New gearbox modulator fitted to V12

[Brakebuster]

since getting my V12 back on the road the gearbox has been getting smoother every trip out, but in an effort to improve it even more i purchased a new gearbox vacuum modulator from jaguar and i fitted it last night

simple enough operation which required jacking the car up safely and securely on axle stands, sliding under the car, identifying the existing mustard coloured Holley branded modulator and removing the retaining bracket thats held in with a half inch bolt ,
the modulator simply extracts with a ' pop ' as the seal is broken from the O ring , with very little gearbox oil dripping from the aperture ,

the existing vacuum hose was hard , though not perished, but i bought a new one anyway ,

the new modulator is significantly smaller than the original Holley unit and is finished in an electroplate effect, but is still made in the USA , and is a perfect fit ,
installation was to simply push back into the gearbox , ensure a snug fit and then offer up the retaining bracket and tighten down the bolt ,

Replacing the vacuum hose is easy enough , but made easier by attaching to the old hose under the car, and then pulling through to the top of the engine from above, this is fitted to the back of the right bank inlet plenum , where two other hoses are fitted ,

with all the installation completed i now have to take her for a test drive to ensure everything is working fine,

i have retained al the original parts......just in case


the reason behind the change is that my unit is 24 years old, and the car has done 105k miles , and for the small cost i feel it was worth the effort, and the vacuum diaphragm is known to get weak and slightly alter the gear changing point compared to engine load

BB

[davidr]

I recently replaced the vacuum modulator on my 1989 V12 and what a difference it made! Following a tip from David Marks at the XJS Seminar, I bought an adjustable one to make the gearbox more responsive and the car more driveable.
It certainly worked! The car would previously wallow about in top gear and refuse to change down unless I changed manually with the gear lever or booted it into kick down. Bear in mind that this a 1989 car that is supposed to have a more responsive gearbox setup! Now it hangs onto 1st gear for longer under throttle but will change up normally if you're light on the pedal. The best bit is that where say accelerating out of a 40 mph limit (or even 30mph!) the box would stick in top, it now happily drops to 2nd at half throttle and pulls away much better. Great improvement and makes the car feel much more responsive. I would liken it to having a more modern gearbox in sports mode where it'll hang onto gears for longer and change down more readily.
Fitting it was as easy as BB says above. It took longer to get under the car than to actually fit the part. It did take a bit of time to get the adjustment right on the modulator. It adjusts by pulling the vacuum pipe off and turning a screw inside.
I started off with the screw adjusted too far in so it hung onto 1st gear for too long and the shifts were too jerky. Gradually adjusting it out and trial and error got me to the right setting. Good thing is that you can set it how you want.
I was so fed up with my "V12 sports car" being less responsive in town traffic than my diesel Land Rover that I was seriously considering a manual conversion. The improvement is sufficient to put that thought on the back burner!
And the cost of this transformation? Best £20 I ever spent on the car!
Picture of the part below - purchased on eBay

image.jpg (13.51KiB)Viewed 3286 times

[BigCatXJS]

Excellent stuff - I have had my adjustable modulator in place for about 4 years now and have not regretted it

Just watch-up for wheel spin in the wet - managed to induce a multiple down-shift from 3rd to 1st once at around 40 mph when joining the M40 in the rain!