I was having trouble getting heat the other day and went trouble shooting today. Turns out the "climate control" is working fine on auto or manual. Here's the perplexing bit. When I call for heat from the system it comes out fine in back and foot wells, but remains cool through defrost and dash vents.

Perhaps its gremlins?????

 

It would help to know what model and year car we are talking about. I suggest you put it in your signature.

 

Bo,

I've moved your question from General Tech Help section to XJ6 & XJ12 Series I, II & III section. The heater design has changed over the years and members here with the same model as you have will be able to offer advice on this issue.

Graham

 

A couple things......

In a sense the system is *designed* to heat and cool simultaneously. All incoming air is first routed thru the evaporator core for refrigeration and then routed thru the heater core to be heated as required. This is accomplished by a couple of cylindrical flaps.

Of course when full cooling is called for the flaps prevent incoming air from going thru the heater core. But, in anything less than full cooling mode, the air is blended thru both.

In your case I suspect the upper flap is allowing some refrigerated air to bypass the heater core and go right into the cabin.

The two cylindrical flaps each have a servo motor that rotates them. If my theory is correct then your upper servo motor is operating correctly

The system has several tiny wiring connectors. A good place to begin is finding and cleaning these connectors (recognizable by the tiny wires!) as they often corrode. I wanna say the system has about 5 of these connectors. Most are easy to spot. One is wayyyyy high on the left side of the main climate contraol casing. Also find the main system grounds and clean them. On an XJS these grounds are on the vertical dashboard supports. Not sure where they are on the Ser III V12.

I've brought a couple wonky Delanair systems back to proper operation just by cleanng connectors and grounds.

If no joy then we'll have to dig deeper.

All that being said, where is your upper/lower bias control set? This is the slide lever below the radio. If set to "colder" then the upper air *will* be colder than the lower air....as that's what the bias control does. However, this is a subtle adjustment offering only a few degress difference between upper and lower air temps. It wouldn't normally allow upper air to be outright cold if the lower air is warm or hot.

Cheers
DD

 

Thank you for the suggestion Gus...I thought it was in my signature???

 

Thank you Graham!!