As some will have noticed, my wife and I are currently traveling in UK in our 1977 XJ-S
We found the AC to be ineffective and visited a Kwik-Fit last Friday and had the AC charged. 950 gr of R134a as the system has been converted. All parts are unchanged, just new refrigerant and PAG oil. Has worked well since 2017, but had lost 2/3 of the charge now.
Weather then changed to rain, wind and cold enough to want some heating, and then issues came:
Cooling works
Heating works when we start out, and then goes away after some time. In the beginning after 30-60 minutes, now after maybe 5 minutes.
Fan works, until heat disappears. Then low fan works, but sometime low fan also stops working. Defrost fan always works. Turning fans off stops the fan for a while, then it suddenly turns on at full speed. I then have to live with low fan and a cabin temperature well below comfort.
Today I unplugged the compressor clutch, and that of course stopped the cooling, but then caused the windows to become foggy as it has been raining all day, and we were three adults in the car in rather damp clothes...

The control unit has been replaced by a modern electronic thingy.

What effect will a bad temperature sensor in the heater hose have ? If I should make a work-around: Can it be done by unplugging or shortcutting the sensor ? I do not really have a chance to find a resistor to replace it...

I suppose it is not the heater valve on the bulkhead, that acts up, but could I cheat the valve to be open all the time by unplugging the vacuum line (and plugging it) or by adding an alternative vacuum source to it ?

I still have around 1000 miles home, so some solution would be nice. The car gets freezing could as it is now...

 

The default is the heater valve is always open, vacuum closes it. My guess is it's the amplifier, those are known to fail. Or is that what you mean by the control system has been replaced?

The temp sensor should be an inhibit switch to prevent the fan from running until the coolant is warm. The terminals can be joined and then that indicated to the system that the coolant is at temperature. I don't think that's your problem though.

 

According to FEN AIR who do conversions to R134 to xjs and other classic cars
800 - 850 grams works out to be optimum charge. Could those extra grams affect it at all
do you think?

 

The amount of refrigerant would have no effect on the fan speed or temperature set points in the cabin. At worst, the cooling performance would be reduced. The XJS system is actually well designed against overcharging, as the fuel cooler absorbs heat from the refrigerant to help boil off any carry over liquid, plus the GM A6 compressor has a sump where any liquid can accumulate and boil off. The A6 is unique in having a sump, the vast majority of compressors don't have that. It's part of the reason they are so long lived.

As a reference point, my 1994 XJS calls for 900g +/- 50g, so I'm not worried about 950.

 

You might want to check the cabin temp sensor plumbing too. Not sure where it is on the early XJS, but on the Series III saloon it's a hole under the top of the dash crash roll, and a rubber hose takes cabin air to the sensor. If that becomes disconnected, the sensor reads the warmer air behind the dash due to engine heat soak and turns up the cooling.

 

Thanks

It is the amplifier I have replaced. I still have the original unit in place, so i can try switching back.

I will try shorting the inhibit switch, that is easy to do on the hotel parking. It does after all act kind of like if the coolant is too cold for fans to run.

 

This may help? It's both the electrical and vacuum schematics for the Delanaire mark 2 system.

 

Leo
This is not something you are liable to be able to fix on holiday. Undoubtedly you have either an amplifier or a problem with the electro-mechanical cam-control module. JagAire can supply new/recon units no bother. On defrost (which I seem to think you said is working) you should get full cooling and full heat over-riding all else. If it is working, then this will dry you out at least.

 

Thanks.

Yes, defrost works, but only with ice cold air. No heat is available after the controls cut heat off after a while, and that means the car is not really driveable as temperature gets well below comfortable levels, even when fan is set on low.

i will try to switch to the original amp as it is one of the things, that is possible to do. Same goes for shorting the temp inhibit switch, it is doable while on tour...

My guess is that the error is either in one or more of the sensors or in the amplifier as everything actually works, the problem is just that the system decides to act up when the inputs are ‘right’. I do not see how the cam mechanism should be able to make this happen ?

The weirdest thing is that the increased cooling seems to have had bad influence on the fan operation. That puzzles me...

 

Defrost should give full heat, so something isn't right. Unfortunately, I don't know enough about the Mark 2 system to say what.

Have you replaced the heater valve? The earlier Series 1 XJ and DS420 valves were normally closed and open with vacuum, the Series II and III and XJS are normally open and vacuum closes them. I wonder if the wrong valve was put in?

 

Heater valve has not been changed, and it must be working as it gives heat for some time until some parameters cuts it out.
I could try to disconnect the vacuum line (and plug it) to see if that gives me the heat I’m longing for...

BR Leo

 

Leo
You may be right, but the cam controls the heater flaps, and if these are in the wrong place, you will not get any heat. There is a potentiometer on the end of the cam which takes the signal from the amplifier, and if this acts up (mine did and it was replaced by a JagAire reconditioned unit) all sorts of weird things happen.
To test the heater valve, just see if the output side is hot once the car is up to temp. I would be amazed if hot air is not getting to the heater matrix.

 

The mechanisms are at least not stuck. I can hear the flaps moving around as I change temperature or chooses defrost.
I have now detached the vacuum to heater valve, but you’re right: I will be able to detect flow by touching the hoses before/after the valve. I think outlet goes to right side of the car, but by touching both sides I don’t have to be sure...

BR Leo

 

Good news. Disconnecting the vacuum to the heater valve has resulted in heat being available all the way from Stansted to Brighton, I guess that’s around 150 miles.
Sunny and 22C, so heat was not needed, but I tested if it worked several times, and it did !

 

OK Leo, the fault is therefore that the system is calling for full vacuum to close the valve all the time, regardless of the temp selected. It should call for vacuum only when full cold is selected. I would start by checking the vac lines to and from the reservoir and the non-return valve, but it is probably something more difficult in the unit itself...