Cold Feet at Speed, SIII, Manifold Possibilities

(David Jauch) #1

This is more for reference.

  • I had cold feet all winter long. Drivers side only. The duct blew warm air, there was another source involved.
  • It was not the ducting from the blower which is directly above the footwell.
  • I found the brake light wire jammed in the recirculation flap and thought I had it.
    Didn’t fix it enough.
  • I found that the foam which is glued to the flap had a crease which prevented the flap from sealing. Is there a well-bonding replacement?

…And then, with the small leak in the system that needs to be discovered yet, and the source hooked up to the ported port on the throttle body instead of the manifold, the actuators were (presumably) never strong enough to pull the flaps shut all the way.

It’s not as cold as it was, but I think things got much better now. Is there a simple, understandable chart of what vacuum colour does what, etc.? I understand the system well enough, but not the routing and colours etc.


(Gary Crosby 75 XJ6L, 85 XJ-S, 09 XF Supercharged.) #2

Here are two vacuum reference diagrams…let me think on this

(Gary Crosby 75 XJ6L, 85 XJ-S, 09 XF Supercharged.) #3

Cold air to the feet…the lower heat flaps are operated by a vacuum actuator on the left side of the heater box.
Either a vacuum leak at the actuator or the tee on the green line under the dash pad are a couple places to check. Cold to feet can also be caused be improperly adjusted heater flaps.

Recirc flaps are actuated by an electric vacuum solenoid on the left hand side activated by pink wire…then vacuum is directed to left and right recirc flap vacuum actuators.

Hope this helps



(David Jauch) #4

It sure will, I’ll look into it. The defrost flaps stay open, but the footwells blow warm, too, maybe the hose to the defrosters or the switch itself is damaged. That can explain why both flaps stay open. I have warm air at the feet, but a leak had allowed outside air in. Green line sounds good.

I think that one problem is the leak somewhere, causing this, and then there is the weak vacuum caused by that leak failing to provide enough power.

Thank you,


(Paul M. Novak) #5

Each of the two heater blower motor assemblies has two flaps, one that brings in outside air through the plenum downstream of the air grille in front of the windscreen and another at the bottom of the blower motor assembly to recirculate cabin air. These flaps are mechanically linked in each blower motor housing and have a foam seal to prevent leakage when the flaps are closed.

When I disassembled my Series III XJ6 parts cars and removed the heater blower motor assemblies I found that some of the flaps had badly deteriorated foam that prevented a proper seal and at least one had a broken plastic linkage that prevented the outer flap from closing fully.

Your cold feet may be due to broken flap linkages or missing sealing foam on the heater blower assemblies. The only way to determine this for certain is to remove the blower motor assembly for a look. Alternatively you can visually inspect the outer flaps inside the plenum by removing the air grille in front of the windscreen while operating the climate control system from full heat to full cold and removing the underscuttle to see if anything is amiss with the lower recirculation flap while operating the climate control system from full heat to full cold. You should see these flaps opening and closing fully if they are working properly.

If any of these flaps are not sealing correctly air may be bypassing them and entering your cabin resulting in your “cold feet”.


(Frank Andersen) #6

Some initial vacuum diagnostics, David…

The defrost vents are kept closed by vacuum - and should stay closed at all switch positions except ‘Def’. This is affected by a microswitch on the functions witch. It is very(!) important to sort out why there is no vacuum to the defrost vents…

The centre vents are opened by vacuum, controlled by servo operated vacuum valve - when the servo is in the ‘cool’ position. Ie, vent positions is an indication of vacuum state of respective vents - but does not clarify why vacuum controls fail.

Generally, in ‘Def’; all internal vents should be closed - except the defrost vents (and the manually controlled side vents.

A suspected vacuum problem should be checked out on hoses in the engine compartment - before the more complicated internal checks. Externally, this is easily done by judicious clamping of hoses while observing vacuum with a vacuum gauge. This may reveal leaks, and possible locations of suspects for further checks…

xj6 85 Sov Europe (UK/NZ)

(Gary Crosby 75 XJ6L, 85 XJ-S, 09 XF Supercharged.) #7

Defrost flaps are controlled by a vacuum switch (valve) located on the rear of the mode control assembly. The defrost micro-switch on the mode control simply inserts a 27K resistance into the feedback circuit to the amplifier…sending a high resistance triggering the amplifier to move to the full heat position and switching fans to high.

The defrost vacuum line is connected via a tee into the main servo feed line behind the mode control switch. It is not uncommon for this tee connection to be knocked loose by anyone working replacing a stereo etc.



(Frank Andersen) #8

You are of course right, Gary - but also; in ‘Def’, vacuum is removed from all actuators. Closing all other vents, and opens the heater valve - done by the servo, of course…

But checking out the reason for the uncalled for open defrost vents is a good starting point…

xj6 85 Sov Europe (UK/NZ)

xj6 85 Sov Europe (UK/NZ)

(David Jauch) #9

Which is what he said; switching to DEF vacuum to the green lines is cut (I will look at the switch connectors first; seems good but can’t be the main issue as there is still a significant air bleed to the feet. It could be the weak vacuum failing to achieve both fully closing and opening respectively, but that is not too logical to me and I will have to look into it).
Simultaneously the switch overrides the temperature control and the servo cycles to full heat. The microswitch works. The servo should close the center flap which must open for full cold. I have to check this.

The white line, when clamped, stops the hissing so I already found the reserve tank and engine bay, check valve side to be functional.

