Thank you, that's something to get me started. Can you clarify a couple of things? From what you said I'm getting that the monovalve needs 12V on one pin, and ground on the other for it to be closed (which would stop the coolant from going in, which would stop the heater core from blowing heat). Is that correct?

What I'm interested in (at the moment anyway, for the summer already underway) is for the valve to be closed when the wheel is on min... so is the PBU is the problem, because it's not sending it the solid open signal?

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Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled