Heater control?

[Atommik]

I've got a 1999 Suburban 4X4 with the 5.7 engine. The heater control that changes/blends air flow from defrost to middle vents to floor, only will change from middle vents to defrost. No air flow from the floor vents. The fan is working, heat is good, but almost no air at all from the lower vents. It just keeps coming out of the defrost vents when it is turned to floor. Is the blend door, vacuum controlled or electronic? Or is it more likely the control knob since the blend door does change from defrost to middle vents? Any ideas on how to test it?

[usmcchevy]

Anyone?

[clinebarger]

The Blend door controls the temp, The Mode door controls were the air comes out. Blend & mode doors are contolled buy a electronic actuators that are controlled by the A/C control head.

The mode door actuator is located on the left side of the air distribution case.(pic)

1. At the mode door actuator, check the Brown wire for 12 volts.

2. Check the Black wire for a good ground.

3. Check the White wire voltage from the control head, it should be about 1-volt in defrost, 12 volts in A/C, and 6 volts in heat mode.

4. If voltages check OK, replace the mode actuator. If not OK trace wires to control head, verify powers and grounds, replace control head.

[Atommik]

Thanks for the info and diagram. I saw the actuator motor while I was doing a quick check last week. Now I know where to start. Thanks again!!

[Atommik]

Okay, now I'm more confused...I checked the voltages and everything was good. Now I have air coming through the floor outlets in the heater box, however, If I have the heat control turned all the way to the hottest setting, the air barely comes out of the floor ducts. As I turn the control cooler, the airflow gets stronger. Are the blend door and mode doors some how connected? Or is there an electrical goof up here? The mode door actuator does not move when I change the temp control, but as I said the airflow drops to nothing when the control is moved to it's hottest setting.

[clinebarger]

There are 2 mode doors.....Most likely one is broke & the Mode Door Actuator caused it, Pretty common issue on a GMT400 truck. replacing the actuator & broken mode door will solve your problems.