LQ4-E39

iTrader: (7)

ok, I am somewhat confused on the functionality of the pressure switch/sensor, specific to a 2003 Express Van PCM. all the literature i've read on here and most other sites regarding LS swaps refers to just one pressure switch in the system, with a 3-wire connector. However, when I looked up the wiring diagram for HVAC for the '03 van, it calls out 2 pressure switches, a high and a low.

Diagram taken from Chilton manual on the local library website:


Here is my question:

^Above diagram was taken from a Chilton wiring diagram for a 2003 Express Van and calls for a high pressure switch, and a low pressure switch.

I am using a 2003 Express Van PCM, and the LT1Swap Diagram for the 2004 Vortec that I used flawlessly to wire the whole swap calls for a 5V reference, a pressure SENSOR signal, and a low reference (omitting the Clutch relay control which obviously is similar between the two)
http://lt1swap.com/2004vortec_pcm.htm

However if I go back and look at the LT1swap site for a 99-02 truck, it's again calling for a high/low switch signal.
http://lt1swap.com/99-02_vortec_pcm.htm

So the question: who's right? Is the '03 Van wired like a 99-02 truck, or like a 04+ Vortec (truck) ? I don't want to hook up the switch signals to the wrong pins on the PCM. Obviously there was a lot of changeover going on at GM at this time between the A/C pressure stuff as well as the DBC -> DBW changeover.

Thanks in advance!

LQ4-E39

iTrader: (7)

also, what's going on at pin 17 at C2? is it looking for A/C request signal or the high pressure switch signal?

aknovaman

iTrader: (7)

They are both correct. The silverado monitors the pressure (Variable volts out proportionate to pressure)and turns the compressor off and in based upon the pcm file settings. The express does it with open and closed switch.

aknovaman

iTrader: (7)

Pm me your phone if you want to discuss.

LQ4-E39

iTrader: (7)

PM sent.

ls1nova71

iTrader: (4)

Vans are wired up just like the 99-02 trucks, the C2-17 gets its power through the high pressure switch on the back of the compressor, that power comes from the 12 request wire (switch on dash).

LQ4-E39

iTrader: (7)

^ That's what I needed to hear. I guess the LT1swap site is wrong.

ls1nova71

iTrader: (4)

LT1Swap doesn't really go into detail on how to retain the A/C, but more on what can be safely removed from the harness, which is correct from what I see. What part is it that you think is wrong?

LQ4-E39

iTrader: (7)

I have read from many sources that the vans use separate high/low pressure switches, but like you said the LT1swap site specs out a SENSOR, obviously incompatible. I'm just trying to find out which setup is correct, and have yet to find a Van-specific PCM pinout that shows the same pinout as the 99-02 (at least for the A/C signals).

Of course, this site is probably referring to 04 Vortec TRUCKS, which are similar in engine wiring but different in A/C wiring.
http://lt1swap.com/2004vortec_pcm.htm

LQ4-E39

iTrader: (7)

I get that the switch is inline with the A/C request - does it get a ground at all, or is it strictly:

A/C request wire (from IHKA HVAC panel) into side 1 of switch, then side 2 of switch back to PCM

How is the low pressure switch wired? The above diagram says it goes to the a/c compressor clutch pigtail?



also, the signal that the PCM provides from C2 GREEN 43 that triggers the relay, is that a 12V+ or a ground? From a quick search it looks like a ground but I want to be sure.

aknovaman

iTrader: (7)

Based upon my experience, here is my understanding of how the Express manual AC system functions.
The dash switch applies +12v when you turn the dash **** to any setting except VENT or HEAT to provide de-humidification (AC).
The dash switch voltage goes through the HI pressure (cutoff) switch to the PCM AC request pin 17.
The PCM raises the idle speed to compensate for the AC compressor load.
The PMC generates a LOW (ground) on pin 43 which closes the AC relay contacts.
With the relay contacts closed, the contacts apply +12V to the dark green wire to the compressor.
The other side of the compressor goes through the LO pressure switch to the PCM pin 55 which monitors whether the compressor is cycling.

The compressor runs till it reaches the HI cutoff pressure level.
At that point, the HI switch opens, deactivating the relay and compressor.
Once the pressure goes below the HI cutoff threshold, it's contacts again close activating the relay and compressor to build pressure.
The low pressure cutoff switch is in line to prevent the compressor from running if the system leaks all the freon out, resulting is a burned out compressor.

Eric can verify this is correct.

LQ4-E39

iTrader: (7)

^great schematic of how it works. Thank you.

ls1nova71

iTrader: (4)

The way Anthony described it is pretty much right, but the compressor isn't tied into the low side switch, it's just a reference for the PCM. Older non PCM controlled A/C set ups however do go through the low pressure switch and it's used to cycle the compressor. You can wire up the high and low switches in series and eliminate the PCM control, it won't hurt anything. The most you get out of allowing the PCM to control the A/C is for it to have the ability to shut the compressor off when necessary, like high rpm and WOT. It will bump the idle too but this really isn't necessary due to the fact that the IAC or DBW will compensate for any additional load on the engine almost as fast as the A/C is turned on. You only really need to bump up the idle on applications where there isn't a way for the engine to compensate for additional load, which is basically carb'd engines.

Here is a wiring diagram I have for a 2004 Express van, pretty much the same as the one you posted I think.