Installing a vacuum operated heater core bypass

[cdm1993]

Vehicle is a 1997 GMC k1500 5.7 Vortec. My A/C is marginal at best (A/C system was recently been overhauled), and I think it could be a significant amount colder if I block off the heater core that runs hot coolant through the cab via a vacuum operated bypass valve that operates when "Max A/C" is selected. From what I have read, some earlier models have this feature already built in. I have been looking at other sites and forums that have done this, specifically these:

http://www.dieselwarden.net/ford/coolbypass.html

http://www.ford-trucks.com/forums/957375-yet-another-mod-done-pic-heater-core-bypass-valve.html

What I need help with are the vacuum lines. Coming out of the passenger side firewall are a bunch of wires. Which one is the vacuum line for "Max A/C"? I believe that line actuates the heater blend door. If I can find the line for max A/C, I can place a T fitting on it and run it to the bypass module which would keep the heater core from circulating 210* coolant through the cab with the A/C on.




A closer view of the wires.


Thank you.

 

[mk378]

I think the HVAC controls are 100% electric on that truck. So you need either an electric valve or a manual one. Manufacturers moved away from vacuum controls in the 1990s.

You can use a solenoid valve under the hood to connect a vacuum valve to the intake vacuum when bypass is called for.

 

[SHOZ]

Max AC workings should all be under the dash I would think. Cars I have seen have one vacuum line running to the CC controls then it branches off from there to the other functions.

 

[Chris142]

Yes all electric.not any vacuum in that dash. Probly a broken blend door or motor

 

[zzyzzx]

Originally Posted By: Chris142
Yes all electric.not any vacuum in that dash. Probly a broken blend door or motor


I agree with the above comment.

 

[SHOZ]

You can add a manual pneumatic toggle switch if you want to operate the vacuum valve.