The havc system, on my 1992 Suburban, has failed to defroster mode. No matter how I manipulate the heater controls all I get is defroster. The A/C works fine as does the heater with the exception that what comes out comes out the defroster. The hot to cold adjustment works fine.

The button "graphic" that you get, when you push the button to direct air out of the vents and gradually to the floor( blend ), flashes all of the little arrows simultaneously rather than one at a time as the mode changes from vent to floor.

That is to say that when the system works properly the lights flash one at a time as the blend goes from vent to floor. Now all of the lights flash continuously.


The only recent change to the system was a new heater core about a month ago. The system failed about a week ago so hopefully I didn't wreck something changing the heater core.

I have searched high and low for a vacuum source, unsuccessfully, which leads me to believe that the system is electronic. In any event whatever motivates the mode doors has failed. I'll take any diagnostic advice anyone cares to give. I have checked all fuses on both sides with a 12v test light and all are good.

Thank you all,

Bill

Bill:

Have you tried to post over at Fullsizechevy.com? Those guys are the truck equivalent of this site. Bet someone over there could help. Sorry no help from me but maybe the site reference will get answers/suggestion.

Durand

Bill, Try disconnecting the positive battery cable,with the key off,to reset the a/c control module.If that does not fix it, replace the control panel assembly(module). It is about $200 from the dealer. Steve.

Bill, Try disconnecting the positive battery cable,with the key off,to reset the a/c control module.If that does not fix it, replace the control panel assembly(module). It is about $200 from the dealer. Steve.

Did that. No difference. I was afraid of that. :(

Is that the digital display control head? If so try this before you replace the head. Pull it and rub an eraser on all the contact points. This worked for me for quite some time before I replaced the head on my 92 gmc 1500. Incidently it set me back less than $100 when I finally did replace it. If this is a different setup and it is vacuum controlled like a ford it will go to defrost if there is a vacuum leak at the control head.

My 90 is on it's 4th control head (last two were used) and now sometimes all the up arrows on the temperature control flash at the same time but it is still working ok so far.

The last one I had would work on what ever it had been set on the last time the truck was driven so I always had to guess, "am I going to need heat or A/C the next time I drive this truck" because by the time I would get it working I was at my destination.

Thanks guys. Its definitely not vacuum operated. I'll give the "eraser" trick a try.

Is that the digital display control head? If so try this before you replace the head. Pull it and rub an eraser on all the contact points. This worked for me for quite some time before I replaced the head on my 92 gmc 1500. Incidently it set me back less than $100 when I finally did replace it. If this is a different setup and it is vacuum controlled like a ford it will go to defrost if there is a vacuum leak at the control head.

Ok. I did as directed. Removed the heater control unit and cleaned the contacts. Now ALL of the display is lit continuously and the system is still defaulted to defrost. Just to see if it would do anything I disconnected the battery momentarily, no help.

Don't know what I did to make all of the indicators stay illuminated but at this point I think its new controler time.

Thanks for the help guys. At least I have a solution to deal with. :)