I have a situation maybe some of you can help. My 1991 K1500 4x4 would not light up or operate the heating controls this morning. I drive the truck every day and when I parked it last night it was working fine. I have checked all fuses and they are good. I have taken the old control box out and tried two different ones at a local junkyard today and they didn't work either. The plug into the back of the box shows 12 volts on a volt meter up to that point. I have both a Haynes and a Chilton repair manual; one says I should have a "maxi-fuse box" under the hood and I don't; and the other says I should have an old school slide control for my heater and I don't. The schematics are different in both books also; and neither seems to be in line with the truck. What in the world has happened to my heater?

 

try disconnecting the battery a few seconds..

 

Give up on replacing the controller. I went thru three on my 91 and it STILL kept crapping out on me. The problem is elsewhere.

While we are in winter......
My temp fix: Until you get a chance to track down the problem, either:
* Pull the unit out of the dash so that you can disconnect and reconnect it at will. Doing this will usually reset it and it will work again (usually until you restart the truck).
* This worked good for me....I used wires as jumpers and managed to get the defroster on and the fan up to "hi" by plugging wires directly into the wire harness. One will be power and one will be a "control" to move the blend doors. I can't remember where they were tho.

Or....
When the weather warms up (unless you have a garage) I'd look for a bad ground, loose wire(s), damaged wire(s), bad relay etc..... I never solved the problem 100% on my truck but I tracked down fuses and replaced the control unit several times with no luck.

 

The early 90s trucks seemed to have alot of ground issues, so defintely start looking there. Add another ground wire to the stock ground, but ground it in another location and see what happens.

 

Thanks to all who replied. I found it this morning while checking different things with a voltmeter. Behind the glove box there is what looks like 3 relays. Inline prior to that there is a small plastic clip with three wires in it. One of the wires is a "hot all the time" wire. Apparently mine had melted almost to the point of fire ay some time and decided to lose connection that night. I cut the wire and wire nutted it together and all is fine now. Maybe sombody else can use this information.

 

If you start having headlight issues, the same type problem may be going on. Mine had a melted switch from a bad ground, as well as the same with windshield wipers. Just bad grounds during those years.

 

Im having the same problem with my heater. I tried a different control box and it didnt work either. I will look behind the glove box for those relays. mine is a 90 k1500. the fuse for my dash lights and taillights blow sometimes too. that might all be due to a bad ground or short i guess. thanks for posting this, that heater has been driving me crazy!