Hi Keith,

I've got a similar problem on a 230TE without aircon.

A 12v feed goes from the fusebox to the terminal 2 of the duovalve, and the feed continues from there, if I remember rightly, to the auxiliary circulation pump. When the control unit is calling for heat, terminals 1 and 3 on the duovalve will be at 0v.

If you unplug the duovalve, do you get full heat out of the vents? You may be able to feel the temperature of the outlet hoses to determine if the duovalve is closing fully. Have you tried tapping the solenoids?

Also does your auxiliary pump work (not that that necessarily has any bearing on the problem)? Under most conditions, you should have system voltage across the pump. If not, you might have a control unit fault, assuming that 12v is arriving on the electrical supply side of the pump, and that the wiring to the control unit is intact.

Sorry not to be more scientific. I may well be looking for a duovalve and an aux pump....

Krolroger (UK)