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Jaguar V12 Supplementary Air Valve

Jaguar V12

Solenoid Air Switch


Supplemental Air Valve

For anyone who thinks British car owners exaggerate when talking about the excellence of the repair documentation available for our automobiles, reading this page should dispel any such doubts.

Whatever the part is called, that's it, the cylinder with a barely readable "12V" and a symbol that looks like a circle with an arrow through it.  It is attached to the inside of the front end of the right side air filter housing with a grommet, and the other end is connected to a hose to the bottom center of the right side intake manifold.  It has two wires coming from it.

Apparently, that grommet only looks like a bellows on Kirby Palm's car; everybody else has a simple grommet, and that's what's shown in the Parts Catalogue.

Yes, the overrun valves have been removed from this car, and aluminum plates mounted in their place.  That's a rubber plug with a bolt in the middle where the hose to the overrun valve is supposed to connect.

Les Marston (raggedrunner at mac dot com) tells us about the symbol:  "This was the logo of a British company called Tecalemit."  You can visit Tecalemit's web site at  They also made the fuel tank changeover valves for the Jaguar XJ6 and XJ12.

OK, now about the name of this part.  In section 19 of the Supplement to the Repair Operation Manual, ©1982 (as well as Supplement A of later editions) and in Figures 13.9 through 13.12 and 13.29 in Haynes manual #478, this thing is called a Supplemental Air Valve.  In section 17 of the same Supplement and Figures 13.31 through 13.34 in the same Haynes it's called Item H - Solenoid Air Switch.

By the way, Figure 13.29 in the Haynes is a very good illustration of the part, except the one in the photo above doesn't have the mounting tangs shown in the Haynes.

The location is another source of confusion.    It's described on page 17-3 of the Supplement as being "Bolted to top of RH Inlet Manifold" but there's no such thing bolted to the top of Palm's RH inlet manifold and nobody has yet reported finding a part that fits this description in that location.

Besides the name and the location, there is confusion about which cars it's found in.  The legend for the EFI wiring diagrams on pages 19-1 and 19-2 in the Supplement indicate it only exists on 'B' Emission cars, UK and Europe; Haynes manual #478 indicates the same thing in Figures 13.9 through 13.12.  But the photo above is Kirby Palm's '83 US-spec, which isn't supposed to have this valve.  In fact, it appears that all H.E. models have it.

There's yet more confusion about how it's operated.  Page 19-2 in the Supplement and Figures 13.9 and 13.11 in the Haynes seem to indicate it's considered part of the 'B' Emission EFI system show it (and a vacuum changeover switch) being controlled by an oil temperature switch.  As yet nobody anywhere in the world has reported finding an oil temperature switch on their XJ-S.  It's unknown what that vacuum changeover switch does or if it even really exists.

Section 17 of the Supplement, Figures 13.31 through 13.34 in the Haynes indicate it's part of the vacuum advance control system and is operated by a "Timer/Relay" or "Time Delay Module".  The "Electrical Guide", publication S57 (reportedly a US-only publication) confirms the timer scheme, indicating it's a 45-second timer.  Reportedly it's a 45-second timer in the US-spec cars and a 15-minute timer elsewhere.  The 'B' Emission vacuum advance system with the 15-minute timer keys off coolant temperature, not oil temperature.

The lower half of Fig. 24-4 in the Electrical Guide also provides an excellent illustration of the part -- this time without the mounting tangs.

On later cars, Jaguar decided to use this same valve to boost the idle when the A/C compressor engaged.  It would sound simple enough to simply connect a wire from the compressor clutch over to this valve, but unfortunately the original timer scheme didn't operate the valve by applying 12V; rather, 12V is applied whenever the fuel pump is powered and the timer merely makes or breaks the ground circuit.  So, to get the 12V from the compressor clutch to operate the valve, an "idle relay" was installed on top of the radiator.  There's a diode in the wiring to prevent the A/C coming on from energizing the other portions of the cold-start system.

