Hi,
I'm looking at buying an '85 XJ-S, and the only thing on the car that does not work is the heat. The fan blows fine through all speeds, the AC works and is charged (blows cold), but when I move the knob to 85 it never gets hot.

I read somewhere this may be due to the heater core being bypassed - the next time I go to look at the car I'll be sure to check that. Are there any other possibilities? Most importantly, are any of them particularly expensive or involved to fix?

I do all my own work and always have. Also, this car would come with an '84 parts car so even sourcing parts to fix the issue will not be a big deal. Just want to know if I'm going to end up pulling the motor or tearing the entire dash and column out. I did a quick search but turned up nothing specific to my problem.

Also, when I moved the fan knob from fan speeds to defrost, I got 99% of my air still from the dash vents and very little from the defrost vents - in fact I had to pull over and wipe down the windshield. Is this an issue or is this just me not knowing how to operate the system?

No change in the heat issue when moving the lower slider knob to either side.

Just to be safe though, can someone walk me through how to properly operate the system? What exactly is that blue/red slider under the radio doing?

Thanks a lot,
Darel

 

These are very common problems.

There are a number of possible causes with the heat not working. I'll deal with the 2 most common.

First there are 2 modes manual and auto, put into manual by pull the temp rotary control out.

There is the vacuum actuator under the bonnet about the middle of the V and up against the firewall, this sticks, mine was, with the engine or AC off this is open when you turn the AC on and full cold in manual this will close, shutting off the water to the heater core. This is easy to check.

Next are the blend flaps in the AC/heater unit. If the heater tap is ok check these. To do this pull the fuses for the blowers you will find these in the fuse box under the dash there should be on each side of the car, with the fuses out turn the heater control from full cold to full hot and you should be able to hear the blend flap motors run. This could be simple and cleaning the connectors and grounds will fix it or it could be major surgery to pull the AC unit.

Either way use it as a bargaining tool. With a spare car depending on the price it could be worth while. Although probably not to fix this problem as almost every XJS will or has at some time had this type of issue.

The slider is to enable hot air in the foot well and cold air in the face. Does it work very well not really.

I would focus more on the areas where the XJS rusts. If you can get a relatively rust free car you are miles in front. To give you an idea my car came from inland Australia very dry but it still had some rust in the buttress and under the front guards. Next would be mechanically sound.

cheers
Warren

 

Duh, Warren beat me by 1 minute, ENJOY.

I reckon the heater tap is jammed in the closed position, quite common.

The by-passing of the tap/heater it not so common.

The tap is at the rear of the engine bay, and on a V12 is slightly difficult to get at.

Check that the tap arm is in the "down" position, which is ON.

There could be a myriad of things causing this issue, but the tap is the most common.

The slider is to alter the temp coming out of the centre vent very slightly, and is related to the "blend flaps" inside the system.

 

Yeah I used a unit off a Commodore (GM) as mine was jammed on

 

Actually, no, not on an '85. The pull-for-manual-control feature is found on the '87-newer cars with the Delaniar MkIII system. The older MkII system operates significantly differently although I certainly agree that checking the heater valve is the best place to start.

Cheers
DD

 

I just tried my heater for the first time yesterday with the same result - in manual and auto modes. Is the trouble shooting the same for a '94 4.0L?

 

Great news, thanks guys. The heater wasn't going to be a deal-killer but it's nice to know the most likely suspect is a quick and easy fix!

 

I suspect at least checking the heater valve would be the same.

I dunno if/when the Jaguar stopped using the Delanair system on the XJS. I'm under the impression....and that's all it is....that it was changed to a Valeo system.

Yes? No? Hopefully someone will chime in.

Cheers
DD

 

Haven't checked yet because it warmed up but supposed to get cold again soon. So to check the heater valve mentioned (assuming it's the same on the six) it should not have vacuum when the heat should be on correct?

 

That's correct.