I have a 78 xj 12 that has full heat all the time. I bought a used control unit to study and rob parts from. I have swapped parts out checked all vacuume lines and fuses and I just can't get the upper vents to open and the heat to turn off. Any help that anyone can give me would be great.

 

you might need a new A/C Amplifier, system in Heat Mode all the time is the classic symptom of a bad amplifier. It is located (buried) behind the radio, unless it has been replaced before. Remove the driver's side Floor Vent Panel, (left side of the radio), look for a cream-color round hex plug with many small gauge wires; Follow the plug's wire harness to the back. It's a black or brown box held in place by a spring bracket.

the only way to find out is by substitution. The replacement is not cheap but you might buy one anyway because sooner or later you'll need it.

John's Cars in Dallas sells an Australian replacement that is shaped like a cigar and is known to be better than the oem part. You have to call them on the phone to get it.

don't bother trying to remove the old amplifier, just get a new one, disconnect the two round plugs from each other and connect the new cigar-shaped amplifier to the plug.

Jaguar conversions, swaps, parts, service and advice - from John's Cars in Dallas.

 

I have a used complet unit and I tried replacing that amp and still not working, maybe the used one was bad. Who knows

 

When you turn the temp selector from hot to cold do you hear a faint whirring sound? If so, the system is at A) least fundamentally operational, B) is trying to comply with your request, and C) the servo motor works.

No noise requires approaching the problem differently although, in either scenario, the amplifier may be faulty.

Amplifiers are a known problem and often fail. It's entirely plausible that the used climate control system you bought has a dead amp.

Cheers
DD