HVAC PROBLEM

[CVX20]

Before anyone jumps on me for not using the search feature let me just say that I've read every HVAC related post that I could find.None of the threads seem to be exactly like my problem.
I've got a 97 GMC E-cab 4x4 5.7 L with approx. 250 k miles on it.Recently my defrost decided not to work with the heat on.It will blow on the windsheld if the temp is turned to cold.The air flow does change from floor to dash to windshield on cold so I'm assuming the blend doors are all working.If I look up above the gas pedal it appears that all the blend doors are working but with the temp on high I don't know if they're going to the proper position.I read on here that there is a way to sync the doors/actuators.Could someone outline the procedure for me or point me to the info if it's listed somewhere.
If this sync doesn't work I leaning toward the control head being bad.Any thoughts on this?Am I right in my thinking or could it still be a door issue?

Chris

 

[great white]

We usually don't jump on guys with "search search search" here, so rest easy there my friend.

The mode door appears to be working if you can go from floor to defrost.

Can you get any heat at all on any setting? (IE: floor dash etc)

If not, then the temperature door (selects heater core or AC) is suspect. If you look under and see the blend door actuator move but still no heat, the door may have come adrift inside.

here's a pic for reference so we're talking about the same thing:

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If the temperature door actuator is moving, then the HVAC head unit is doing it's job. That means you've either have no heat at all in the core or the actual door inside is not moving with the actuator.

 

[CVX20]

That's the thing.I can go from hot to cold or cold to hot and I can control where the airflow comes from but I get no flow to the winshield if the temp is set to hot.If I turn it back to cold then I get airflow to the windshield.If I switch back to hot I get airflow everywhere but the windshield it seems.Old girl has good heat but I haven't worried about the AC for a couple of years.

Thanks for the reply
Chris

PS Going back to work on the 17th Dec(been off with a broken wrist)so if I don't get it sorted I'm thinking about setting it to defrost and then disconnecting the mode door actuator.Been fairly cold here and pretty hard to drive with no defrost.

 

[great white]

Let me check the troubshooting tree in my FSM and get back to you tomorrow.

Something is not quite right with all this.....

 

[CVX20]

That's why I was thinking the control head had gone a bit brain dead.Do the actuators send a signal back to the control head to let it know the position it stopped at or is that a bit to complicated for this system.

Thanks
Chris

 

[great white]

That's why I was thinking the control head had gone a bit brain dead.Do the actuators send a signal back to the control head to let it know the position it stopped at or is that a bit to complicated for this system.

Thanks
Chris

There's actually very little to go wrong in the HVAC head. It's mostly just PCB, switches and a few resistors....