On your truck, you should have a small plastic item called a vacuum check valve located near the brake
booster. If the check valve does not work, then the vents will shift around under load. You need to find the vacuum line near booster, look for inkline check valve. It is in the same vacuum line going through firewall from engine bay to inside of vehicle. you should only be able to blow through it one way. see what you can find. LMk thanks
The heater/AC system changes to the defrost mode during a low vacuum condition such as during trailer towing, hill climbing, and acceleration.
1. Connect a test vacuum supply to the vacuum line going to the heater/AC controls.
|A. ||On AB vehicles, locate the vacuum connection in the engine compartment tucked into the wiring harness clip above the middle of the radiator. Disconnect the black rubber source hose going from the engine to the black plastic heater/AC system hose. Connect the test vacuum source to the plastic hose. |
|B. ||On all other vehicles, remove the vacuum check valve from the vacuum supply line near the brake booster and connect the vacuum source to the hose going through the dash panel. |
|2. ||Pull between 10 and 20 inches of vacuum (Hg.) with the test vacuum source. |
|3. ||With the test vacuum source shut off, the vacuum reading should remain constant for one minute. |
|4. ||If the vacuum reading dropped within one minute, there is a leak in the system. Locate and repair the leak as outlined in the appropriate service manual before proceeding to the repair procedure. |
|5. ||If the vacuum reading remains constant for one minute, proceed to the repair procedure. |
1 Valve With Reservoir, Vacuum Check (all Vehicles) (NNN) NNN-NNNN2 Connectors, In-Line (AB Vehicles) (NNN) NNN-NNNNAs Required 3/16" Vacuum Hose - serviced in bulk (AB Vehicles) (NNN) NNN-NNNN
This bulletin involves inspection of the heater/AC vacuum system and installation of a revised vacuum check valve.
|1. ||Locate where the black rubber vacuum source hose connects to the black plastic heater/AC hose. Cut the rubber source hose 2 inches from the connection and install the new vacuum check valve (PN(NNN) NNN-NNNN. If the rubber source hose is small (1/8 inch), use two in-line connectors (PN(NNN) NNN-NNNN and two 2 inch pieces of 3116 inch hose (PN(NNN) NNN-NNNN to adapt the new vacuum check valve to the smaller hose. |
|2. ||Trim the vacuum hose so that the new length is the same as the old. |
|3. ||Install the vacuum hose back into the wiring clips taking care to locate the check valve where it will not rattle and is not exposed to excessive heat. |
THE ORIGINAL VALVE, LOCATED UNDER THE INSTRUMENT PANEL, SHOULD BE LEFT IN PLACE.
ALL OTHER VEHICLES
|1 ||Install the new vacuum check valve assembly (PN(NNN) NNN-NNNN, in the vacuum lines at the same location as the original check valve. Discard the original check valve. |
Reimbursable within the provisions of the warranty. TIME ALLOWANCE:
Labor Operation No. 24-50-47-90...............................0.2 Hrs. FAILURE CODE:
X9 - Vacuum Leak