low, and it determines when to engage the AC compressor. This is why on 03+ trucks the AC compressor is disengaged under WOT (and other conditions the PCM deems necessary).

This is my assumption my impression is that the direct control of the AC Compressor Relay & input from the AC high pressure sensor are a couple of the big changes in the PCMs, you can't just install a pressure sensor in place of a safety switch and expect the older PCM to work, even if you reflash the PCM. Again that is my impression, someone more expert than I will have to corroborate that.

What we did, I created a relay setup to kick the fans on high when the PCM Fan 1 (pin 42) setpoint was reached, and I wired the low efan relay to come on with the AC compressor clutch relay. It's not ideal I know.

Can you provide some links or search that I can use to get the older PCM to operate the fans on both pin 33 and pin 42?? I would really like to study & revist that again on the S-10.

Mr. P. :)

[Chevy366]

Look here and see if it helps : http://www.lt1swap.com/2000harness.htm, there is a section on 99-02 PCM pinout and 04 pinout , I think there is a way .
Here are some more : http://www.ls2.com/boggs/torques/00pinpcm.htm , http://forums.hybridz.org/showthread.php?t=95493