A coolant vacuum valve switch is a type of electronic engine switch that commonly functions as the heater control valve for many road going vehicles. It is a simple electronic switch that uses a vacuum operated diaphragm to open or close the switch. Once activated, this switch will allow coolant into the interior of the vehicle’s heater core so that the heater may function and blow warm air. Because the coolant vacuum switch essentially functions as a heater control valve, when it fails, or has any issues, it can disable the heater. Usually a bad or failing coolant vacuum valve switch will produce a few symptoms that can alert the driver of a potential problem that should be serviced.

1. Heater blows lukewarm air

One of the first symptoms of a potential problem with the coolant vacuum valve switch is a heater that blows only lukewarm air. If the coolant vacuum valve switch has any issue, it will causes the valve to only open half way. This will restrict the flow of the hot coolant to the heater core and reduce the effectiveness of the heater.

2. Heater does not blow warm air at all

Another symptom, and a more serious issue, is a heater that does not blow any warm air at all. If the heater control valve fails completely in the closed position, it will not be able to allow any coolant into the heater core so that warm air can be produced. At that point the only way to restore functionality to the heater is to replace the switch.

3. Coolant leak

Coolant leaks are another more obvious symptom of a problem with the coolant vacuum valve switch. Because the coolant vacuum valve switch functions as a heater control valve, it comes into direct contact with coolant and is susceptible to leaks as a result. The vacuum valve switch will usually leak at the threads, or directly from the housing, due to cracks from heat or excessive corrosion.

The coolant vacuum valve switch is an important component as it plays an essential role in the operation of the heater and is also tied into the vehicle's cooling system. For this reason, if you suspect that your coolant vacuum valve switch may be having an issue, have the vehicle inspected by a professional technician, such as one from YourMechanic, to determine if the car needs a replacement coolant vacuum valve switch.

This article originally appeared on YourMechanic.com as Symptoms of a Bad or Failing Coolant Vacuum Valve Switch and was authored by Ed Ruelas.


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