When I drive my 1986 XJS XJ-SC in automatic mode, it accelerates like grandma's Cadillac, not bad but it doesn't exactly knock your socks off. Now that I found someone that rebuilds those transmissions for a reasonable price, I finally dared to shift it myself. If I shift into first, then second, then third, she finally goes like a V12 should go and I will for sure end up with a speeding ticket. :-)

Really dumb question: Am I supposed to get off the gas when shifting or do I just keep the pedal to the metal? I don't really want to burn up the tranny this month.

Thanks,

Andy

 

Leave the car in first gear & keep your foot on the gas. If the car is running right it will shift by itself from 1 to 2 @ about 6220 rpm. You will probably hear the tires chirp as well. This is how we could tell how well the engine was running. If it starts to hold back or shift out earlier check the weights in the distributor as well as the catalysts.

 

A V12 with TH400 won't shift by itself.

In an auto keep the pedal to the metal.

Mine was like that a real dog in D it was a little better with the 3.58 diff but still not great. With my 4L60e the thing goes like stink, it'll chirp the tyres into 2nd, I have shift points programed about 6000rpm.

 

Warren, a TH400 will shift from first to second by itself even with the gearstick in first. Gearstick in second, mine at least, will not upshift until the stick is moved to drive.
For the OP, lightening the governor weights (B&M do them) does wonders for gearbox response on the TH400, and also make sure the modulator is not toasted AND that the vac tube to the manifold is really top quality silicone. Just changing this during my rebuild made a huge difference to the responsiveness of the downshifts from cruise to accelerate.
Greg

 

Mine never did that, engine would go to 6500rpm in 1st, maybe mine was not working properly.

 

Iirc mine did the same...

 

Mine upshifted on its one 1 to 2 only though.

I did read a very long time ago that it was a TH400 thing, and not a Jag thing, governed by line pressure and modulator vac, and convertor protection.

If the governor plays up, and they do play up, line pressure would be affected.

Modulators, and the vac hose to the manifold is a well known problem area.

 

As designed, and if unmodified, the TH400 should shift to 2nd even of the gearshift is left in 1st, as others have mentioned. However, many 'shift kits' eliminate this characteristic...and shift kits are often installed by transmission repair shops during overhaul.

If left in "D" the transmission may well upshift too soon under WOT, at about 4800-5000 rpm. I've experienced this on a number of V12s. Jaguar itself recalibrated shift points circa 1989; there's even a TSB on the subject.

The lazy 400 can be tweaked to become much more tolerable in the XJS application....which suffers from lots of weight, lack of low RPM torque, and tall 2.88 gearing. On my '88 XJS I had a shift kit, adjustable modulator, 2500 stall converter, and modified kick down. This made the transmission more responsive. WOT shifts, if left in "D", came at about 6200 rpm.....about 1000 rpm higher than before. Plus, WOT kick down to 1st gear was available at a much higher road speed.

The kick down mod is nothing more that operating the circuit from the enrichment switch.....which closes at about 75% throttle (versus 100% throttle of the regular kick down switch). This might be too aggressive for some but I liked it.

Cheers
DD

 

It makes you realize how little 1st gear is used. What a waste.

Gotta be careful though, at full tilt mine always breaks traction shifting from 1st to 2nd on its own.

I believe the video on Harry's Garage of the yellow XJS he manually shifts the automatic trans the entire time he drives it.

 

Mine did the same, on heavy throttle, enough to scooch sideways a bit. A handful on wet pavement....where the scooch became a 45º angle or more. Yikes !


Cheers
DD

 

Without a doubt, that tyre chirp when the transmission shifts from 1st to 2nd when you put the gear in 1st manually, is one of the most gratifying experiences when driving the XJS.

For a car so refined and smooth, it's nice to know there's some bite and growl when you have to poke the beast.