I replaced the heater coolant vacuum valve on the '89 XJS after noticing I could pry it closed and open, but the vauuum hose, which did draw, would not operate it. How can something that simple be that hard to change?!- Got it change though, and happy that the lever did now work when I turned the temp control from cold to hot, and with the engine shut off, I can hear a slight whrrrrr in both directions when rotating the control knob from cold to hot and back. There is definitely hot coolant now flowing through it, but still no heat to Mumsie's feet, central vents, or the defrost.
We went for a nice 500km ride down across the US border into Maine on Saturday, car worked super except my wife wanted heat and I could not provide.
Before I delve for days into the archives, where should I head next in the search for the reason I'm not getting heat? BTW, the fans work good.

 

Greg,

You are stating that "hot coolant" is getting TO the core, so that means the tap is dropping vac and moving to the ON position. Since I am gathering this has been an issue for a while, maybe the core is blocked. Feel both the core hoses at the firewall, and establish them both being the same (close to) temp.

If that is so, then the core is deemed OK, and I reckon you got a "blend flap" jammed inside the mysterious box of bits called the "climate control unit", and since I reckon an '89 is a Delanair 3 it is ECU driven, with small stepper motors at each flap axle point. The Delanair 2 was Vac/rod & linkage driven, and once set properly very reliable.

I have NO hands on with the Delanair 3 unit, so I will bow out gracefully, and wait for someone with that expertise to come along.

 

The whirring indicates that the blend flaps are working, they are drums actuated by direct acting servos (on the RH side of the unit IIRC). If the check of the in and out hoses from the heater core reveals a temp difference as an alternative to a blockage you could have an air lock in the heater core after replacing the valve. It is some time since I did this but I remember it was hard to get rid of. I did the simple thing: Draining a significant amount of coolant and refilled/bled as per book: Fixed. You could of course drive it up a steep incline and with water valve fully open (just pull the vac hose if unsure) and run at 2000rpm for a minute to see if that helps.

If you can tell us the circumstances leading up to the "no heat" situation it would also help.

 

Greg, good remarks from Grant and Per.

I suffered the blocked heater core on my XJS....then after fixing that I still had no heat because the system wasn't properly bled. Thus my lessons on V12 cooling system bleeding began. Ah, memories :-)

All incoming air is refrigerated first then heated as needed by rotating a drum (as mentioned above) to direct the air through the heater core. I have to wonder if one of your drum-rotating servo motors is dead and you're hearing the other one whir.

The drums are horizontal in the case. The servo motors are on the right side of the case attached to the "axle" shaft of each drum. On the left side of the case the drum axles are attached to feedback potentiometers. I vaguely recall that each axle is visible at the feedback pots. If I'm correct then you can operate the temp knob and observe whether or not the axles -- and therefore the drums -- are rotating.

Cheers
DD

 

I always blow out heater cores when I have the disconnected. I just fixed a xj8 doing this and felt bad for the customer because she said she's gone without heat for 2 yrs and thought the repair would be expensive.

 

Sorry for not detailing my problem more, I have owned the car since April, and had no heat ever since owning it. -If I can find the other end of the heater coil hose coming through the firewall (and can access it with my BananaHands), and if I cannot feel any heat coming through it since I do have heat at the heater control valve, I will try to flush it out both directions with a common water hose. If no luck there, I will begin to find the heater box and drum controls. -Wish me luck.

 

+1 for a thorough bleed. I recently had no heat immediately after a coolant system refill...... I had to bleed it 3 times before I got heat back.......

 

Used to have an old (non jag) car which the heater wouldn't work after flushing etc. Finally used a 2/3 stage flushing system (Holts?) where you put a cleaner in run it for a while, then drained it out, put another part in and flushed it out again and finally another part which you left in there. Worked a treat, I was finally warm.......till a lorry wrote the car off!
I think its equivalent is still available as Holts 2 part radflush.