Good afternoon,
I own a 1996 XJS and it has the best heater of any car I've ever owned. In fact it works so well you cant make it stop heating the car! As you can imagine this made driving it in the summer unbearable roof up and nearly unbearable roof down. How do I make it so that the heater isnt jammed on and I can use my air conditioner? Any Ideas?
Thanks,

 

Sounds like your heater control valve at the rear of the engine at the firewall is stuck. It defaults to on, and shuts off with vacuum. Apply vacuum to it and see if it actuates.
It could also be a control flap issue on the main Delanair unit. On your version, they are servo actuated. But check the heater valve first.

 

+1 for the above suggestion. Start with teh real basics such as checking the operation of that heater valve as they eventually seize and fail.

Also, do all your tests with the aircon setting turned off and also don't have the fan speed setting in Pos 2. Otherwise it will run in auto mode and will affect the settings. You want to remove all those variables that are controlled through the Auto & Aircon configuration.

Remember also that "aircon" doesn't mean Cold. It does have the ability to lower the temp if required. But it also is filtering and drying the air. In the UK, I rely on the aircon operating with high heat much more to remove moisture from the airflow to the windscreen in the winter, rather than lowering the temp.

Good luck with your investigations.

Paul

 

If you order a replacement heater valve, I have found that 3 out of 4 of them are defective. They will leak right away.

I would recommend ordering the cheaper, and seemingly much more resilient plastic one. I believe it is AC Delco 15-5827.

Cheap, somewhat easy to replace. You will lose some coolant, so be ready.

 

Thank you! I will give that a shot!

 

It needs a coolant flush anyway^^