But that's not all.  Jaguar decided to boost the idle when the A/C came on in gear, but not to boost it when the shifter is in N or P.  Since there was already a safety switch on the shifter, this was wired to a second relay called an "idle override relay" that prevents the solenoid air valve from energizing when the shifter is in N or P.

If your supplemental air valve isn't working, you might consider squirting some carb cleaner through the center.  It tends to get jammed up with goo.  Attempts to get it apart are likely to be destructive.

If it's toast, Iain Burgess (Iain dot Burgess at leicon dot com dot au) suggests you consider replacing it with a similar valve from a Toyota.  Here's what the A/C idle boost solenoid from a Toyota looks like:

There's a green-and-silver decal on the other side -- the side the valve is sitting on in this picture -- that says "Nippondenso" and the number 084600-5510.  This one is a junkyard part, but presumably you could buy one new at a Toyota dealer.  Note that this valve is apparently not used on all Toyotas with air conditioning; you might want to print out a copy of this picture to take with you.  If you don't get the connector with it, you can use two insulated spade terminals.

It looks nothing like the Jag part.  It does largely the same thing, though, with one notable difference: the Toyota part has that white plastic knob by which you can adjust how much the idle is boosted when the solenoid is energized.

The existing bracket can be bent, cut, and drilled and this valve bolted to the inside of the RH air filter housing -- or anywhere else for that matter.  The hose barbs are 8mm (about 5/16") which makes them a hair smaller than the 3/8" barbs on the Jaguar valve -- but the passage through this valve is bigger than the one on the Jag valve, so you should have a suitable range for adjusting the idle boost.  Perhaps the most challenging part of the retrofit will be sealing the original opening in the air filter housing while providing a barb to connect a hose to.

Here's how Palm mounted the Toyota valve on the inside surface of the air filter housing:

That's the hole for the original SAV to the left, not yet plugged.  The plug for the overrun valve connection is partially obscured.  The inlet hose barb on the Toyota valve is jammed through a rubber plug in the hole that once held the waste line from the air pump.  This hole should be plugged on all cars; even if you have to maintain a working air pump system, this line should be relocated to the other side of the filter housing or simply dumped overboard.

Simple 90° mounting bracket -- but it's not necessarily a simple job to attach it to the solenoid.  The strap around the coil was removed by bending four pairs of tabs straight.  Then the mounting lugs were cut off, leaving a simple U-shape except that it was twice as thick on one side where the mounting lugs had been attached.  This side was drilled and attached to the bracket with short self-tapping screws, short enough that they don't protrude through and jam into the solenoid coil.  Finally, the strap was reinstalled on the solenoid and the four pairs of tabs bent back into place.  This job could conceivably been done without removing the strap from the solenoid, but it would have required utmost care to avoid drilling into the solenoid coil.

Those three screws holding the bracket to the air filter housing are installed with lock washers, self-locking nuts, Loctite, and the exposed threads beyond the nuts are deformed.  If you don't understand why, perhaps you should skip this particular modification.  Another idea would be to use sheet metal screws instead of bolts and nuts, thereby not having any potentially loose objects within the air filter housing.

Here's how the valve mounted on the air filter housing looks installed on the car:

Looks OK.  Probably would look better if the bracket were painted black.  Remember, the OEM valve didn't exactly look like a neat and professional installation.

Not hooked up yet -- note the bare wire ends at the extreme right.

In this position, the connector on the solenoid clears the manifold by about 1/8" -- plenty.  I think this location would clear an overrun valve and connecting hose just fine.

There are pros and cons to this mounting location.  The pros include the hose routing; the hose to the manifold is a simple U-bend, and there's no hose to the air filter housing at all -- the intake is via the barb protruding directly into the housing.  The biggest con is the adjuster location; it's nearly impossible to get a hand on it.  But mounting the valve where it'd be easy to adjust the idle would result in big looping hose routings.

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