Bricknose Blend Door Issue?

[Bruno2]

i have a 91 F150. I am having an issue with the blend door not operating. Also, there is a vacuum issue. When accelerating to get on the highway I lose air from the dash vents. It defaults to the defrost and eventually comes back. What operates the blend door when the switch is moved from hot to cold? I get the same temp no matter what position the switch is in.

[grumpin]

Vacuum loss while accelerating is most likely a vacuum leak.

[ArdWrknTrk]

Look at the vacuum motor beneath the passenger hood hinge.

Follow the brittle plastic hose into the harness bundle beneath the heater hoses connection.

This often cracks, and my solution is to splice it with rubber vacuum hose or small engine fuel line (that yellow/green Tygon you'd use for a chainsaw or blower)

The climate control vacuum starts from a port at the back of the intake, goes to a splitter (manifold) on the firewall/cowl, then there should be a line with a one way check valve.

Depending on your engine it likely goes to a vacuum reservoir (a black tin can or plastic ball on the passenger inner fender)

The tin cans are notorious for leaking seams or rusting out on the bottom.

From there the line should pass into the cab with that same bundle I spoke about above, where it comes out to the fresh air/recirc door in the plenum behind the blower motor.

Hope this helps!

[Gary Lewis]

Look at the vacuum motor beneath the passenger hood hinge.

Follow the brittle plastic hose into the harness bundle beneath the heater hoses connection.

This often cracks, and my solution is to splice it with rubber vacuum hose or small engine fuel line (that yellow/green Tygon you'd use for a chainsaw or blower)

The climate control vacuum starts from a port at the back of the intake, goes to a splitter (manifold) on the firewall/cowl, then there should be a line with a one way check valve.

Depending on your engine it likely goes to a vacuum reservoir (a black tin can or plastic ball on the passenger inner fender)

The tin cans are notorious for leaking seams or rusting out on the bottom.

From there the line should pass into the cab with that same bundle I spoke about above, where it comes out to the fresh air/recirc door in the plenum behind the blower motor.

Hope this helps!

All of the above. And if the check valve isn't there or isn't working then when you accelerate the HVAC system loses vacuum. But if the check valve is there the HVAC system, being sealed, keeps its vacuum and it doesn't change modes.

[Bruno2]

All of the above. And if the check valve isn't there or isn't working then when you accelerate the HVAC system loses vacuum. But if the check valve is there the HVAC system, being sealed, keeps its vacuum and it doesn't change modes.

Thanks Jim.

[Bruno2]

Thanks Jim.

Gary, the truck did not do this before we did the AC job on it. We also changed the evap coil. I am thinking something got broken (frail little spaghetti line) or possibly something wasn't reconnected. It warrants further investigation.

[Gary Lewis]

Gary, the truck did not do this before we did the AC job on it. We also changed the evap coil. I am thinking something got broken (frail little spaghetti line) or possibly something wasn't reconnected. It warrants further investigation.

Yes, that line is frail. So check it out.

[FuzzFace2]

Vacuum loss while accelerating is most likely a vacuum leak.

Well not really.

There should be a check valve between the vacuum supply and the holding tank to keep this from happening.

My check valve is in place but is bad and at times I can hear the air flow change where it is going.

So just because the valve is in the line check to make sure it is working.

And yes that tin can can also rust out and cause a vacuum leak, been there done that also.

Dave ----

 

[85lebaront2]

Gary, the truck did not do this before we did the AC job on it. We also changed the evap coil. I am thinking something got broken (frail little spaghetti line) or possibly something wasn't reconnected. It warrants further investigation.

Blend door is a purely mechanical cable system, open and remove the glove box door and look at the top side of the casing just to the left of the heater core access door. There is a lever or cam and lever that works the blend door. On the vacuum issue, use a small vacuum pump and go to the splice between the controls and HVAC system, apply vacuum to each port until you find the leak.

Black is vacuum source and white is fresh air recirculate actuator, both of these run out with the wiring harness through the top of the outer portion of the evaporator casing. If there is a vacuum reservoir on the inner side of the evaporator casing then there will be a tee in the black line.

This is from a 1990 EVTM but should be the same:

[myrl883]

Blend door is a purely mechanical cable system, open and remove the glove box door and look at the top side of the casing just to the left of the heater core access door. There is a lever or cam and lever that works the blend door. On the vacuum issue, use a small vacuum pump and go to the splice between the controls and HVAC system, apply vacuum to each port until you find the leak.

Black is vacuum source and white is fresh air recirculate actuator, both of these run out with the wiring harness through the top of the outer portion of the evaporator casing. If there is a vacuum reservoir on the inner side of the evaporator casing then there will be a tee in the black line.

This is from a 1990 EVTM but should be the same:

The temperature blend door is cable operated on these trucks. Vacuum operates the mode and recirculation doors only. It sounds like you have two separate problems.

Loss of vacuum will send all of the air to the defroster. This is a safety feature in case of malfunction - you'll still be able to see where you're going...

The plastic lines get more brittle with age, and the white tube to the recirc door is especially prone to disintegrating with age. They will usually just crumble if you pinch them. Replace/repair as necessary with small diameter rubber hose as recommended earlier.