Series 1 heating and Ventilation

I want to get the air flow better in the car and nothing of the vacuum system was working so I started checking this morning and found the leak, Now with a vac gauge in the line into the modulator air flap quadrant control,(1 in the drawing on O4 in the SM) the system has about 12 inch HG when the engine is running. Connecting back up the flap actuator mechanism (2 in the service manual O4 drawing) the flap now opens and closes as it should under vacuum. When trying to get the control panel reinstalled (I have taken out the radio because there’s no antenna) I discovered a “spare” vacuum tube in there with no obvious place to connect it. There are two to the Modulator, heat control (3 in the drawing) as there should be and no other vacuum controls as far as I can see in there.All the pipes are on the Heater sensing unit (4 in drawing) and the 4 way pipe connector has all its pipes.

Anyone know what this extra pipe may be or is for? Its black rubber identical to the pipes used on the other controls and it seems to have nothing else connected at the other end wherever that may be.

Any advice appreciated.


Your problem is located here…

Hahaha, I remember when you posted that image before, Roger. I have it too, of course, in the manual, but I never paid much attention to it. But I realized that I don’t know how the thingy that attaches to the plenum works, or what it is supposed to do.

As you know, I removed it and simply connected the vacuum source, via the two controls, to the scuttle flap and the heater valve, which worked fine. I assumed that the thingy has something to do with regulating the heat by modulating the source vacuum sent to the heater control knob (and hence on to the heater valve). But what that regulation is supposed to be, I don’t know.

I suppose it’s intended to somehow close the valve if the coolant is hot and open it if the coolant is not so hot–all for a particular position of the knob. So setting the knob to, say, 5 would open the valve somewhat more at that position with lukewarm coolant, and not as much with hot coolant. But if true, I still don’t know what the thingy actually does via its various vacuum taps. And, being “open loop” (no sensor of cabin temperature) it’s hard to believe that it does much.

IMHO, the bloke who designed the wiper parking mechanism finished that job at noon and had the afternoon free to tackle the heater. :slight_smile:

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Have you verified the connection from the heater sensing unit and the water valve, Ian…?

By the way; the heat sensing unit is rather important - it modifies vacuum according to in-car temps. It is a unit that doesn’t age well…

And as general observation; counting the number of connected hoses is insufficient - it must the correct hose to the appropriate spigot…:slight_smile:

xj6 85 Siov Europe (UK/NZ)

Hi Frank
Yes I’ve verified all the pipes in the back of the control; panel, all are there, all are apparently in the right places and the system holds vacuum. I read the section of the brown book on the subject of the system and have tested everything I can test except the heat sensing unit since it isn’t purely a valve as the other things are and there are no instructions as to how to test, merely replace if faulty. Any ideas on how to test it?
Summer here now and hot so no need for the heater. I’m taking the car for a tour around Ireland at the end of this week which will put a few miles on it and we may have a cool morning somewhere when I can test the heater as well. I should add that although the fresh air flaps now open fully under vacuum I didn’t notice any real difference in the airflow especially through the dash vents. I can’t go fast where I am but on the highway in England en route I’ll do the limit and see if there is much air. If not I’ll have to investigate further when I get back. Keeps me occupied at least.


The fresh air flap is mechanically linked to the defroster vents, opening them with fresh air and closing them with recirculated air. Also, the air flow from the scuttle goes into a plenum that supplies the fans–not directly to the vents. For these reasons, sensing a change in vent air when the scuttle is open is complicated. IMHO.

I don’t think it is ‘testable’ as such, Ian - nor repairable; indeed to be replaced if malfunctioning…

Somehow, inside the unit is ‘something’ changing with temp - altering vacuum levels and distribution. I seem to recall that someone, far in the past, actually dismantled the thing - as replacement no longer exist. But I don’t recall the result…

In theory; if you cool the unit it should increase air flow temps - and heating the unit should cool air. Sort of confirming that the unit is actually working…sort of… But it is likely slow reacting…

It is advisable to use a thermometer to verify temps - human senses are not really absolute.

However, as Roger also implied - there are other ways to skin this cat. The sole purpose of the sensing unit is to alter setting to maintain constant cabin temp without driver inputs. Bypassing the unit for manual control may requires some ingenuity…

xj6 85 Sov Europe (UK/NZ)

Thanks for the replies. When I get back from my trip I’ll take it out and see if I can work out how it should work,
Has anyone found any better motors to replace the asthmatic fan motors in the heating/ventilation circuit which seem to use more of their energy making noise then shifting air. Are series 2 or 3 motors any better and will they fit?


@Roger_Mabry has. Also, note that the motors run in opposite directions and it’s possible to get them swapped or one of them wired to run backwards. Earlier motors had field coils and were different left and right. Later S1 motors were permanent magnet, and ran in either direction depending on hookup polarity.

I found AC Delco motors (PN 88960338 15061 made in Canada)and their metal fans produced about 40% more air… but you have to make a 1/4" thick “gasket” out of silicone in a tube and let it setup… then install the fan(s). Make sure you test the fans when installing as they are a tight fit.

When using AC you should run them on the lowest setting as moving more air heats it up.

Evening chaps, new owner here, getting to grips with the cliff-face of knowledge that is Series 1 ownership. This thread caught my eye because (a) I cannot persuade the ventilation system to produce any heat, and (b) sliding the air supply from ‘car’ to ‘air’ produces an interesting rushing (or sucking) sound deep in the bowels of the centre console. The result of a vacuum leak, perhaps?

