Vacuum and AC/heat controls

[chris989]

Hello all,

89 v2500 burb,,, 5.7.

The AC/heat works correctly but when pulling a long grade and low on vacuum the AC max stops and the air discharges at my feet. The vacuum pulls the AC vent doors into position for max.
It is only a issue towing and long grades.



Is there a check valve somewhere that has failed to hold the control setting?

Is the canister under the hood (removed) supposed to have enough reserve?

I don't have any idle/vacuum issues so i am guessing the ac/heat control is leaking???

THanks,
Chris

[Dead Parrot]

Pretty sure the canister serves both as a reserve and a check valve. If things are working correctly, you should be able to park the vehicle and several hours later change the controls and they should move without starting the engine. Only good for one or two changes.

With no check valve, the AC controls will respond to low engine vacuum by returning to their 'rest' positions.