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There is a vacuum reservoir that helps prevent this situation from happening. The symptom occurs because when you open the throttle, the manifold vacuum decreases (and can even fall to zero under heavy throttle). Since the vent doors are controlled by vacuum, when the vacuum goes away the vent doors return to their resting position, which is defrost mode. There is a check valve installed to prevent the vacuum from bleeding off. The vacuum reservoir was designed to close the check valve during brief periods of low or no vacuum. If this valve has gone bad, or there is a crack in one of the vacuum lines connected to this reservoir, or the reservoir itself is damaged, your issue occurs. This vacuum reservoir is usually located beneath the battery tray, near the auto trans dipstick against the firewall, or inside the front bumper on the passenger (right) side, but it could be in other locations. It usually looks like a canister or a black plastic bubble.
all my vac lines go thru the firewall cant seem to find this check valve it must be in the dash yay for me
got any info on removing the dash 94 ram 1500The valve isn’t under the dash. It is in the engine compartment between the canister and intake manifold. Although the location varies, once you find the manifold port (or vacuum canister), follow that line to the tee and the check valve will be off of that. Example pic below.
found it, it was laying on the back of the engine where part of it was damaged due to heat, something melted into the side of it
big help on this sir thank you so much!!!!There was a TSB on this http://dodgeram.info/tsb/1996/24-01-96a.htm
That TSB doesn’t apply to the OP’s vehicle since he found a check valve already installed. His check valve was damaged and that is what was causing the issue. Thanks for posting, though.
well I replaced the check valve and it didn’t fix my problem so apparently I have a vacuum leak in the dash, I checked the vacuum line all the way to the canister it is solid …
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Do you have a hand operated vacuum pump? If so you can check the reservoir for a leak, and also you can attach the pump to the line going to the dash to see if it holds vacuum. Also, make sure you installed the check valve in the correct orientation. If there is a directional arrow on the valve make sure it is pointing toward the manifold port (away from the A/C). If there isn’t an arrow, with mouth suction, the air should flow toward your mouth, but when lightly blowing the air, should be stopped.
no vacuum going into cab so I went under the dash to the first junction and found the supply vacuum leaking, it just pulled out of the junction so I used a little silicone back away from the opening and pushed it back together, let it set up and it worked like a charm, you have been a great help, thank you so much!!!!!!
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