Heat/cold knob not working

1998 OLDSMOBILE 88
120,000 MILES • 3.8L • 6 CYL • 2WD • AUTOMATIC
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BARBWIR3
  • MEMBER
  • 1 POST
The knob where you select defrost or heat is not working.
I can hear no vacuum noises when I turn the knob like I used to. I hear nothing moving under the dash when turning the knob. The AC light works and the rear window defroster works. I feel the issue is vacuum related.
Please give me troubleshooting advice.
Dec 11, 2017 at 1:28 PM
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STEVE W.
  • ✓ASE Certified Mechanic
  • 15,200 POSTS
  • ASE Certified Mechanic
As this is a mode issue you would want to look at the number three actuator in this image. It is just above and to the right of the drivers side foot well. Remove the panel under the dash and you should see the actuators above you. Cycle the mode control and see if the actuator moves. If not you could unhook the hose to it and apply vacuum from a hand pump and see if it moves then. If it works with an exterior vacuum supply you will need to check the line for damage or blockage back to the control head. There are also seals in the control head that could be leaking and preventing the vacuum from reaching the mode actuator. Or the programmer could have an issue.
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Dec 11, 2017 at 4:28 PM
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