Page updated on 05-24-2018

Heater control valve

Tony Viv
07-01-2007, 10:59 AM
Still trying to diagnose my A/C problems mentioned in another thread I started checking the heater control valve.

There is vacuum going into the electric piece on the firewall, but there is no vacuum coming out of that line which goes to the heater control valve no matter what settings I put the control head on. I checked the valve itself and it will hold vacuum and the rod moves.
At what setting does the electronics apply vacuum to the control valve?
Even if the blend doors are closed, if the heater control valve is not closing could it cause warm a/c?
Is the valve normally closed with no vacuum?

Thanks all

07-02-2007, 06:27 PM
Do you have rear a/c-heat?
When you talk about "heater control valve" where is located exactely?

If you have a rear system there is a "heater bypass valve" located in the engine compartment on the pass side forward of the firewall. Not sure what controls the vacumm to that but I'll look into it some more.

Heated engine water flows thru the front heater core all the time. The temp control door is what determines hot or cool air coming into the cabin. There is a "recirc door" but I cannot find the location of it in my "genuine '98 GM manual". It says that it's on the plenum.

Not sure why you can't get the temp control motor off. I looked at my '98 Sub and it looks very accessable (sp). If and when you do get it off just use a flat blade screwdriver to check the freedom of the door. If it moves free and the motor runs and is engaged with the door it should be good.

All this stuff ( recirc, temp, a/c and modes) go thru the dash control panel and there have been a lot of posts and fixes by replacing the panel. I'm like you, don't want to throw parts at problems without knowing that the money spent has a really good chance of fixing the problem (although I do have some rather expensive tx case parts as spares that did not fix a simple wiring problem)!!!

So post back and lets see if we can get you fixed up..............Steve

Tony Viv
07-02-2007, 09:19 PM
Stickman thanks for replying I was begining to think no one would.
My tahoe is a 2 door without rear heat, I am going to try to enclose images of the 'heater control valve that I speak of along with the electronic unit on the firewall that the vacuum line goes to from the heater control(bypass) valve. Like I stated the unit on the firewall has a vacuum line going into it that has vacuum but coming out there never is vacuum going to the valve, no matter what setting the control head is on.
I have also tried to get the best pics I could of the hot/cold blend door actuator, directly behind and below that steel rod with the welded tab is the 2nd bolt to get out( not the screw that is easy to get at). it is so tight not even a 1/4 in drive with universal joint can get in there. The pic are taken through the removed glovebox and ashtray power outlet area. The circle in the middle of the motor moves between both alignment marks, but Im not sure if the gear is really engaged
Let me know what you think and thanks again for your help .jpg .jpg

07-03-2007, 06:26 PM
I have looked thru the '98 C/K Truck manual and can find nothing, other than a replacement guide, for the water bypass valve. Nothing on why it's there, what it does or what controls it. So???? Also the pic's you posted showing the temp door motor make it look like a real pain to get to. My '98 Sub isn't even close to what you're dealing with.

With all the grief you're going thru I would suggest replacing the switch module. Maybe with one out of a "pick-n-pull" just to see what happens. Sorry I can't be more help, but I'm stumped.

07-24-2007, 09:42 AM
sounds like the same problem I'm having. All parts look the same and are in same locations as well. I noticed that the heater control valve closes when you turn the temp gauge to cold and otherwise remains open. simple test to check valve, remove upper vac line and attached to valve, it should move.. By the way, what solved your problem. I too can watch the motor move between the alignment marks. Thank you for your help in advance..

Add your comment to this topic!