No heat due to bad heater control valve
In some cars, the heat is regulated by a heater control valve that controls the flow of hot engine coolant from the engine to the car’s heater core. On older vehicles, the valve is operated by a cable, while newer vehicles used a vacuum actuator controlled valve.
A heater control valve can fail in several ways. Since a vacuum operated valve defaults to the fully open heat-on position when vacuum isn’t applied, a loss of vacuum is the first thing to suspect in a “heat always on” condition. The loss of vacuum can be caused by a break in a vacuum line or a faulty vacuum switch. To repair a faulty switch, you sometimes have to replace the heater control. Depending on the amount of vacuum, the vacuum motor will close off the flow of engine coolant fully or partially. However, even if the heater control supplies vacuum the mechanical portion of the valve can fail, preventing the heater control valve from closing. Or, rust or sludge in the cooling system can block the valve and prevent it from closing fully.
On a cable operated system, the cable can break or detach from the control levers, leaving the valve in the fully open position.
How to test a heater control valve
If you have No heat or the heater isn’t hot enough
Adjust the heat to the full on position with the engine at operating temperature. Then check the temperature of the hose on the inlet and outlet sides of the heater control valve.
The inlet and outlet hoses should be the same temperature. If the inlet side is warmer than the outlet side, the valve isn’t opening or opening fully. Remove the vacuum line from the vacuum motor and check for engine vacuum. If there’s no vacuum, conduct the steps shown below. Next, try moving the mechanical linkage by hand to open the valve. If the temperature still doesn’t change on the outlet side, suspect a stuck shut or clogged heater control valve.
Heat always on or air conditioning cools at idle but not at highway speeds
Pinch off the hose on the outlet side of the heater control valve (the hose closest to the firewall) and drive the vehicle at highway speeds. If the blower stops blowing hot air, you’ve found the problem. And, if the air conditioning runs colder with the hose clamped off, the valve is causing the problem.
Troubleshooting vacuum motor problems
If there’s full vacuum going to the vacuum motor, then the vacuum diaphragm may be leaking or the mechanical valve is rusted or clogged and must be replaced. However, if the vacuum motor isn’t getting full vacuum, consult a shop manual and conduct the troubleshooting procedure to determine why vacuum isn’t reaching the vacuum motor. The problem may be as simple as a cracked vacuum hose, broken vacuum switch, or faulty electronics.
© 2014 Rick Muscoplat
Posted on by Rick Muscoplat