You know the sound...the one made by the mid-eighties XJS climate control system, a collection of dash-internal flaps, serpentine vacuum hoses, and a vacuum-powered drive motor whose operation parameters might best be described as black arts.

Upon resurrection from the barn, the system was completely inoperative, and stuck in the mode of defrost albeit with no blower power. This was actually great for the season, for as I have come to expect, the heater fails precisely at winter solstice and the air conditioning resumes, followed by the reverse at summer solstice. Not bad these days to have the warm V12 air coming up from the windscreen vents.

No position of either knob resulted in any action.

I checked the fuses, and found the de rigeur suspect 50A blower fuse to in fact be suspect. Measurement with a DVM confirmed the fault, and one new fuse later, the blower was blowing--so now I had the option of actual blowing heated defrost air.

So today, after driving the car and fixing minor faults for the past few days, I happened to twist the RH knob to "auto" momentarily....and voila! The old, familiar sound of the vacuum motor whirred to life spontaneously! This was as exciting to me as a motor on the Mars lander coming back to life would be to JPL bench jockies.

The next thing ya know, I've got full A/C through the facia vents, AND the ability to return to defrost! I popped the bonnet, and sure enough, the GM compressor is spinning! However.....

The big black cat is full of tricks and surprises. Although the flaps are straining and groaning to do what they are told... alas. The winter solstice thing can't be escaped.

The system is stuck in full cooling mode, no matter where the left hand knob is directed.

So here I am, December 29th, and no heat available. Going to seek professional help from a power greater than myself.

No doubt the control unit will be 7.5 million USD, and the labor to fix it twice that. Nevertheless, I won't stop until full climate control is restored.

Any suggestions where to start? Things to avoid?

Thanks,

Lou

 

Nicely worded Lou;-)

You can find info on this in the XJS book contained in the FAQ at the top of this XJS forum. From about page 517. The article is long.

Here is an extract:

TOO MUCH COOLING: Mike Morrin says, “If the system works properly for a while (10 minutes) and then drifts slowly towards full cooling, then the fault is likely to be in the plumbing to the in-car air temperature sensor. This is a reference to a couple of cases I have heard of where the rubber connector inside the dash had come adrift so that the air temperature inside the dash was being measured. There is no cold air supply inside the dash, just heat soak from the engine and heater unit, causing the measured temperature to creep higher and higher, and the A/C to pump full cold into the car.”

Hope this is of some use to you.

 

An easy place to start is to see if the heater valve is working. It has an external actuator arm so it'll be easy to tell.

While a helper turns the temp control from cold to hot you look to see if the valve operates. If it does not operate it is either stuck or it is not being commanded to operate.

To see if it is being commanded properly turn the control to full cool....there whould be vacuum at the hose. When the control is turned to full hot, the vacuum should disappear.

Sometimes a stuck valve can be freed-up with whatever your favorite aerosol solvent is at the time.

In case you don't know the valve is almost dead center on the firewall, below the level of the large balance tube at the rear of the engine.

Cheers
DD

 

I can't be of any help, but I just had to say you spin a hell of a yarn, Lou!

 

Thanks for the help and the compliments gentlemen!! I'll do some reading.

Today, it decided to go back into allowing heated water into the heater core, and defrost was back.

But the resumed operation of the vacuum motor servo whatsis was just a tease...it's gone again.

I think a great modification to these cars would be a simplification of all the dashboard warning lights...replace all of them with a single warning lamp that reads "$$$$".

I replaced the transmission-driven speedometer sensor with a new one, and the speedo is back fulltime.

 

Later models have that, as well as, being equipped with GPS showing you the nearest bank, tow truck service and and most expensive mechanic within 100 miles.

 

Now THAT'S funny, Jag Po'!!

 

On the trail of this problem, more clues, more symptoms.

