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Is My Monovalve Bad?

While the system has multiple points of failure and changing the mono valve isn’t a guaranteed solution in all cases, the monovalve is a common failure part and when they fail they often exhibit telltale symptoms.

  1. No heat at highway speeds or high engine RPM. Slowing down or shutting off engine and restarting temporarily remedies the situation.
  1. Erratic operation, such as sudden onset of hot air when A/C is called for. Poor and lukewarm heat when full hot is called for. Unstable automatic climate control operation.
  1. A/C not working well, despite system being properly charged and condenser get hot to the touch.
  1. Heat not working at all, despite normal engine operating temperature.

Old Bosch Monovalves can sometimes be rebuilt, but you’re better off retiring them for newer technology. See diagram. Here are some of the common failure points that plague the old valves:

  1. Weak return spring or poor quality aftermarket return spring. The OE design has physics working against operation and reliability, which is not ideal. At high engine RPMs, if the valve is closed and needs to re-open, the return spring is doing all of the heavy lifting in fighting the coolant flow and pressure. If the spring is worn out or poor quality (inferior aftermarket parts), it won’t allow the valve to re-open unless the engine RPMs drop. If the spring is made too strong, the valve cannot shut with a good seal, and performance of the AC and automatic climate control suffers. It can also increase resistance and shorten coil life.
  1. Torn diaphragm and worn stopper or poor quality aftermarket part. If these parts are worn, valve function will be erratic. There is also an equalizing vent inside which allows coolant to pass into the solenoid chamber. If the screen or vent is clogged, the valve won’t operate properly.
  1. Solenoid coil failure. As these units age, the coils fail from heat, corrosion, and coolant leaks. When tested with a meter, this can manifest as either total continuity loss or a high electrical resistance failure.
  1. Aging electrical connectors can fail and cause short circuit. Terminals get corroded, break, and/or fail to conduct properly and increase circuit resistance.
  1. After too many rebuilds, actuator housing screw bosses get stripped, and/or screws corrode and break inside the housing.