My heater valve (original Volvo p/n 9447896) failed. One of the plastic hose fittings barbs fractured radially and I lost coolant. I replaced the heater valve with a Motorcraft p/n YG-136. The question I have is, when is this heater valve supposed to actuate? It seems the temperature control **** on the MCC can turn the heat on and off, but I don’t see the heater valve actuate. However, I believe the temperature control **** does operate a damper shutting off the air flow. I tried to turn the AC on and I still did not notice heater valve arm movement. Is this Motorcraft p/n a viable substitute or do I have another problem?

 

I dunno any motorcraft number, but vacuum closes the valve, no vacuum opens it. my 1992 740 had a chevy style valve when I got it, and the heat just didn't work as well (25% heat was nearly as hot as 100%), I replaced it with a VDO valve, and got much smoother control over how much heat.

the temp **** should only affect how much heat you get, the air direction **** adjusts what flaps are open. note that heat is never sent out the center console vents, those are always fresh air.

 

Pierce, mainly I am trying to understand what switch or button triggers the vacuum?
Is the heater valve either full open or full close, or can it be in between?
Does the temperature **** provide an adjustable vacuum pressure?
Does pushing the AC button to the AC off position turn the vacuum on to close the heater valve?

The Motorcraft p/n has an external linkage arm that I'm thinking adjusts the heater valve/vane. I don't see it move whether I move the temperature control **** or turn the AC on or off. However, the temperature **** is able to adjust the air temperature coming out of the vent. I don't understand how that is happening since the actuator arm is not moving to close the valve.

 

Actually hot air does come out of the center vents of 740/940s... At least in all the cars I have ever had my hands on.

 

the temp **** controls the vacuum, the AC can be on with full heat (often used for rapid defrost).