Intermittent Cabin Heat - Second Generation Climate Control

Applies to late 123 and 107 chassis and all 126 chassis 1981 to 1991

If you own a 1981 to 1991 123, 107 or 126 chassis - you know your heater motor is working ok - but the heat seems to come and go, then a failed monovalve may be the problem.

Torn Mono-valve Diaphragm

Problem & Solution

Common among these chassis:


This little valve controls the amount of hot water going into your heater core. The seal for the valve is made of rubber. With age this rubber splits and will cause your heater to go wacky. It will generally get hot when you want it to be cold and in some cases it acts like it has a mind of its own. Failure of the mono-valve has often led to some big repair bills for the unknowing. This part is very inexpensive, can easily be replaced by yourself, and in 7 out of 10 cases will most likely solve your sporadic heat problems. Our kit comes with installation instructions.

The monovalve housing is located on most models in the engine compartment  just forward of the firewall right of center.