99 Tahoe A/C blows warm on hills

[chris peter]

My 99 Tahoe A/C works fine at idle or just going straight down the road but if you push on the gas just a little or go up a hill with the cruise on it seems like it switches over to heat until you let off the gas. I don't think its the throttle A/C cut off because this is light throttle. This is the 5.7L old body truck with out climate control. I think when the engine vacuum drops the heater water control valve is some how opening or something like that. Any Ideas please.

[j cAT]

Originally Posted by chris peter

My 99 Tahoe A/C works fine at idle or just going straight down the road but if you push on the gas just a little or go up a hill with the cruise on it seems like it switches over to heat until you let off the gas. I don't think its the throttle A/C cut off because this is light throttle. This is the 5.7L old body truck with out climate control. I think when the engine vacuum drops the heater water control valve is some how opening or something like that. Any Ideas please.

when you accelerate the engine vacuum drops...this is why there is a check valve and a vacuum tank that feeds the ac controlling doors [old style ].....vacuum lines need to be tested with vacuum guage..usually at engine the lines crack / swell and make for leaks...

with T fittings you can tap into the lines and check this out ...open throttle and view the change in vacuum ..

vacuum at idle should be about 17-22 inches..on opening throttle should drop to about 13inches ..

carb cleaner usually eats away at these rubber parts ..

[chris peter]

Vacuum at idle is 21 inches and the needle is steady. when you push on the gas it drops but comes back up when engine rpm levels off. I just noticed some kind of vacuum switch on the fire wall it has an electrical connection on it and the vacuum line comes from the engine goes to this thing and then to the heater water control valve. Could this be my problem part what is its function?

[j cAT]

Originally Posted by chris peter

Vacuum at idle is 21 inches and the needle is steady. when you push on the gas it drops but comes back up when engine rpm levels off. I just noticed some kind of vacuum switch on the fire wall it has an electrical connection on it and the vacuum line comes from the engine goes to this thing and then to the heater water control valve. Could this be my problem part what is its function?

that switch maybe working at the incorrect vacuum causing this..bypass the switch to see if your problem is resolved.

[chris peter]

I pulled the vacuum line off the water valve put a screw in the vacuum line ran the truck and got heat and no cool air at all. This maybe the problem part.

[becerrda]

Originally Posted by chris peter

I pulled the vacuum line off the water valve put a screw in the vacuum line ran the truck and got heat and no cool air at all. This maybe the problem part.

did you ever fix this problem

[chris peter]

Yes the repair was to replace the center Heater Blend Door Actuator. This one is located in the center of the dash and moves as you change the temp knob. Very hard to get to. 2 screws are easy to get at the 3rd one I drilled a 1/2" hole through dash and bracket that was in the way in front of the screw and then used a 5.5mm socket and got it out otherwise you have to take the dash out. The recirulation blend door is another hard one to get out I have not been able to do that one.