1999 GMC YUKON SLT heater control and temp problem

I am having an issue with my 1999 GMC Yukon Slt 4x4.
The heater controls that change from defrost to feet isn't working. The actuators have been replaced and the the climate controls have been replaced. Also, the A/C button doesn't light up. Mechanic doesn't know. He said if it was a year older or a year newer he could narrow it down.
Also, I am getting hardly any heat coming out. Maybe around 70degrees in the interior at normal operating temp. Radiator, water pump, heater core, thermostat, all hoses and heater blower motor has been replaced. No leaks anywhere. Only thing I can think is the heater control valve, but I am growing tired of throwing money at this thing. I would greatly appreciate any leads to what these issues are and a solution on fixing them.

Have you checked the fuse for the climate controls? That may be the problem.

Also check the coolant level for the low heat temperature. Low coolant can make the heat not be as hot.

black2002tahoe said:

Have you checked the fuse for the climate controls? That may be the problem.

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Yes. I have checked them and they looked good. I might just replace them anyways. but, I'm sure its something else that is making everything i mentioned happen. Its probably something simple. Hence why I am here to get more opinions. Thank you for the suggestion.

black2002tahoe said:

Also check the coolant level for the low heat temperature. Low coolant can make the heat not be as hot.

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coolant is topped off. No leaks at all. I have replaced everything to do with coolant when I did my water pump. And just had the intake manifold gasket done as well.

Shayne said:

I am having an issue with my 1999 GMC Yukon Slt 4x4.
The heater controls that change from defrost to feet isn't working. The actuators have been replaced and the the climate controls have been replaced. Also, the A/C button doesn't light up. Mechanic doesn't know. He said if it was a year older or a year newer he could narrow it down.
Also, I am getting hardly any heat coming out. Maybe around 70degrees in the interior at normal operating temp. Radiator, water pump, heater core, thermostat, all hoses and heater blower motor has been replaced. No leaks anywhere. Only thing I can think is the heater control valve, but I am growing tired of throwing money at this thing. I would greatly appreciate any leads to what these issues are and a solution on fixing them.

Click to expand...

Also check vacuum line above fan underhood. May have to replace it if it's a loose fit.

The floor-defrost is controlled by the actuator near the gas pedal. There’s a notched silver dial on it. Watch that dial as the panel is switched from defrost to floor to dash. The notch should roughly align with some marks on the actuator and rotate from about 11 to 1 o’clock.



Compare the linkage to the photo below.



If that actuator doesn’t move, pull the connector on the actuator. There are three wires - one for ground, one for battery voltage and the third is a signal. With a multimeter, check for 12V on battery and then watch the voltage on the signal wire as the panel knob is turned. See what you’re getting. If it doesn’t change voltage, then the panel isn’t sending a signal.

The heater door actuator functions the same and is behind the ashtray. Watch the dial on it and see what it does.



To test the heater water valve, there is a metal rod plunger on the end of it. Push it up and down to see if it moves. Without vacuum (a/c panel temperature is not on MAX) the valve will be open. Unplugging the vacuum line from the bottom is another quick check. If you have vacuum on it with the temperature dial not on Max, then the panel or the solenoid on the firewall is the issue.