94 B3000 SE Heater blowing cool air

[Richard]

Heater is blowing cool air. Water pump less than 1 yr old. New thermostat. Thermostat changed after found that heater was blowing cool air. After changing thermostat and topping up coolant, on short test drive heater was working great. While still on test drive, changed to A/C, but it did not get very cold. Went back to heater, and went back to cool air again. Noticed after changing thermostat, with heater on and moving lever to defrost, the A/C compressor would engage. Anyone have any ideas? Thanks, Richard.

[Richard]

Up-date on heater problem. Removed heater core and flushed with clean water (was very dirty inside). Heater is working much better now. Takes awhile to warm, but seems to be better now. Side note, does not have a Heater Control Valve as is stated on auto-zone trouble shooting web-site.

[hobie237]

Have you checked the thermostat? Or just changed it? They're designed to fail open, as that's the less-dangerous failure mode, but that also leads to the symptoms that you're describing. Is the motor operating at proper temps?