98 k1500 a/c heat vent control problem

I have a 1998 Chevy K1500 6.5L turbo diesel and my problem is that the a/c heat vent control isn't working properly. I have spent 2 full days trying to fix it myself but with no look. The vents wont change from floor to any other position the knob twist but no effect and the a/c and recirc air buttons don't engage or light up. What I've tried is to replace the control unit but it made no difference and to check the mode door motor, works but only when the fan/blower is turned off so it will change the vents but as soon as you put the blower on the vents go back to floor, also I bypassed the control unit and hot wired the a/c to see if it still worked and with the blower going and the control unit bypassed the a/c blows ice cold. so my only guess is a relay. This is the hvac control unit with 3 twist knobs and a button for a/c and a button for recirculating air. I replaced the control unit when I first got the truck 2 years ago b/c the controls would stay on the windshield vents and wouldn't change to any of the other vents and the a/c button wouldn't light up or engage the a/c. Replacing the control unit fix it all. But now it is something different any help or input you can give is well appreciated.

If the mode door motor only works with little or no air flow over it- there in lies the problem, the mode door motor is stripped, weak, or the door itself is bad. Pull the mode door motor off and check motor and mode door operation separately

I have found 7 or 8 C/K trucks of your vintage, that the lever that drives air to the defrost pops off the defrost door- is yours okay? The lever is attached to defrost door just above gas pedal- and shares its motor with the mode door operation.

Also common on your truck is the temp door actuator issue- but you've not said the air temp is an issue, only the air flow....

In the photo below, be sure that both linkage arms are attached- circled in red.

I already pulled the mode door motor apart and its fine, it works fine when the blower/fan is off but when you turn the blower/fan on it loses all power and goes back to floor vent almost like the blower/fan is stealing its power.

Pvt Bates

The air flow alone is forcing the door to the floor, something is amiss, the door axle is cracked, or actuator is stripped, or not all your levers are attached. Nothing will steal its power....

I took the mode motor off so it wasn't even connected to the vents and had it plugged in and with it in my hand case open and case closed it worked fine but in both cases open and closed when I turned the fan/blower on the motor and gears went back to the floor position even when it wasn't connected to the vents.

With the a/c light and recirc light not coming on, this alone is good enough reason to look for another control head- I've replaced them already for inop a/c button (only thing wrong) already. The control head is a known issue on these trucks, since it is easy to replace, I'd opt for another boneyard unit- and try to get a couple more years out of it- until it needs to be replaced.

A new one would be big bucks.. it is up to you.

i tried that this morning and it has the same symptoms as the one that was already in

Reading from my '98 C/K service manual: The mode door function is disabled with the fan in the off position and enabled with the fan in any run position. This is all controlled by the logic in the HVAC control module. This same logic also disables and enables the Recirc & AC buttons. In your case it seems to be doing just the opposite. Which leads me to think that the replacement HVAC control module is either bad or not the correct one. Or there are some wiring issues. There are many parts that are VIN specific.

Where is the HVAC control module located. Also do you think it could be a bad relay.

Where is the HVAC control module located. Also do you think it could be a bad relay.

It's the dash unit, which you say has been replaced. All the logic is inside of it.

Yes it was replaced yesterday and it does the same thing as before so do you think i could be a relay

The relays only control blower speed- not air flow position or temperature-- so no,a relay is not at fault. The control head is-- you need a verified good control head to prove it, evidently you have a used one with the same issues as yours.

Ok so today I talked a guy into letting me try his control head that he is using in his truck he usually parks next to me in are apartment complex. I tried it out in his truck works fine put it in my truck same thing that has been happening put it back in his truck works fine.
Not the control head thats the problem.

Well, keep going, borrow his mode door actuator and do the same test. Also study your wiring connectors and harnesses at the same time you are in there-there may be some connectors or pins or wires shorting out- there are not that many components to swap out.

How about the control head ground? perhaps your control head is trying to ground out through some other method because of a poor body ground, or one that is not attached. Check ground G202 on left side of dash behind driver's kick panel behind parking brake..see if it is attached, broke, corroded.

Would the mode door motor or wires effect the a/c and recirculate air buttons.

I doubt it, but I've been wrong before...

Did you check the ground connection?