'98 K1500 Heater - All or Nothing

My '98 Z71 Silverado has developed a problem with the heater control. When the temp knob is below the 11 o'clock position, no heat at all. Rotated to or above 11 o'clock, it's full hot. No in-between. Even with the a/c running in the defrost mode, if I turn the knob past 11 o'clock the heat will run me out.

It also emits a strange whistling noise occasionally in the vicinity of the tranny hump.

Any help appreciated.

JRS

It could one or a combination of several things.

1. The Temp/AC/Fan/Mode controller. (in the instrument panel)
2. The Temp Control door motor.
3. The Temp control door (blockage from debris).
4. The noise needs to be pinpointed closer, although that would be the area of the temp control door and motor.

[quote=JRSpann

It also emits a strange whistling noise occasionally in the vicinity of the tranny hump.

Any help appreciated.

JRS[/quote]

inside the air duct in this hump area is the temp door control..the noise is most likely as was mentioned debris interfering with this doors movement which controls the temp...the heater core is always hot, no control to the hot coolant flowing thru it..the temp door does it all...

this door control maybe defective as well..

Thanks for the replies! I'll start digging into it tomorrow and see what I can eliminate.

Thanks again!
JRS