A few months back, I mentioned how my aircon was no longer blowing out the central vent. The Jaguar Engineer Factory expert diagnosed it as a faulty vacuum resistor, but being on a call-out, he didn't have one with him. Instead he simply bypassed the resistor, connecting the vacuum hose directly to the solenoid, as shown in the first photo.



I needed to order a heater core and a few other things from David Manners, so tossed in a new vacuum resistor, using Jaguar Classic Parts' diagrams to determine the part number.

This is what arrived. It's clearly the right part number, according to JCP, which shows the same resistor part for both solenoids. So why is the old one yellow and the new one blue? Hmm.


Yellow = old part / Blue = new part

Anyway, I gained access to the solenoids, which in terms of swearing and blood was only a PG job, perhaps a bit PG13 when it came time to easing the foot duct out, and put the new resistor into the vacuum circuit. Very easy.



Tested the aircon, and . . . nothing came out the central vents. Removed the resistor and connected directly, and got plenty of air. Swapped the resistor around end to end in case that was somehow an issue. No air.

It's not a big issue in the grand scheme of things, as the central air vents do work without the resistor, but the flap slaps noisily when turned off, and I thought a simple, cheap repair would fix it. As the flap works perfectly (or seems to) without the resistor, my main question here is, is this in fact the correct part? The different colours make me wonder if different colours mean different amounts of resistance. The yellow one has "1C 15" engraved on top, while the blue one has "AISIN" which is a major Japanese automotive parts manufacturer. So have Jaguar Classic Parts got it wrong, has David Manners, or is something else entirely likely to be the issue?

 

Most unusual that nobody knows anything about vacuum restrictors and aircon.

Anyway, I've been doing some more research, and it seems that despite what the Jaguar Classic Parts diagram says, it's a different part entirely. The old restrictor I removed is yellow, which is Part CCC3537, as seen here: Jaguar XJ6 XJ40 A/C Vacuum Hose Resistor - Yellow - JLM1303 CCC3537

It's interesting that in this diagram for the XJ6 the same basic setup is shown with two different restrictors.
https://www.jaguarclassicparts.com/u...s/vacuum-hoses


However, before I spend more money, I would like to know if:
(a) this is in fact the issue - in other words, how to test if a vacuum restrictor is actually working; and
(b) if anyone know what the difference colours in all the restrictors mean - are they signifying varying levels of restriction, in which case if blue is more restrictive than yellow, then obviously it won't work.

 

If I understand their purpose, which I may well not, it is to damp down the vac signal to prevent the "clonk" you reported that occurs without it when the internal flap slams shut or open against its spring.
If this is true, you should be able to suck the end that is connected to the vac source and after a bit of a struggle, pull air through. One might be more resistant than the other, or work in a different direction.
If you cannot suck hard enough, find a vac pump and see if that does it. Just an idea, no experience to go on, or actual knowledge!

 

Thanks. I agree with you, that it's to prevent the clonk slapping sound. I'm just not sure of the actual structure inside. I've given them both a suck (blowing does nothing), and it does seem as if the yellow one is a tad easier to suck through. Good hint. I'll try again when I can hear better (this room is a bit noisy now).

Although if they're a simple diaphragm or sheet with holes in, they couldn't really fail as such.... Actually, on doing more research, they're also known as vacuum delay valves, and probably use a sintered metal (porous metal) disc to slow passage of air. This disc can get a bit clogged with lint and debris over time.

Anyway, today I tried removing the vacuum tube from the solenoid while the aircon was on, and the flap slams shut, then opens again when I reconnect it. So the system seems to be working. I can feel the vacuum when I put my fingertip against the tube. It's just that the blue resistor resists too much and the yellow one is resisting too much, I guess.

 

When testing did you allow much time for flap operation? The centre flap does take a little while to respond normally with damper in place.

 

I thought I did, but there's no harm in giving it a bit more time. Even with the resistor out of the circuit, the flaps can take a while to move from, say, defrost to cool. Sometimes I've thought it was stuck. In fact it might be a bit gummed up.