1990 Non A/C, My truck lost heat in the spring and now its getting time to fix it. Blows air still, just cold air. I took the glovebox out and slid the cold-luke warm lever back and forth and I can see the cable actuating a rod but I'm not sure if thats the only thing that is supposed to happen. I thought I read somewhere that there is vacuum in the mix somewhere but that might be for A/C only.

 

Check the two heater lines going to the heater core at the firewall. One will have a vacuum actuated valve, it could be stuck closed. I suggest ditching it and replacing that hose with one long hose or put together a ball valve shut off or the like.

 

May have a cloged heater core. Not unsual. Take the control valve out of hose in engine compartment and see if opens. Then hook hose back to heater tube at cowl. Refill coolant and see it water flows. Hose will ge hot is flowing and you will have heat inside cab. No heat equal plugged heater core.

 

Check for the heater control valve in the heater hose. Try bypassing it. If that does not work the heater core could be plugged. Get thetruck warm and check to see if both hoses are hot. If one is cold do the above. If they are both warm it could be a blend door problem.

 

Also make sure the cable from the temp slider has not come loose.

 

The heater control valve is turned off by vacuum . . . with engine running switch heat from OFF or MAX AC to any other position. . does the linkage change position?? If so it is working. If not, you have a vacuum contol system problem.

Snap heat control lever from HOT to COLD fairly quickly. Do you hear the blend air door smack it's stops?? If so it is working. If not, fix or adjust the cable.

Now if the above items work, engine is hot & the heater hoses are not both hot, you probably have a plugged heater core. Unless you have no fan belt on your engine. ;-)