Engine/Transmission (1994 - 1998) A/C Control Check

[DavidShelton]

My A/C and Heat controls don't seem to work in my 1994. Air blows from the defrost vents no matter what position the controls are in. I checked the vacuum line under the hood and it will not hold a vacuum, but the pump holds steady at 30hg if I plug the line going to the dash. The temp control does work and blower motor works. What is the best way to troubleshoot the dash control and related system?

 

[hsmith]

You may well have oil contamination from the vacuum pump getting into the control system. Remove the switch and see if there is evidence of oil. If so, probably has also damaged some of the servos.

There should be a check valve at the pump to prevent this, but mine did not come with one. It was added later.

It is blowing out the defrost as that is the default position for the servos.

 

[Missouri Mule]

DavidShelton, quite often the vaccum line going through the firewall has a hole burned into it from the exhaust manifold/down pipe.



When I was in default mode(defrost) I had a pinched line under the dash.



The selector knob tabs can be broken leaving the vents in the last place that was selected.



But it seems that you mite have a whole in one of the vaccum lines.

 

[chopperman]

Does your cruise control work, 4X4 also? If not you have a leak. Try checking under the drivers side battery tray. Corrosion from the battery will destroy vaccum lines. Also if you have vaccum inside the cab there is a vaccum actuator under the dash up from the accelerator pedal. It changes climate contol functions from defrost to your feet, feet and body ect. Accuator is about thirty bucks from the dealer kind of a pain to put in but thats what was wrong with mine. Try trouble shooting your vaccum lines under the hood first. Good luck, Brian

 

[DavidShelton]

I checked all of the vacuum lines under the hood and found all were OK. The leak is somewhere under the dash. Next I'll look at the stuff under the dash and try to isolate the leak.

 

[hsmith]

Doubt you have access to a smoke machine. Maybe a friend that still smokes, heaven forbid. A lung full of that nasty stuff blown into the line going through the firewall while you look inside may help.

 

[DavidShelton]

I was thinking I would try to isolate one item at a time and test with my hand held Mightyvac pump.