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Jaguar V12 AAV -- Selecting a Donor Thermostat

Jaguar V12

Auxiliary Air Valve

Selecting a Donor Thermostat

Ideally, you should replace a tired thermal bulb in an AAV with one that works the same way.  Richard Guevin suggests you check out the "thermal pellet" offered by Corea Electronics Corporation.

If you just plan to replace the thermal bulb in your AAV with one from a thermostat, you are going to need to buy a thermostat with a suitable bulb in it.  This page is to provide some guidelines towards that purchase.

The first criteria is that you need the coldest thermostat you can find.  In general, insist upon a 160 degree thermostat.  You want the AAV to have moved its piston all the way to the fully closed position by the time the engine thermostats start to open.  This is also a good reason not to be operating your Jag with thermostats any colder than 180 degrees.

Thermal bulbs are marked on the bottom to indicate their temperature range.  The ones that come in the AAV are marked "-30º", whatever that means.  The ones in thermostats are usually marked 160, 170, whatever.

The next thing you need to look for in a donor thermostat is a bulb that is approximately the correct size and shape.  Unfortunately, this really complicates matters.  It'd be really great to be able to tell you to just go out and buy a 160 degree thermostat to fit such-and-such a make and model car, but that won't do it.  Different thermostat manufacturers will make thermostats that all fit the same car using radically different thermal bulbs!  That's why the two examples given on the accompanying pages list the specific brand and part number of thermostat used rather than simply what type car they were intended for.

It would also be nice if the donor thermostat has a bulb with a flange with a bevelled bottom like the original.  Neither of the examples on the accompanying pages has such.

It would also be nice if the plunger sticking out the top of the bulb was configured properly to push on the bottom of the AAV piston.  Good luck; this is not likely to be a feature found on a thermostat.  But it's not terribly difficult to finagle something.  The examples on the accompanying pages took two different tacts here.  If worst comes to worst, you can have a machine shop make up a contraption on a lathe to press onto the end of the plunger.  If the bulb fails again later, you can then buy a simliar thermostat and transfer this device again.

Another desirable feature would be a thermostat that it's not too difficult to get the bulb out of.  Fortunately, they are generally not too difficult to rip up.  The hardest part is often removing pieces that are pressed onto the bulb itself.

Finally, of course, there's price.  As long as you're looking for any thermostat with a suitable bulb, you might as well try to find a cheap one.  Even the expensive ones are an order of magnitude cheaper than a new AAV, though.

For those of us trying to devise a scheme for use by anyone and everyone, availability is also an issue; ideally, a thermostat needs to be common enough that a reader can take the part number and trot on down to the local auto parts store and buy one just like it.  For the individual's purposes, however, whatever one you find will do; you don't care if anyone else can find one like it.  However, the selection is still probably going to be limited to thermostats that are carried in stock so you can look at them, since you probably won't want to special-order a thermostat on the chance that the bulb might be suitable.  The pictures in the catalogs are not likely to be good enough for you to be able to tell for sure; in fact, they quite often use the same photo for both regular and heavy-duty thermostats for the same application, even though the bulbs are likely to be quite different.

Two examples are provided on accompanying pages showing the rebuild of the AAV baseplate using thermal bulbs from thermostats: one using a Robertshaw thermostat, and one using a CarQuest thermostat.  It's suggested you read about these projects before you go shopping for a donor thermostat.


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