Formosa_Oolong

Hi All,

Happy New Year!

I have a 1991 560 SEL. Recently, it began only intermittently heating cabin air with the heater on. Usually air blows cold. Blower works.

After research, I decided that it might be the "mono valve" and inspection revealed a badly torn large rubber diaphragm. I thought, "Aha, that's the problem."

I installed a new aftermarket replacement mono valve and tested the system again. Alas, the problem was not fixed.

Voltage at the monovalve input control terminal while car is running is about 13.3 with climate control system off (or temp on Max Cold) and about 0.13 - 0.15 with heater engaged - that is, any temp above Max Cold when it's cold outside. These readings are consistent.

Sporadically, immediately after heater is turned on, there is a sound of rushing liquid behind the dashboard and then for 1 - 2 minutes the heater blows nice heated air. But soon the air cools to ambient temp even while the mono valve control voltage remains in the 0.13 to 0.15 range. More commonly, there is no sound of liquid movement when the heater is switched on and the heater just blows cold air.

Does this suggest that the replacement valve is faulty?

Thanks very much for any help.

Russell Ormerod

Sounds like a flow problem! Is there an aux water pump fitted? Does it work? Is the coolant system clean and free of rust? Thermostat working as it should?

Formosa_Oolong

Thanks for your reply Russell. The only question for which I have a good answer is that regarding the thermostat. It was replaced not long ago and it is working. I don't know whether there's an aux water pump or where it should be located (or how to test its function). Coolant seems fairly clear.

One other thing I've noticed is that I am often able to get warm air at idle but when rpm's rise during driving or even when I increase rpm's in Park, the air temp drops rapidly.

Russell Ormerod

A quick check is to run the motor to operating temperature with heaters on full hot then feel the water hoses temperature in and out of the fire wall, they should both be hot. If it has an aux pump you will find it in one of the heater hose pipes. Maybe do that test then increase the rpm and see if the outlet hose cools.

Ric Bayl