Does anyone know what the water valve vacuum line do? On my 89 XJS the vacuum line is missing. Where does it attach too and should I reinstall it?

 

The vacuum line runs from the L/H side of the aircon unit out through the firewall to the heater valve. Line is coded red and vacuum closes the heater valve. The source of the vacuum is from the rear of intake manifold to aircon unit via a reservoir tank mounted below ABS pump.
Try looking around ABS pump for red plastic line, if it's a marelli car probably in the bundle exiting the flexible rubber protector.

 

Baxtor thanks for the advice. I wanted to scope out things as you suggested. Apparently, someone purposely disconnected the heater valve. What I found were three vacuum lines coming through the fire wall at the same place. A solid red line just as you said, a black with yellow strip line and black with white strip line. The red line is hooked up to vacuum port on the engine as is the black and yellow one. Someone put a screw in the end of the black and white line plugging it up and left dangling. I don't know what all these lines for but why would someone rerun and plug the vacuum lines. Any idea? Any advice would help. Thanks.

 

Is the car marelli ignition?

 

I don't really know. I know nothing about Jag's except that I always loved the XJS and always wanted one. I found this convertible by accident with 32,000 miles on it sitting in someone's barn for the last 25 years. Now if we were talking mustangs, well that's another story... Anyway how can I tell? It's VIN # is SAJNV4849KC164226 if that helps.

 

Marelli was #156989 on so that makes yours marelli ignition.
One of the other vacuum lines runs into the ignition ECU in the footwell of your car ( passenger side) not sure of which line it will be, the other will be the feed to the vacuum reservoir. It's easy enough to get to the ECU so you could get in and check which line is plugged in at that end.
Total of three vacuum lines, one intake manifold to ECU, one intake manifold to vacuum reservoir and one air con to heater valve.

 

I will trace tomorrow the lines. Any idea why someone would play with the different vacuum lines?

Thanks for answer to the ignition.

 

Hard to say. Might hazard a guess when you have identified which has been plugged etc.

 

Probably trying to fix a problem ...or should I say mask a problem?....by eliminating/bypassing the function of xxx-component or system.

The heater valve closes with vacuum applied and opens with vacuum removed. So, perhaps there was a control problem and the vacuum hose was removed so as to ensure the valve stays open to provide cabin heat? I dunno. Just a guess.

Cheers
DD

 

Someone went through a lot of pain to do what they did. I'm going to trace the lines as Baxtor recommended so we can figure out what to do next. I get plenty of heat but maybe that's the problem with the a/c. Fully charged and I only get 60 F out the vents and that's on a good day.

 

The system is essentially a blending system of cold and heat by way of blending flaps internally and it's possible one or both of these have stopped working leading to too much heat. Pevious owner may have disconnected water valve vacuum line thinking (incorrectly) that would shut off the heat.
If that's the case you could as a stopgap fix simply connect heater valve to manifold vacuum supply thereby closing it whenever engine is running.

 

Baxtor do you think this could be affecting the performance of the A/C blending warm air when the a/c is on?

 

Yes it could. Air is passed through both a heater matrix and an evaporator and blended to give the required temp. If the blend flaps are not operating correctly it's possible for example for too much hot air to enter the mix.
The attached diagram shows how these blend flaps operate.
Of course your problem might be something completely different.

 

Wow thanks for the info. Off under the dash I go...