XJ6 & XJ12 Series I, II & III 1968-1992

What does the Heater Control Valve Do?

 
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Old 03-29-2019, 06:24 PM
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Default What does the Heater Control Valve Do?

What does this do? Mine is all rusty. My A/C heats fine, but doesn't seem to cool well. If this is bad would it account for that problem?

 
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Old 03-29-2019, 06:39 PM
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In stock configuration our cars basically heat and air condition all the time. Pneumatic flaps divert the air from all heated air to all air conditioned air to (most of the time) mixing the hot with the cold. So the amp, the servo and the flap system are really determining the temp of the air coming out. If they are not working well or consistently you can force the heater valve closed so it is mixing cold air with ambient air (instead of hot air) so the result is that it produces colder air at the vent.

The hot water valve pictured is mainly used to close when the engine is too cold to warm the coolant. Others can jump in to correct me if I am wrong, but that is my understanding.

Jeff
 
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Old 03-29-2019, 06:57 PM
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there are two types of Heater Valves or "servos" as they are also called in the UK:

1) one is NORMALLY CLOSED / OPENS WITH VACUUM,

2) the other IS NORMALLY OPEN / CLOSES WITH VACUUM.

The Series 2 and 3 XJ use Number 2.

Prior to that, all Jaguars used Number 1 including the Series 1 XJ.

I understand Number 1, Normally Closed, because it is logical, i.e., when Heating the interior of the car is needed, you select Heat and Temperature which Opens the valve via Vacuum and lets hot coolant flow into the Heater Core , also known as Heater Matrix in the UK.

I do not understand Number 2, Normally OPEN, because it means that hot coolant is always flowing through the Heater Core even in hot weather.

Someone else can explain that concept.

But mainly the Heater Valve allows or prevents hot coolant from flowing into the heater core, it is a mechanical "door" operated by the force or action of vacuum to open or close the "door".

if the valve looks bad, replace it.
 

Last edited by Jose; 03-29-2019 at 07:02 PM.
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Old 03-29-2019, 07:36 PM
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Jose is almost there.

When the demand for COLDER air is asked for, the internal system will close the heater tap, thus stopping fluid through the core. If that tap fails to close, as yours is, the core is always seeing HOT coolant, and hence your heater is working just fine, but when Summer hits, that tap needs to close for full COLD air to be available. The system can only divert so much HOT air.

And YES, if that tap was replaced, your A/C would more than likely work much better.

The UK summer is not as savage as ours, and we really know when that tap has jammed open, as that is the default setting, as Jose has explained, as in his #2.

Again, down here, any Jag presenting with A/C not as cold as it should be, the tap is #1 go to, before regas etc etc is even considered.

Also, there is a pad style" temp sensor on the Inlet pipe to the core, up behind the glovebox of a RHD car, that prevents anything happening until a predetermined temp is sensed in that pipe. Many of us bridge that sensor, so the system re-acts quicker.
 
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Old 03-30-2019, 05:35 AM
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Thanks Grant, I now understand the Series 3 heater valve a little better.

The heater system in my 1965 S type is so simple compared to my 1984 XJ-6. I obviously prefer the S type!!
 
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Old 03-30-2019, 06:07 AM
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Agreed, my S Type and MK10 are soooo bloody simple.

A/C is simple also, "all windows down, 80MPH, not so bad". HAHA.
 
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Old 03-30-2019, 07:21 AM
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I gave up on air conditioning the S type, it has a very smart ventilation system, particularly when the rear door's Quarterlight Vents and front Scuttle are open. Open the Rear Seat Vent and you gain about 10 horsepower! The syphon effect I call it !

The XJ-6 doesn't come close!
 
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Old 03-31-2019, 05:40 PM
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Like most of the old-type heaters, they used valves to control the amount of hot coolant flowing through the matrix and hence control heater output. On my 1977 MG Midget, the heater valve as per factory fitting had no control in the cabin, you had to stop the car, open the hood, move the valve, get back in the car and drive on !! I now have a new-type valve with Bowden cable. It saves a bit of effort. On Jaguars air conditioning came later, but the heater valve remained.albeit controlled by a vacuum line, not a Bowden cable . On modern Jaguars, as with most modern cars, the heater matrix is hot all the time, and when summer comes and it's hot outside the car, is closed off with a flap. Climate control systems use flaps to blend hot and cold air as need to maintain cabin temperature.
 
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Old 03-31-2019, 09:09 PM
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I remember the Rover P3 that I rewired for a guy, had similar to the Midget, but it was more like a gate valve tap, and bloody HOT when touched to turn, oh the days of early motoring.
 

Last edited by Grant Francis; 03-31-2019 at 09:39 PM.
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Old 04-01-2019, 09:45 AM
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I heard decades ago that the conversion to normally open heater valves was to allow the valve to be "exercised" so that it wouldn't stick. If a Series 1 owner doesn't use heat most of the year, the valve is never exercised. If a Series 3 owner doesn't use heat (or climate control generally) for most of the year, then vacuum is applied to the valve each time he starts the engine, and the valve is off just like a Series 1 is with no vacuum. But, when the S3 is shut down for a while, vacuum bleeds off and the valve of the now cold engine opens, exercising it.
 
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Old 04-02-2019, 05:25 AM
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Originally Posted by Grant Francis View Post
.....The UK summer is not as savage as ours ......
That'll be right.

It's usually a washout.

Graham
 
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Old 04-02-2019, 02:17 PM
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I recall our family 38 dodge that took us through WWII and beyond. A heater, yowee!!!
Knob on the dash to pull the valve open, or close it. A rheostat for fan speed.
And a screw down on the valve on the engine. Screw it down for summer, open for winter. the summer/winter valve. The unit a neat box with two doors under the dash at the fire wall..


I added a S/W valve in my XJwuzza six. On he line away from the vacuum operated valve Mild weather valve closed. No circulating hot water in the matrix. Power vent, nice....

Open in winter for nice hot water in the matrix. great heat output with a 190F t'stat and the great liquid circulation of the LT1. And it makes BMEP's quick!!!

Carl
 
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Old 04-07-2019, 04:13 PM
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Hi All. So I changed that valve today. That wasn't fun. I didn't know how to get it off the firewall. Eventually figured out that I needed to take the air intake louver (where the windshield wipers are) off and then I could unbolt the nuts and bolts holding the bracket and wiggle it all out. BIG PROBLEM is that the new one, out of the box leaks!!!! Like a sieve. I've checked my hose clamping and it isn't coming from there, rather it is coming from that Phillips head screw that holds the pivot on the opposite side shown in the picture above. Un believeable. This was a part I got from XKS and is manufactured by a company called MTC( google MTC 7945 and that's the one).

So I'll need to replace it all over again. I'd prefer to buy a better quality part. None of the parts sellers say who the manufacturer is. I don't want to get another crappy part. So I'm wondering if someone knows a source for this part that might be better quality. The Jag Part number is C41051. I know about David Boger at everydayjag, But I worry there about getting an old part that'll fail as well.
 
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Old 04-07-2019, 04:16 PM
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XKS sells garbage parts. You are not the only one they have ripped off.

buy from a reputable business like SNG Barrat USA store.
 
 
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