The heater in my 89 only works on the defrost. Whatever setting it is set on the air only comes out the defrost. Is there a vacum hose that could be broke? if so where would it be? or could it be something different?

 

1st, check the hose that goes to the vacuum canister on the front right fender. You can see the hose near the bat. mine was broken, it runs over to the manifold. next check the smaller hoses that engage the 4wd if you have it, the are hard plastic and go into the dash behind the bat side.

It is more than likely a vacuum line, good luck finding it.

 

Thanks. I found a broken vacuum line underneath the coolant bottle. I have not fixed it yet. Could that be it or does that one go somewhere else?

 

There is a line coming out of the firewall,that goes to the climate controller,and to the heater control valve,if either of those are disconnected it can cause your problem.

 

Thats probably one of them. I replaced that entire hose, it goes over to the manifold, that made all my settings work, but then under load it would revert back to the defrost till I let off the gas, so i did some more searching and found one of the ones going into the firewall had a small hole worn in it, its pretty reliable now and the 4wd works.