Blowing cold air.

[tomford]

I Don't know what to search, have an 87 K-5 and the previous owner has the heater core changed, and now the temp lever doesn't change from cold to hot air. It just blows cold. Any ideas?
Thanks
Tom

 

[diesel4me]

If it has A/C ,its likely the vacuum controls or actuator isn't closing the blend door in the heater box--could just be a vacuum hose came off,split,etc,the control valve unit at the dash,or the "can" is defective and not closing the door..some years of A/C trucks had a hot water shut off valve on the intake that is plumbed into one of the heater hoses--if that stays closed you'll get poor or no heat..

Non-A/C trucks have a manual cable that opens and shuts it,that may have broke or came off the lever on top of the heater box..if that blend door doesn't close all the way,cold outside air comes blasting in,and the heater core doesn't do much !..

Oh yeah--you can open the glove box and squeeze the box part away from the dash where the latch is and open it "all the way" to gain access to the top of the heater box..in case you didn't know already...you should be able to see the cables or actuators on top of the heater box...A/C trucks had another actuator in the center right above the floor outlet for the heat,to change from floor to defrost mode too...

 

[tomford]

Thanks. It has AC so I'll check the vaccum lines.

 

[Blue85]

The change between hot and cold is not vacuum actuated, even on A/C trucks, at least in the 80's. There is a cable from the dash temp lever to the blend door. The little plastic tab does like to break and can be replaced cheaply now (search for the source if you find that problem) or repaired. However, unless the blend door is really jammed all the way to cold you should get some heat. The vacuum actuators select things like defrost and fresh/recirc.

I would check coolant flow issues first, starting with coolant level. If both heater hoses are hot near the firewall, then you know the problem is under the dash. If not, make sure the heater core is actually plumbed in and not blocked with valves, as a previous owner may have rigged a bypass for the sake of AC.