Ok so I have posted before about my "heat not working" however the next time I drove the xjs the heat was fine. What I've found is that if the temps are around the mid 50s I have good heat. Below that, like this morning when its 46 I have no heat. It took about 5miles for my windshield to defog! Both heater core hoses are as near as equally hot as you can tell, and my rad hoses are hot too. Also, my heat valve lever is moving automatically as it should, so I think it's working. I guess it could be a blend door, but why would it only really happen below a certain temp?

 

What happens when you put the heater to defrost?

 

Sometimes mechanical bits (like blend doors) don't like cold weather and refuse to cooperate....but 46ºF (8ºC) isn't all that cold.

The intellectual part of the system might not recognize coldness below 47ºF, due perhaps to a temp sensor problem, and thus doesn't see the need to give heat.But defrost mode should always give heat....which I suspect is why Greg specifically asked about it.

One possible clue is air flow. When heating is called for the system directs almost all air to the footwell outlets. If your setting calls for heat, and the airflow is directed to the footwells, it can be assumed that the system understands the request and is trying to comply....but for some reason the air isn't actually being heated.

What year is the car? Does it have the Mark II system with the manual temp control feature. If so, does using the manual control make any difference?

Cheers
DD

 

I did have it on defog the whole time. Mines a 93 and I pretty much leave it on the manual option. It is an interesting thought about the temp sensor, but not sure where to look. I would think no matter the temp, if on manual, it would blow full hot no matter what. If I switch off of defog the airflow changes to the other vents. I do have the fuse out of the passenger side AC blower due to a parasitic drain, but again, I have heat above 55ish

 

If one fan is disabled while these effects are happening, you may well be sending air from the hot box out of the disabled fan's intake under certain conditions. The aircon unit's hot and cold boxes must have air pressure from both fans to send conditioned air into the cabin properly. The more so if that side's fan has the intake air or cabin air temp sensor in or around it. i would fix that fan before doing anything else.

 

How hard are the fans to replace?

 

Driver's side a pain, passenger's side a lesser pain. Doable in about a morning per side.