Rick's Free Auto Repair Advice

no heat Chevy Blazer no heat oldsmobile Bravada

I don’t have heat in my Chevy Blazer, no heat Oldsmobile Bravada

Readers write to me asking where the heater temperature control valve is on their Chevrolet Blazer and Oldsmobile Bravada. BUT, GM’s term for that “valve” is really misleading. It’s not a valve at all.

The hot coolant comes into the heater box and into the heater core. The blower motor blows air through the heater core. GM uses a temperature door to regulate how much heat gets into the passenger compartment. Most manufacturers call this a “BLEND” door because it blends hot air from the heater core with outside air to get you the right temperature air.

In this vehicle, that door is opened and closed by an electric actuator motor that sits on top of the heater assembly. That’s where all the confusion comes in. GM calls that motor a heater temperature control valve, making you think that it’s a plumbing style valve. It’s not. It’s just an electric motor that opens and closes the blend door.

The other “motors” on the heater assembly are vacuum operated motors that open and close “MODE” doors. They open and close the recirculated air damper, and move the air from the heat/vent/defrost positions.

To test the electric BLEND motor, use a digital multimeter and attach the red probe to the light blue wire at the connector. Connect the black probe to ground. Turn the ignition to RUN and measure voltage at the blue wire as you change the heat setting from hot to cold. The voltage should change from 1 to 12 volts. Then place the red probe on the brown wire. Leave the black probe at ground. You should see battery voltage on this wire. Next, leave the red probe on the brown wire and move the black probe to the black/white wire. You should see battery voltage.

Blazer Bravada heater control wiring diagram

Blazer Bravada heater control wiring diagram

What you’ve just checked is the varying voltage coming from the control to command the motor to open and close (the blue wire), battery power coming to the motor (the brown wire), and ground (the black/white wire).

If you get those readings, then the control unit is working fine. If you don’t, then you either have a broken wire to the brown wire, a bad ground or a problem at the control head.

If all those readings check out, remove the electric motor and move the linkage by hand. If the door opens and closes smoothly, you’ve got a bad electric motor. If the door binds, you’ll have to repair that.

99% of the time, it’s a bad electric motor. I’d bet money that the blend door is closed and that’s why you’re not getting heat.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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