Not the right time of year to be very concerned, but it did get a little chilly this morning so I upped the temp, only to find it didn't produce any heat! I ran it up to full heat and still nothing. Luckily, cold air definitely works.

Would it be coincidental that I just went through my center vents to remove the broken vane holder plastic and redo the foam trim? I noticed the center vent temp modifier wiring had been taped down, and I didn't bother to do that again, just stuffed it back in as it was.

Where should I start diagnosing? The heat valve? There are no issues with the coolant system that I am aware of. Everything is like-new under the hood.

 

Most likely it’s the heater pump that’s packed up. This is near the bulkhead, just before the heater valve.
Good wright up here
http://jimbutterworth.co.uk/8jagpump.htm

 

Thanks! I found the Jaguar docs for diagnosing the HVAC systems and have determined so far:
1) Face vent differentiation potentiometer is faulty (panel code 24)
2) Heater Pump appears to be locked up as you stated.

I'm not sure about the heater valve yet. Is it easy enough to witness cycling, such as when going through the actuator cycling in the panel testing? At this point I couldn't tell. Since this is supposed to be OPEN on fault, I'm not as worried about it yet.

Great write up, thanks! That should help me get to the right resolution.

 

SUCCESS!
I had a few old DC motors in the garage and scavenged a couple nice brushes from one of them. Took a lot of work to get them sanded down to 4x4x10 mm , but I completed the job and everything is back up and running! Now my car can go from ice cold to steaming hot in seconds again :-)