61 MK10 heater plumbing

Hello Brains trust. thought i will chuck this one out there for clarity.

heater pluming, from aluminum manifold over the carbs that has the thermostat at the other end, thought a vacuum actuated valve, into heater core. out of heater core down in to metal pipe and forward and into the water pump. that is what it looks like to me after thinking and looking at the parts manual. can any one confirm.

the confusion comes about because i have a bypass vacuum valve fitted. unlike the manual. or is there a configuration i am missing??

As a 420g noobie, id say, that the system is exactly how you described it, according to the manual… I studied the manual few weeks ago just to confirm myself, that somebody installed the valve on my car in the wrong direction…

thank you, a picture speaks a thousand words.

ok so i have previous owners creative repairs to undo. and a vacume valve to locate.

thankyou Marek

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When I looked into this a few months ago, I was told that these valves aren’t available, and old ones usually leak, prompting many to bypass or place a manual valve.
I did buy this item:


I haven’t installed it yet, but will be doing so as soon as my radiator returns from the rebuilder. I have seen a photo of it installed on a Mk10 on the Saloon Data site. I haven’t worked out whether it is correct as NO normal open or NC normal closed - I think it should be NC. If you do find a source for the original valve, please let us know.

correct normally closed. needs vacuum to open. i have just been playing around with it. thanks for the link

You can use this more modern valve, but it needs to be connected this way. Doing it the opposite way wont allow you to get hot air inside the car (guess how I know)
image image image

thank you very much, that all makes sense now.

as a side note, it maybe an idea to upgrade your hose clamps when you get a chance,
i don’t trust that type, i only use SS the ones with the thread slots, better bite and doing work lose.

But sheesh you under bonet looks nice, i have a long way to get to that!

thank you

Peder, do you have a source for that valve?

Ron smiths post is a link

Peder’s valve is different than the one I posted. Also, I checked the one I linked and it’s NO instead of NC. I believe it could still be used, but off and “on” settings in the cabin would be reversed.

i just ordered this https://www.ebay.com.au/itm/HVAC-Heater-Control-Valve-Tap-suit-Jaguar-XJ6-III-1980-1986-6cyl-4-2ltr-4235cc/123367974087?epid=25005062755&hash=item1cb94de4c7:g:CuoAAOSw0dVf9BxY&frcectupt=true

I have a notion that the S2 and later valve works in reverse to the S1/420G/MKX valve

As I live in a very warm climate, I have my 420G heater temporarily bypassed, as the valve leaked.

In these cars, that means you get more heat inside

So in summary, I am not sure whether the MKX valve turns on or off with application of vacuum

The Service manual does not seem to mention this, and I just cant remember what way it is supposed to work.

I do believe original style repros are available, but I will probably fit a generic like Ron

Try David Manners

I guess I can check by depressing the “heat” button, and see what happens to the relevant vacuum tube, which I now have capped off

Peders looks like the original, but I cant quite understand how it is hooked up

Thanks. Let me know how it works.

Many years ago the vacuum operated valve on my 3.8S failed. It was no longer available but I fitted one for a Mark 10 and it works fine, is opened with vacuum from the three button console control. The only difference in the 3.8S one and Mark 10 one is the 3.8S has an attached bracket to hold it in place which is really unnecessary as the short length of hoses hold is very securely in place.

So, I got into that system on the Mk10 today, and I can confirm that for this car the correct valve should be NC - the vacuum control module sends vacuum out to the solenoid at the rear of the intake manifold to open the the valve when HEAT button is pushed.

On a forums search, it does appear that the XJ vacuum solenoid described above is also NO, like mine from e-bay. It would be good if someone could confirm that. What I can say is that a NO solenoid is likely to work with the HEAT switch pushed in to turn the flow of heater water off. That would be dependent on a control valve that works and passes vacuum.

Which brings me to the next question - how leaky are these vacuum operated heat switches normally? When I pull a vacuum with my MityVac, it switches, but definitely leaks down when either on or off - not sure how big a deal that is, but probably not ok. Are they rebuildable? My unit is very clean in appearance, like new, but I suspect rubber internals have not stood the test of time well.

good question

one test, does your plenum chamber lid operate freely ?

Its a tight return spring in there, so if that works well, as mine does, thats a sign of good health

I got rid of the resevac chamber and one way valve…both known failure item

I replaced them with a one-way fishtank valve $5, (to the disgust of at least one poster), but its been working fine for years

I will have to attend to my heater valve

I notice from another forum I am one that where it is cold, often get rid of the heater valve, and allow full flow all the time

my plenum chamber doesn’t operate at all, although I can manually open it (yes, strong spring). However, the hoses are old and hardened, so I will start with replacing them. A full test of the vacuum system will have to wait for the rebuilt radiator to be available so I can run the car.

Given the warm climate, you might fit a manual valve that would allow you to have heat on the rare occasions you need it. Mine now has a common one that I believe is still available, but the handle is stripped off. I’ll send a photo tomorrow. Where I live, we have a short and sometimes hot Summer, wet and cool the rest of the time, so the climate system needs to work. I guess I’ll pull out the control unit and see if it can be disassembled.

here is the manual valve that came off my car - defunct, but I have seen these for sale. Also, the new heater valve, which will require the heat switch to be “ON” to stop flow. I’ll change it if I find a NC valve.