Series 1 - Vacuum Flap & Heater Control Modulators - Repair Tips?

(Edward Banks) #1

Hi Jag-Lovers,

The previous owner of my 1970 S1 played around with the HVAC system in the car and has made a mess of it; I am keen to return it to the original configuration.

Both the Flap and Heater Control Modulators don’t seem to function correctly. I know many people recommend replacing them with various third party offerings but it would be my preference to keep the original components and make them work as best possible.

I have pulled the flap control modulator apart and the rubber diaphragm inside seems in reasonable condition but I am unable to get it the reduce the vacuum passed through and in turn, close the recirculation flap actuator.

Has anyone develop a method or have any tips for restoring the operation of these components?

Sincere appreciate any thoughts.



(Robert Wilkinson) #2

Ed, I’m one of those who has tarted up this system in my Series 1, so I am of limited help.

Most folks have trouble with vacuum leaks, or getting vacuum at all, but you seem to have the opposite problem. Your modulator (lever above radio) may not be bleeding vacuum properly. Just providing a variable restriction (like a water valve) doesn’t work with vacuum, because even after shutting the vacuum source off, the distal part of the circuit (the vacuum solenoid that operates the scuttle flap) remains evacuated. Another possibility is that the vacuum source is too powerful. It’s supposed to come from a regulator controlled in part by heat sensed from the heater core. I doubt if that’s the problem, though.

The heater control is similar. Both are “shunt” regulators in the sense that they are sourced by an enormous source of vacuum (the intake manifold, supplying a large tank) and regulate the vacuum supplied to vacuum motors (heater valve, scuttle solenoid) by providing a controlled leakage path–a variable amount of vacuum is leaked, with the rest supplied to the motor.

You can try temporarily the following. Insert a vacuum “tee” in the line just before the shuttle vacuum solenoid. With the tee open, the solenoid should not pull in. With a finger over the tee, it should regardless of where your control lever is set, according to your description. Now, restrict the leak provided by the tee–put a screw in it for example. If you can get a “just right” leak from the tee so that the lever modulates the scuttle flap position, you can either leave it or tweak the modulator to have the same leak.

It’s great fun to tart it up, though. Have you noticed that moving that fresh air flap also messes with your defroster vents?


(Edward Banks) #3

G’day Robert,

Thank you so much for the response, it has certainly helped with the process of thinking the problem through. :slight_smile:

Unfortunately, the previous owner really did make a mess of the system, many of the vents are disconnected, so opening the scuttle flap currently has very little effect on anything. Thought I’d start with these components as the appear the most controversial! Once somewhat operating, I’m sure I’ll be able to source the remaining connecting components and then learn about the intricacies of the system.

Out of interest what did you replace the scuttle flap controller with? I’ve seen the 3rd party rotary heater valves but haven’t come across a solution for the flap.



(Robert Wilkinson) #4

Mine actually worked, but it was too large physically as I was routing an air duct above the radio. So I replaced it with a small vacuum toggle valve made by Clippard (available from Amazon but cheaper elsewhere). In one position it applies the vacuum to the vent solenoid; in the other, it blocks off the vacuum whilst connecting the solenoid to atmosphere. I flattened the switch handle a bit by filing it and attached it to a flat piece of metal shaped like the lever on the original–so that it would accept the black thingy. Looks stock, but it’s on-off, no in-between. I don’t miss the partial function; not sure if it even worked on my car.

BTW, Roger Mabry did this as well. He also replaced the heater control as you describe and is very happy with it–requires a matching motorized heater valve. I you do that, you could use an electrical toggle switch to control a remote vacuum solenoid for the scuttle (also made by Clippard). The advantage then is you don’t need any vacuum plumbing in the car at all! I would have gone that route if I had been smarter and a bit richer.

(Frank Andersen) #5

Do you have the schematics of the lay-out, Edwards…?

Like with electrics - unless you have the diagrams, and a PO have messed up; getting back to the original ain’t easy…

A central part is the heater sensing unit, which applies and releases vacuum to the flap actuator and water valve according to cabin temp, heat control modulator and flap actuator module settings…

The heater sensing unit is the usual source of problems; it’s unlikely to be available - and there are no readily available description on how to restore it. And testing it is more complicated than testing the other components…

Fully functional, the system is designed to maintain the temp by regulating the air flow and water valve. Without the ‘brain’, the heat control unit, the system cannot operate automatically.

Its’ possible, if the heat control and air flap modulators are in working order, to operate the system manually. But the flap actuator and water valve must of course also - be working, and vacuum supply verified…

In short; getting back to the original configuration will require identifying each component and a lot of testing…so…?

xj6 85 Sov Europe (UK/NZ)

(Roger Mabry) #6

Robert and I have modified out S1 heating and AC to work much better than stock… no stock parts are available new since the factory ran out of spares during production…the plastic nipples broke off (now we fix them with small brass tubing epoxied in their place).

