1995 E320 Heater Valve Diagnosis

[thayer]

Confirming failing Heater Valve prior to dropping the $500 on the part.

Symptoms

Monday, exterior temp is 80 degrees, engine at operating temp. Max heat from any and all vents at all pushbutton settings.

Tuesday, exterior temp is 38 degrees, engine at operating temp. Cold air from any and all vents at all pushbutton settings.

the 95 has the monovalve/auxpump combo unit.

In my searching I found that the monovalve should default to open allowing coolant into the heater core under monovalve failure.

Tuesday continued: I pulled over and unplugged both plugs at the valve without turning off the car. Zero change in vent temps (cold air all pushbutton settings). Plugged the valve back in. Very confused at this point.

Went for another drive, cold air.
turned car completely off, unplugged both plugs at the valve.
Turned car on and HEAT!
Turned car off and plugged in just the tall cylinder on the unit - HEAT!
Turned car off and unplugged tall unit and plugged in short unit - NO HEAT!
Turned car off and reversed it Short unplugged and tall plugged for the winter.

So, in conclusion, I am thinking that the short cylinder is the mono valve and the tall cylinder is the aux water pump. (this also makes sense as a pump would need a larger housing due to a pump being more complex than a valve)

The remaining question I have is that if the monovalve is failing, why would I need to unplug it to get heat if its natural state of failure is open. When I plug it in it closes and when I unplug it it opens during cold exterior temps but in warm exterior temps its open all the time.

Fan speed, flaps, and temp wheel all function beautifully.

What controls the voltage sent to the monovalve?

[Zulfiqar]

try tapping on the heater valve with a small hammer and see if it starts to behave.

my W124 did that too and the tapping fixed it. It gets stuck with wrong and neglected coolant.

Also make sure the thermistor next to the dome lamp is not covered in dirt/lint. The pushbutton unit measures cabin temperature from it.

[thayer]

I did give it a tap with the hammer yesterday and no change. I'll check the interior temp sensor's dust level. I hadn't really gone into that one due to the fact that today the fan speed changed as the cold car became warm. Obviously with the monovalve unplugged and open, it was getting max heat from the core.
wonder how the coolant system would react to an acidic type flush. may try that next. Pretty sure when my I lent the car to my mother, the coolant light came on and she filled it with green coolant instead of water.

[thayer]

Well, I swapped the heater valve with a junkyard heater valve and there is no change in behavior. My next question is what directly affects the opening and closing of the heater valve

[thayer]

So today on a little drive I put the climate control on auto, center button, and 75 degrees. 45 degree ambient.
Result was low fan and pleasant air.

Moved temp wheel all the way to cold: result was max heat
Moved temp wheel all the way to hot: No heat.

At least the monovalve is operating, even if it is the opposite of normal

Up is down. Right is wrong. Cats and dogs living together, mass hysteria.

[thayer]

OMG

So I swapped the wires between the aux pump and mono valve.

Everything works fine now.


please find a gif that represents how I feel right now.