I just picked up a 1976 XJ6L & I also own a 87 XJ6 and both have the same heater problem, NO temp control (set at high, hot, defrost), so my question, where does one start; with the dash controls or the heater valve?

 

You say you have no control no matter where you set the dial. Does this mean you have constant heat, or no heat?
EDIT:
You mention the fan control, but what happens when you twiddle the temperature control? Try rotating it to both extremes, 65 -85 and back. Sometimes that wakes mine up to fly right.
(';')

 

First.....

When you change temp settings do you hear a faint whirring sound?

If you gradually go from full hot to full cold does the airflow change from the bottom outlets, to the side outlets and, finally, on full cold, to the big center dashboard vents?

The answer should be 'yes' to both questions as it indicates the system has rec'd and understood the request for a temp change...and is trying to comply.

The heater valve should be open at all times except when full cooling is requested....at which time is closes. Have a helper adjust the temp setting while you observe the actuator arm on the heater valve. No movement means the vale is stuck or isn't being commanded to move.

Sometimes the innards of the valve fail...rendering the valve useless even tho the actuator linkage moves as ordered...but I wouldn't worry about that possibility quite yet.

Lastly (perhaps I should've gone here first!) I'm not quite sure if I understand the exact symptom. Are you saying the system is stuck on "high, hot, defrost"? If that's the case you may have an amplifier and/or other electrical problem.

Cheers
DD

 

Remove USA passenger side vent outlet finisher that is on the transmission tunnel. Just two screws. Using a flashlight and lookup up, observe if the the metal rods move in response to setting the extreme heat then extreme cold setting.

If yes, then check your vacuum signal from the engine and check valve. should have vacuum in foot well bulk head adapter on the larger of the two metal nibs.

If no movement on the rods, then using a DVM check the resistance on the cabin temp sensor (under dash over hang), also check the resistance on the ambient temp sensor in the passenger side blower, and check the Resistance on the temp setting pot, should vary as you move it from one extreme to the other. Often the thermistor in the blower fail from vibration.

It unlikely the transition motor failed at one extreme so it most likely moved there on its own from a failed sensor or failed control unit (brown box on driver side behind heater vent.

Those tests should turn up something, try 'em and report back.