Originally posted by Cameron
According to my Helms manual’s diagnostics, the possible source of insufficient, erratic, or no heat is the following:
Coolant level
Engine overheating
Heater core
A/C electronic door actuator motor
Water control valve
Thermostat
Let the motor warm up to normal operating temperature and feel the heater control valve inlet hose. This hose (at least in the 97) is located on the left side of the firewall in the engine compartment. There will be two hoses going into the firewall. The upper one with the vacuum actuator attached is the heater control valve inlet hose. Feel it before the actuator. That is, further away from the firewall. Is it hot? Now feel it on the other side of the actuator. Is it hot?
There is a complicated (at least complicated enough for me to not want to type all 40 + pages of heater diagnostics) series of test to perform to pinpoint the problem. I would take it to a compitant mechanic. Or, order the Helms manual for your truck from Helmsinc.com
Good luck.