Here is the problem usually reported: Your blower is running and you have good air flow but the air is cold.
It is possible your "temp valve" is stuck in the cool position. (See photo below) This can be caused either by something holding it in that position, or your connecting link is disconnected or not working for some reason.
Here is how to check the "temp valve" for proper operation:
Remove the glove box. You will see a silver rod running side to side. You cannot see where it attaches on the left end but on the right end it should be attached to a arm coming out the top of the climate controller.
With the engine running, and warmed up to operating temperature, change from heat to cooling by using the controls on the dash. The climate controller should move the rod, which moves the temp valve (door).
If the rod does not move the temp valve, the air will not be diverted through the heater core and the air output into the car will be cold.
If the climate controller does not move the rod at all see section 1C-Climate Control Diagnosis of the 1989 FSM for troubleshooting instructions.If the rod is moving and no warm is output to the inside of the car, read the instructions below for adjusting the air mix valve.
Other things to check:
- Be certain fan is blowing fast enough to circulate air through the system.
- Check for bad thermostat not allowing the engine to reach proper operating temperature (170 or more degrees).
- Check heater hoses for heat. If one or both are cold there could be a restriction in the hose or heater core.
Thanks to Barney Eaton for this information.