97 laSabre hvac

[Herb1949]

I have a 97 Le Sabre that the heater doesn't turn all the way off.
It also sometimes goes to the defroster even when heat or vent are on. When it does this it will return to the selected setting after a while.
This is the single control system without the digital settings.

Any ideas on where to start t-shooting?

[imidazol97]

Originally Posted by Herb1949

I have a 97 Le Sabre that the heater doesn't turn all the way off.
It also sometimes goes to the defroster even when heat or vent are on. When it does this it will return to the selected setting after a while.
This is the single control system without the digital settings.

Not sure what is meant by the statement the heater doesn't turn all the way off.

However, the switching to defroster most likely is a flaw in the vacuum supply. The 4 motors that operate by vacuum to direct the path of the air flow operate by vacuum. With NO vacuum they move the air to defroster for safety in case of system failure.

There is a problem with the automatic system where the vacuum for the motor that blocks the air from going out the defroster and makes it come out the vents for AC use typically fails. The plastic connector sucks shut at the electrical control for the vacuum lines.

However, you may have a supply problem with the vacuum lines under the hood. Start checking on the left side where the vacuum comes out next to the big hose for the brake booster. The rubber connectors or line parts can deteriorate with age and heat close to the engine. Then they leak. Add in low vacuum when accelerating and you lose vacuum inside which lets the system go to defroster, but other times enough vacuum is present to operate the motors.

The vacuum line from the driver side of motor goes to the right side where there's a T-connector with rubber IIRC and one line goes to the plastic tank under the front fender behind headlight; the other goes to the inside. I believe this line is easier to spot if the cover for the Relay center is removed by its two clips.

I recall only 1 person has said the one-way vacuum valve in the line before the T-connector had gone bad and would not allow air to flow into the heater end when engine throttle is opened wide. But it might be worth removing it and checking that it only allows air to flow toward the engine end. I believe it's connected by rubber tubes, so those may break when you remove the one-way valve.

Good luck trouble shooting.

[brcidd]

Agreed, with manual system as yours is- go underhood and look for rotted black plastic vacuum tubes- and replace as necessary...