The dash pad and footwell trim come off and expose most of the wiring, I will then come up with one way or another of creating vacuum and find out when and where the hissing is happening (as it should not happen in defrost if it’s a green line, etc.)… and report the results in a few weeks or months :slightly_smiling_face:
Ideally I can disconnect the fans and install an air compressor regulated at very low pressure so I won’t have ‚foreign‘ noises to deal with.

All there is is a vacuum leak in the cabin and a defrost/footwell line not working perfectly.

Thank you both,


(Gary Crosby 75 XJ6L, 85 XJ-S, 09 XF Supercharged.) #10


Just to show my lack of knowledge on the series 3…my series 2 has the fresh air intakes from the headlight trim to the footwell. I understand this was eliminated on the series 3…but I thought it could be installed if the owner made some modifications. So just out of shear curiosity…could you be getting cold air forced in from some previous owner mod?

(David Jauch) #11

The inlets and ducting is still there. Weird, right?
The inlets are blanked off (by a vacuum formed plastic plate I think) at the footwell. The pull air flaps can be retrofitted as far as I know: anyways, I wish I had them but they are sealed well. And I think I eliminated the draught for the most part. The remaining issue might well be a lack of power (the weak vacuum, the leak, the actuators unable to pull them shut) that can’t close the recirculation flaps fully. If that is not it I‘ll check the flaps. Misadjusment can’t be ruled out, but that came come later. I also left out that it blew cold everywhere between 0-110mph and only with fans off and at standstill no air came in. So overpressure in or after the fans is what I need to look at…

Good idea nonetheless. I did weite cold feet at speed because that is when most cold air comes in. It is much less pronounced without incoming air.

(Jochen Glöckner) #12

Indeed David,

in my car there is a point where the footwell is not fully closed towards the sill and/or the bulkhead as well. To combat cold flow of outside air at speed I put some insulation material over the opening - fairly sure that opening shouldn’t be there, but some PO who redid the sills may have been less than perfectionist …

Good luck


75XJ6L 4.2 auto (UK spec)

(Frank Andersen) #13

It’s all a bit complicated, David…

The servo turns to ‘full hot’ as Gary says, basically by simulating a cold cabin sensor - but also; a ‘dump’ valve removes vacuum for the whole system.

Which basically means that all vacuum operated flaps go to their ‘default’, spring loaded, positions. The push-rod operated linkages follows the servo…

As you may have more than one problem; I think it might be ‘easier’ to first find out why there is no vacuum to the defrost vents with switch out of ‘Def’ - which should keep them closed…?

xj6 85 Sov Europe (UK/NZ)

xj6 85 Sov Europe (UKL/NZ)

(David Jauch) #14

Interesting. Luckily enough mine has never been welded or otherwise modified as far as major bodywork goes. I‘ll look into a mismatch between blower and body should the issue persist.

Frank, not at all complicated enough to me. The flaps don’t go to their default position completely if the footwells get air at all time and the window gets more than a bleed. The recirc. flaps work to some extent. The temperature can be adjusted and the servo seems to have full travel. The linkage can move.
DEF overrides the cabin sensor and sets the blowers to high… no problem on my part so far, only the XJ doesn’t know what to do as of now.
By the time I can put in a new crash roll I know how to get it out in five minutes tops.


(Doug Dwyer) #15

Yes and no.

Early Series III cars had 'em. They were eliminated circa 1982.


(David Jauch) #16

The pull air vents?

I think the non-AC cars retained the vents (from memory!) but the ducting was always there, and is so (plugged of course) on my ‘84 with AC. Did air conditioned cars before ‘82 come with the vents?

(Doug Dwyer) #17

Yes, the cars with A/C (Climate Control) had the manual vents until 1982-ish.


(David Jauch) #18


Small update: The white lines merge behind the mode switch on the righthand side of the unit. The white line to the source is fine and holds vacuum.
The white line to the blower flap solenoid holds vacuum too, but I have not checked if that stays this way when the system runs / with ign. on.
The white line to the mode switch is leaking when def. is selected.

Now, I have to remove the radio either way (somebody flipped the rear speakers, left and right :roll_eyes:) and will look at the connection at the switch. However, something did move when I tried and so it could have come off under the crash roll which I‘ll remove next. And I‘ll find a remote stop where I can let the engine idle for a while and test every function; make a list.


(Frank Andersen) #19

When ‘Def’ is selected - the electrically operated vacuum control valve removes vacuum on all lines, David…

The colours of my vacuum diagram doesn’t match yours, but that your defrost flaps stays open at all times seems to indicate that the screen vent valve may have failed. This valve also controls the lower heater flap actuator - the screen vent are kept closed by vacuum and the lower heater flap kept open by vacuum, for heating…

Alternatively, a leak in these hoses/actuators may cause loss of vacuum to these items…?

xj6 85 Sov Europe (UK/NZ)

(David Jauch) #20

Exactly: vacuum loss. Not disappearing fully. The lines might be yellow in yours or Garys diagrams so one can see them.
Here‘s what I think is going on:
There is a vacuum leak in the green circuit. The leak is not big enough to cause total loss of power, but it disables most of the functions of the system as the ‚suction power‘ is limited by the diameter of the main source. Some warm air tends to find the footwells and the rest of the warm air goes to the windscreen. At higher throttle the flaps eventually open fully either way due to the leak. The recirculation not fully closing is due to a fault in the RH fan (the flap being obstructed and not sealing), proven by my passengers who always complain about melting, whilst I‘m wrapping my feet in my jacket.