In the morning, and only after sitting all night, you start the car and rotate the selector knob to defrost. When the car warms up, you get warm air at the defrost vents and a little warm air at the two dash side vents, for as long as you leave the control at defrost.

Then, if you ever switch the selector to AUTO, the warm air goes away, and switching back to defrost will never get your warm air back, it blows fully cold or at maximum, lukewarm. Switching the car off for even an hour will not get your warm air back. It has to be overnight.

I checked for vacuum at the heater valve; full vacuum for cool (65F) is present, and no vacuum for defrost, and no vacuum for AUTO when temp is set to 85F. So the vacuum portion at least to the heater core valve is operating properly.

Is it a telling clue that you can get defrost heat until engaging the A/C in the car and then nevermore? (Yes, the A/C compressor is spinning when in defrost mode, even in the morning.) Where to look next?

Thanks to the sage Zen masters on this board!

------------Later that day-----------

Re the heater water valve on the XJS firewall center: At full cool, vacuum is applied, which moves the lever on the vacuum servo UP, cutting off water flow.

At full heat, (no vacuum) the lever is meant to go DOWN.

My question is: By what means does the lever move DOWN again in the absence of vacuum? Is it by coolant flow action, or is it spring-loaded, or how?

Because mine does NOT return to the DOWN position in the absence of vacuum, as it should.

If you move it down manually with a screw driver (it moves very easily) then you get coolant flow, and heat again.

In the morning, when I first fire up the car, you can hear lots of gurgling under the dash, like the coolant is flowing but maybe with some air in it. ? Coincidentally, in the morning is the only time I can get any heat to come through the system. Once the selector is turned for any A/C, it is not possible to return to heat mode, and I think this valve is the culprit. But I need to know how it works, as above.

If the valve is meant to return to DOWN by means of coolant positive pressure, then is it possible I have low coolant or an air bubble in the system?

 

The heater valve is spring loaded but they can get sticky and weak....which sounds like what has happened in your case.

Air pockets are a bif problem in V12 cooloing systems and it CAN cause lack of heating (ask me how I know)

It takes some patience to properly bleed a v12 cooling system but it isn't hard.

Elevate the entire front end of the car several inches and then elevate the left side a few inches more.

Remove the bleeder plug....top of left radiator tank. There's an access hole in the upper radiator mounting panel

Start engine, top off coolant via the central fill tube. Run engine at 1000 rpm or so.

Wait. Wait some more. Add more coolant if needed. Squeeze the upper radiator hoses with your hands. Wait, repeat, wait, repeat, add more coolant if needed. Wait, repeat

When you get a solid, steady, gushing flow of coolant out of the bleeder hole and have a big mess in your driveway, you've sucessfully bled the system :-)

CHeers
DD

 

Thanks Doug!!!

It appears that the trouble with the heater valve is just as you described, sticky, hit it with more Tri-Flow today and it now is responsive to the spring-load-open.

OK, I'm asking...How do you know?

And, thanks for the bleed procedure. Never heard of this but anything that messes up the driveway has to be too much fun.

 

stopped by for an amusing read (laughing with you Lou, not at you)

waiting for future chapters and/or alternate endings

 

Thanks H2O, I'll keep the cards and letters coming.

Doug, I found the bleed screw on the radiator; that was my big accomplishment last night. You are right, once again, about the weak return spring. When you disconnect vacuum abruptly, the lever snaps back fine, but when the climate control gradually reduces vacuum, it tends to stick about halfway open/closed.

Since I'd rather probably go ahead and mash my hand with a hammer than contemplate replacing this valve, I baptized it in Tri-Flow until there was a pool on the engine and chased it with many sprays of silicone spray for good measure. I am hopeful it will free up with more use.

Meanwhile, what brain damage has the climate control sustained that is making it do this:

Cool air out the bottom (at the legs) vents, while decent heated air out the front side vents and defrost vent? This while set for 85F full heat.

Back in the good old days, you would get heat out these bottom vents when set to heat, so something has given up.