Hello all, I've found old threads on here quite helpful in the past for fixing my 87 XJ12, so thank you for those, but I can't find anything similar to my current problem so it's time for my first post.

So up until today I've had no heat; I was just getting blasted with cold air when I would turn on the heating. Thanks to a thread on here I realized that the inside of my heater valve could still be closed even though it was opening on the outside so I bypassed the valve and I got heat!...well, at my feet and in the back seat, and only for about 5 seconds after switching the knob from high to defrost or vice versa, ugh. If I sit there, flipping the switch from high to defrost, or vice versa, every 3 seconds I get good constant heat at my feet. If I switch from high to medium or low the heat does not come back on.

I'm fine with heating coming from only a few vents, but switching from high to defrost every few seconds to keep winter at bay isn't fun so I was hoping someone could let me know how to solve that. Thanks, George,

 

Looks like I was too quick to post here; I pushed in the heat temperature dial and now the heat stays on. Still, anyone with heating problems should definitely try bypassing their heat valve to make sure that's not the problem before replacing the heater core.

 

Welcome aboard anyhow


Cheers
DD

 

New valves are not cheap !! I was quite shocked to see this sort of price on eBay, but it is a new valve, and as they haven't been in mass-production for a long time, reasonable in the circumstances.

JAGUAR XJ6 XJ12 XJS HEATER CONTROL VALVE C41051 | eBay

 

Can't say about the UK market. But, in USA, generic plastic valves are available and function quite the same. Much less pain in the pocket book as well as no rust!!


My 83 Came here with this type and it does just fine.


Don't recall the default though, vacum on valve closed or the converse??


If the one you have isn't stuck, perhaps, a vacuum check might fix to work.


There is a reserve vacum tank on the firewall!!


Carl

 

do you mean your temp. knob is a push/pull knob ?
if so ... what is it's purpose ?

 

The later (from Aug 1986-on) Ser III cars with the V12 came with the revised 'Delanair Mk III' climate control. One of the features was a 'pull for manual control' capability. When pulled the temp knob simply becomes a hotter-colder control....over-riding the automatic temperature regulation.

The 6-cyl Ser IIIs never had this system. They used the Delanair Mk II system from beginning to end


Cheers
DD

 

With regards to the heater valve... Unless you are a stickler for completed originality or concours, I recommend a modern plastic heater valve. I don't have the exact vehicle model year that the heater valve comes from, but just remove your old, worn out, leaky, rusty, seized or otherwise faulty valve and take it with you to your local parts counter. Don't tell them what it's for, just have them pull a variety of valves and compare the inlet/outlet pipe diameters. Most of these heater valves function on the same premise, no vacuum - valve closed. vacuum - valve open (pretty sure) ... either way, just match it up and make sure the valve functions the same way (use your mouth to apply vacuum)... Good luck, and let us know which way you went!!!

 

Four Seasons 74612 is one of many that will work




To clarify you want a valve that closes with vacuum and is open without vacuum


Cheers
DD

 

^^^ what Doug said

 

Well considering my laziness and ignorance, the crampedness of the v12, and that I never use the air conditioning, I don't see why I should replace the valve.