Welcome on board, Paul

The original S1 heating system were operated by vacuum via various ‘thingies’ - hard to come by these days.

The water valve is opened, for heat, by applying vacuum. The sound, and lack of heat, implies a vacuum hose has likely disconnected/bust. First trace the vacuum hoses, by feel and sound from the water valve to the heater control unit in the cabin. And/or from the flap actuator to the air flap modulator - and see what gives…

The heat control unit reads cabin temp and is controlled by vacuum hoses at the lever (Air flap modulator) and the rotating knob (heat control modulator) and hoses may be disconnected or a fault in the components may cause leaks. Which affects the whole system…

You can connect vacuum directly to the heater valve to get permanent heat, but it won’t solve the problem. There are descriptions in the Archives to ‘modernise’ the set-up if there are unrepairable component faults…

And access to components is not all that easy - and some further checks may be required…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Many thanks, Frank, for the welcome and the feedback.

Given that salt has just appeared on our roads, I have the luxury of the winter months ahead to familiarise myself with the topic in the icy recesses of the garage.


Hi Paul.

Switch off engine, open and close the car - air slider. It will hiss as it’s vacuum activated and bleeds the vacuum off to close. You can see the flap moving if you look down the air intake between the wipers; the actuator is in the engine bay right in the middle. Without a big leak you should have several cycles before vacuum runs out.
The no heat issue is very likely due to the hose at the heater valve next to the throttle linkage. The hose might have fallen off.



welcome aboard! Another nice winter hot cup of tea thing to do about your “new” car is to get yourself documentation, most notably the parts catalogue and the Repair Operation Manual to familiarize your with the way things were meant to be by the engineers at Browns Lane.

As regards heating and ventilation there are schematics of the vacuum system on this site. Before getting any deeper into it, I would try to check the physical basics: is the heat exchanger still connected to the coolant circuit? Any leak there requires a tear down of the entire dashboard assembly, so someone may have cut corners in the past … Even with the heat exchanger connected there may be an air bubble trapped in the exchanger causing a no heat situation.

If you really need to dive in, you’ll have to remove the cheek panels at the center console (easy) and the crash roll on top of the dashboard (easy to pull off, but tough to refit without splitting the vinyl!).

Good luck


75 XJ6L 4.2 auto (UK spec)

Welcome Paul,

We have all been there. First to do. Check the health of your vacuum lines. If they original or even half the age of the car, there is a good chance that they are dried out, brittle, creaked or broken. If you are hearing a hissing sound behind the radio console, it may be somewhat of a good sign as it means you are at least getting vacuum to the controls. If the hissing starts and stops when you slide the fresh air lever, you most likely have a hose broken or disconnects from the control. That control goes to a dash pot under the scuttle air intake and regulates the flow of fresh air into the cabin.
All the vacuum lines, (I think there are 3, 1 in 2 out), pass through the firewall through a small round plate that is located near the passengers left foot. The main vacuum line is larger then the others. To inspect the lines in the cabin you will need to remove the side panels from the center console and pull back the carpeting in the passenger foot well. You will also need to remove the radio mounting plate, as it also contains the heat and fresh air controls.
Remove all the knobs from the radio, heater and A/C controls and pull of the ends of the lever controls. Remove the face plate (not sure if you have to remove the nuts on the radio controls) then remove the 4 Phillips head screws holding the radio panel in place.
Careful work the whole thing out. Keep in mind that there are vacuum lines attached to the right heat control and the top fresh air control. The radio has power, antenna and power antenna connections. There is also a connector attached to the A/C control on the left. Be extra careful with the Fresh air controller on the top of the unit. It is too easy to snap the vacuum nipple off the control. They vacuum controls are nearly impossible to find and very expensive if you can. I have a good method of repairing broken nipples.
You may decide to remove the radio from the bracket and let it sit there. Your choice. Carefully pulling the unit out ill give you the opportunity to examine the health of the vacuum lines and controls.
Trace all the lines back to the firewall. Then trace then inside the engine compartment. Use the diagram at the start of this post as a guide. Some people decide to bypass the temp sensor attached to the heater A/C box.
The next thing you would need to do is check the health of the hot water valve and the fresh air dash pot. Now that I think about it, before you take everything apart in the cabin, pull out the scuttle air intake and check the dash pot with the engine running to see of that is where the hissing is coming from.
When I got my Series 1, I had to replace every vacuum line and control in the system. The vale and dash pot were fine. Half the lines were missing and the other half were dry rotted. I know have a fully functioning heater. The fans are noisy, but at least I get heat. I have replacement fans. But that is for another day.
I hope this proves to be helpful. I’m working from memory and I hope that everything is correct, nore or less.

Hi David

Thank you - I will check later in the week and report back.


Hi Jochen

I have a copy of the brown Service Manual and have been mulling over whether to get a copy of the Haynes Book of Half-Truths, but I’d overlooked the parts catalogue.

I’ve not made a physical inspection of the connections yet - thank you for the prompt to do so.


Hi Lawrence

I am immeasurably grateful - thank you for this.

One point of detail - are the vacuum pipes specific to the system or can they be replaced, if necessary, by a standard product?

Just realised that I have ‘reply-bombed’ this post - I’ll @ in future!


The parts catalogue is online at

Good luck


75 XJ6L 4.2 auto (UK spec)

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