I have used the Clippard valve in two S1 XJ’s using the stock lever and have used the Vintage Heater Servo in both of those cars…an electrically switched water valve with the control replacing the stock unit and it uses the stock knob… no one can tell from inside the car that things have been updated.


I loaded the photo showing the VA shaft sticking out in the stock position with no knob attached to show that their configuration allows the use of the stock heater knob.

(Frank Andersen) #7

Does the system automatically maintain set cabin temp, Roger…?

…or is it more like the DELII modification - bypassing the AC amp with manual control of the servo…?

xj6 85 Sov Europe (UK/NZ)

(Roger Mabry) #8


Yes, the VA Heater Servo maintains the set temperature. Does not mix outside air with the heat, just shuts down to the set temp. The systems are totally separate except they both use the same two fans and vents.

S1 heating is really simple… the real problems are with the vacuum lines that go to plastic nipples on the devices behind the hot radio. The lines get bad (rubber) over time and then changing them causes the nipples to break (45+ year old plastic) that lives behind a radio and in front of the heater matrix and evaporator.

Roger Mabry

(Robert Wilkinson) #9

I didn’t realize that, Roger. On the later systems, there is a temperature sensor that actually senses the temperature of the cabin air. Does yours do that (as you know I’m considering getting one) and if so where is the cabin air sensor located?

(Roger Mabry) #10

What I mean, the system does not change from the setting on the heater control knob…maintains that “number” until YOU change it up/down. All manual operation…

Roger Mabry

(Frank Andersen) #11

That’s what I thought, Roger - cabin temp must be controlled manually. As indeed required with the AC amp bypassed on the DELII - the heater sensing unit being the equivalent…

…and no longer available; your mod is likely as good as it gets…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Edward Banks) #12

Thanks Robert,

This solution does look mighty appealing and seems a great deal more robust! Thank you also for including the link, I had a look on their website but was unable to find the correct component before you sent this through. My scuttle vacuum actuator works perfectly, so this is certainly an option.



(Edward Banks) #13

Thanks Frank,

Yes, I have the workshop manual for the vehicle and also have a reasonable ability in applying the knowledge in the workshop manual.

All the components exist and I have repaired several of the broken plastic vacuum lines with brass alternatives and two part epoxy; these seem to hold a perfect vacuum from my testing. The issue I am having is getting the internals of the valve to operate correctly, even though these components actually seem in good condition - soft rubber, moving central pin and spring.

I will continue to investigate and report any findings.



(Edward Banks) #14

Thanks for the info and image Roger,

Certainly it is visually 100% acceptable! :slight_smile:

Yes, I know the issue with the broken plastic nipples and have overcome it with brass tubing. Will let you know how I go with the valve itself.



(Edward Banks) #15

Hi All,

A well known Jag mechanic in Melbourne (Australia), has suggested that the system, when in good condition, actually works quite well. Can anyone else verify this information?

I appreciate they updated it for a reason but if it can be used than this would certainly be my goal. If I am simply kidding myself then perhaps the alternative suggest by others is a far more sensible pursuit and I can spend the time out on the road enjoying the car.



(Roger Mabry) #16


I believe I have a good repaired valve if you need it.

Roger Mabry

(Frank Andersen) #17

The crucial part for automatic operation is the ‘heater sensing unit’, Edward…

…placed inside the cabin, it somehow manipulates vacuum on the heat control monitor and water valve. Based on cabin temps it changes settings to keep the cabin temp constant without drive inputs.

The common heat sensing gadgets are a bimetallic spring or expanding liquid (mercury?) - no sensing, no variations with temps. But I never had a look inside the unit…:slight_smile:

The system was reputedly well working, so much so that it was retained in the initial AC set-up. However, the weak point was that the system was prone to mechanical/vacuum failures over time…

xj6 85 Sov Europe (UK/NZ)

(Edward Banks) #18

Thanks for that additional knowledge Frank.

Makes complete sense. :slight_smile:

(Roger Mabry) #19

I fixed two broken nipples a different way. My local hobby shop did not have brass tubing of the small size needed… but had inflation tubes that had threads on one end. I just put a small wood screw into the nipple and started some threads… then put Gorilla Super Glue on the metal threads and pushed it in the hole and turned it. When done I put Epoxy over the nipple end for added strength.

Epoxy is not yet on the repaired part in the last photo.

But, I am shocked on the shipping cost to Australia from the US for a 5 pound box of heater
parts. USPS is $82.20 and FedEx more at $180.25… the parts cost is about the same as the
shipping by USPS…

(Tom weckworth) #20

I have a vacuum heater control the nipped are good.