Took the car out last night heat works to restaurant stops working on way home. Blower works blows cold.
Checked antifreeze took a little but still no heat plenty of cold air.
Any suggestions?
94 xjs conv.
Thanks guys!

 

Check the heater valve - its silver and mounted centrally on the fire wall with a supply and return hose to it. That would be my guess, others will know more.

 

Yes, the heater valve is a good place to start.

With the engine warmed up and running have a helper slowly move the temp control knob from cold to hot while you observe the action of the valve. Do you see the little lever/actuator move? if "yes" then the valve is probably OK. I say 'probably' because in some cases the actual internal valve has broken away for the lever-arm....so there's a chance that a moving lever arm isn't actually opening/closing the valve. Rare, but possible.

If the lever/actuator does NOT move then the valve is stuck/broken....or it isn't being commanded to move by the control system.

A stuck valve can sometimes be browbeaten back into operation with the help of some WD40 or similar solvent/penetrating spary.

With the control set to hot, remove the vacuum hose from the valve and put your thumb over the end. You should not feel vacuum. Vacuum holds the valve *closed*. Vacuum is removed to open the valve to provide heat.

Also....

What *does* happen when you turn the temp knob to call up full heating? The system should respond by diverting almost all of the air to the footwell vents. If this happens then you know the system at least understands the request for hot air and is trying to comply.

Also....

What happens when you select defrost? Do you get hot air?


Cheers
DD

 

Please tell me if this is hijacking the thread but I have a similar issue. I get cold air at all settings except defrost. In defrost hot air comes from defrost vents on top of the dash. I can hear a whirring noise when I move the temp selector but have not checked the valve on the firewall yet.

 

If it blows hot on defrost then the hot engine water is getting through to the heater, unless on other settings the heater valve is closing immediately. More likely that when you switch to other settings the actual HVAC unit is malfunctioning. Depending upon which model you have, it would be an electronic failure, or a failure of the flaps to activate properly. On a 1986 you have the Delanaire Mark II. It could be well be an amplifier failure as the defrost setting over-rides the signal, and these are almost impossible to get now, or it could be the flaps not working. In the Great Palm's book there are suggestions to get over the amp problem, which you could at least use to test this theory. There are other bits to go wrong (temp sensors, cam drive thingy that does the flaps, and actually jammed flaps, vac tubes come off vac solenoids etc etc) but I would start with the amp side of things.

Greg

 

Hi all,

I have a 90 XJS convertible. I had to go in to the AC unit over the summer to drill out those drain holes so the AC wouldn't short out. The AC works great. Since the weather is turning cooler here in NM, I noticed the heater isn't working on any setting. I only get cool air and I have never gotten anything coming from the footwells. It when the heat is on, the air comes from the dash vents. I checked the vacum at the water valve and in either hot or cold, there was no vacum.

Any suggestions??

Tony

 

Had no heat, also no ac. Jag mechanic thought it was a vacuum problem affecting blend doors. Heater valve ok and other obvious things checked. Nothing showed up to explain problem.

Car was in shop over a week. Finally the shop owner on a hunch replaced the computer cannibalized from an xj6 he had in yard. After making fitting adjustments, things worked almost...got heat but no different fan speeds. So I found a 92 xjs computer on ebay, had mechanic talk to seller before buying ($350) who also had seller send him a picture before giving me go-ahead to buy it.

Installed and things are all working as new. Shop owner friend of family, has worked on other Jag we owned, and admitted it was 1st time he encountered this kind of problem.

So I'm out $350 plus labor but if car continues to be problem-free, at least re. this problem (yeah I know, selling my boat was to stop throwing $ away but I have a low learning curve) I hope the next time I take it to shop will be for something else.

And for sure there will be a next time.

 

feel both heater hoses and see if there is a significant difference in temperature. If one line is much colder than the other, it isn't circulating properly. Sometimes it is hard to feel in the engine compartment next to a hot engine though....

I thought the heater valve was "always on" and the controls turned it off for when the AC was on? I haven't even looked at mine because I assumed if it is broken then the heater should be working anyways, and the thing is 25 years old, so its entitled to be pretty clogged