My blower fan works properly, however my temperature side seems to do nothing. It always feels like it’s on full heat. It even seems to blow some warm air when it’s totally off. AC isn’t working either, but at this point I’d just like to circulate outside air. I’ve tried the knob pushed in and pulled out. My temp knob turns with almost no resistance. Do they typically turn with similar resistance as the fan control side? I’m wondering if that could be the issue before I dig much further. When I bought the car the left knob struck me as too easy to turn, but at the time I was just happy the heater worked.

thanks

 

Pull the left knob off the shaft. See if the shaft turns easily, or if it's just the knob spinning on the shaft.
The heater water valve uses vacuum to close. If the system has been broken for sometime, the valve itself may be stuck open. The quickest way to shut off heat until your system is repaired would be to clamp off the hose leading to the valve. The valve is located against the firewall, immediately behind the center rear of the engine.

 

Since no vacuum opens the valve, clamping the hose would keep the valve open

To close the valve you'd need to apply vacuum. Just run the hose to a nearby source of manifold vacuum.

Cheers
DD

 

Very little turning resistance on the temp control knob

When slowly turning the temp knob does the airflow change? When heating is called for most the air comes thru the footwell vents. When cooling is called for the center dash vent opens. In blend mode most the air comes from the left and right dashboard vents

Is any of this happening?

If so, it suggests that the system understands the change in the temp dial setting and is trying to comply

Cheers
DD

 

I was talking about clamping the water hose, not the vacuum line.

 

thanks for all the tips, I will take a deeper look after work. There is a change actually when I turn the temp knob. Cold blows much harder through the center vent (still hot) and it moves the force to the floor when I get to hot. That’s interesting I never noticed.

 

If this is the 1990 XJ-S in your signature it has the the Mk3 system in use from 1987 on. Some of the advice here is for the Mk2 system. If you have a pull for manual option on the temperature dial it’s the Mk3 system. This one is much more electronic than the older one. I’d start by cleaning the contacts on the long connectors. You’ll need to remove the panels of the sides of the footwells. Undo the connectors, spray them with contact cleaner, and reconnect them.

 

Ahhhhhhhhhh.......

Now I get it :-)

Cheers
DD

 

Took a quick look and the heater control valve is opening and closing with the adjustment of the temp knob. at this point potentially the valve itself is broken internally. I guess the next step would be to take it out and see if it’s functioning properly, unless there is something else I should consider? I was driving on a cool day and that’s when I thought I was still getting heat. Maybe I just need to fix the AC, as maybe that’s what the vented air feels like? Not a major issue, only time I would like it is during rain with the top up.

 

Yes, it is the 1990 with the pull option for manual control. I will remove the footwell cover and give it all a cleaning. It does have the vacuum on the top of the control valve, but I also noticed wires coming from the bottom of the valve (edit:I must be mistaken on the wires, I don’t see that on the replacement valves) So that must be the MK3

 

Hi,
I have the same system in my '90 coupe.

Like you I have no heat or aircon. The lack of heat is due to a seized water valve. New one ready to go on and the vacuum control opens and closes this new valve nicely as the temperature knob is rotated. The aircon is probably a very different (and expensive) story when I get round to it.

If you can see the operating arm on the underside of the water valve move fully as you turn the temperature dial, I doubt that there is anything wrong internally as it is a fairly simple rotating shaft with a hole drilled through it. A bit like shut off valves on domestic plumbing systems. Suggest that you feel the outlet hose from the water valve to see if it flows hot / cold as you adjust the temperature dial.

The temperature knob on my car has always rotated very easily. I guess this is because it is just moving electrical / vacuum components rather than cables, pulleys and shafts that directly operate the water valve on simpler heater systems.

Good luck fixing your car.

Cheers,
LeeP