'85 XJ-S V12 stock TH400 trans.

Downshift switch works fine. If I am doing 30 mph the car will quite nicely drop down into 1st and pull like a freight train. No issues there.

Now, normally if I were going faster than that the car would never downshift. If I were doing 50 it would easily accelerate but not downshift. I figured this was normal.

Under certain circumstances, if i were traveling at highway speeds, and i floored it, the car would accelerate, then at some point the trans would start to slip and the RPMs very slowly rise faster than the MPH rose. I diagnosed this as a dropped trans filter, added it to my to-do list and moved on.

Today I got curious.

I was making the usual 60-mph 1000-foot twisting elevation to my house and I said screw it. I floored it and when the trans started to "slip", I kept my foot in it. TH400s are cheap to rebuild, so I figured if it was on its' way out, lets help it along.

I was doing about 60 and the car was accelerating slowly, all uphill, and the revs started to rise like normal...kept rising...kept rising...and then...

Holy ****.

The trans downshifted, for the first time ever, at about 65-70 mph. The engine roared (it helped that I had one of those concrete barriers next to me) and leapt forward. Before I even knew it I was doing 90 and had to brake. I actually did one of those giddy maniacal laughs they do every time the drive a Bugatti Veyron on Top Gear UK. Unbelievable knowing what I've been missing all along. The car pulled so hard I couldn't believe it.

So....

What's with the enormous downshift lag? I don't think this is a real tranny slip like a worn trans would do. The car was still part-throttle accelerating while the ten-second downshift was occurring (and yes the entire downshift did take about 5-10 seconds in all). The 2-1 downshift behaves perfectly normally. How do I fix this, because frankly, I've had my first taste of smack and now I need another fix immediately.

Thanks,
D

 

play with your shift points, adjust your kickdown switch, and see about upgrading that ol vacuum modulator.

 

I thought about the vac mod but wouldn't that affect all shifts or even all downshifts? Why just this one specific downshift?

 

If you were at part throttle the then kickdown switch couldn't have been actuated, as it will operate only if you go to wide open throttle. Actually, further than WOT. If you look at the arrangement you'll see that the throttles are fully open before the kickdown switch operates.

Either you buried you foot a bit deeper to get a downshift or it downshifted according to engine load via the vacuum modulator.

Tweak your kickdown switch adjustment a bit as mentioned by M90....that's an easy place to begine. Or reconfigure the kickdown to operate off the enrichment switch....which closes at about 80% throttle rather than 100+% throttle.

Cheers
DD

 

thats a freakin brilliant idea! makes me wish i still had a V12 so i could try it! you should be able to just wire that junk up in parallel, no?

 

Not knowing how many amps the kickdown solenoid draws I used the enrichment switch to activate a relay which, in turn, activated the kickdown solenoid.

Wish I could say I thought of it but I picked up the idea years ago on J-L.

This mod, a slightly higher stall TC, and a decent shift kit made the engine/trans combo much more responsive.....almost to the point of making the 400 transmission suitable for the car, engine, and axle ratio :-)

Cheers
DD

 

That was a typo on my part - the pedal was buried but it was accelerating as though it was at part-throttle because it hadn't kicked down. I know the switch is working when it should, and the 2-1 downshift is fine.

 

whoever thought 2.88's and a 1500 stall was a good idea deserves to be shot.

i run 2.93's in all my GP's with a 3k stall and love them. i have no desire for a lower ratio.

 

Actually that would be a higher gear ratio.

3,000 rpm stall? Wow that engine does not make any torque until that rpm? Must be a wonderful driving car...

 

a higher gear ratio would be numerically lower.a lower gear ratio is numerically higher.

the whole point of a higher stall is to get the blower spinning. its got torque to spare, and then some... which is obvious based on its gear ratio. i can smolder the tires from the bottom of second gear from a roll.


i would like to see a low stall car with sub 3:1 gears perform..... oh wait. lol

 

Actually the TCs used by Jaguar were 2000 rpm stall but, still, the V12 just doesn't make enough low rpm torque to pull 4000+ pounds thru 2.88 gears with any sort of.....well....ooomph. Of course, once some road speed is gained and the engine gets into the power band acceleration improves.

Add to this that an awful lot of XJSs are simply not running right, often without owners even knowing, and you have some rather sluggish Jags out there.

OTOH, those tall gears and 3 speed auto can work well under some circumstances, especially high speed driving. For example, 2nd gear is good to over 100 mph....making it very useful for high speed overtaking.

Cheers
DD

 

Thnking now about dropping the pan since I'm due for a fluid and filter change. I'm going to change out the vacuum modulator and add a Transgo shift kit while its out. Do you guys think this will solve my downshifting issue? Not the shift kit obviously, but replacing the vac mod.

 

Transgo makes the BEST shift kits. i run on in my modified GP.

 

Your lack of 3-2 downshift at WOT might be a valve body problem....clogged orifice or such.... so a shift kit actually might be the fix, directly or indirectly, as installation usually involves dropping the valve body....which you'd clean out very carefully (so as to avoid losing the little check ***** and such)

I don't think the vacuum modulator would make any difference in this particular case, as I think the WOT kickdown is purely electric, but I'd replace it on gen'l principles

Others will chime in. Been decades since I had my hands inside a 400 and even then it was only once, so I'm no expert :-)

Cheers
DD

 

replacing the mod. is a good preventative maintenence on an older trans. i remember just recently a member having blown the diaphragm in his mod. and all the fluid was pumped out and into his V12!

 

Post deleted, wrong forum.

 

I think the kickdown switch is not working, at least not reliably. Probably needs adjustment. The adjustment is a bit tricky, and the design is poor (surprise, surprise!), so it is common for them not to work.

I've had two XJSs, and both came to me with non-working kickdown switches, behaving exactly as you describe.

Without the kickdown switch, the transmission will still downshift sometimes based on the vacuum modulator, but not reliably, as you have found.

 

My kick down switch is not working. Actually the micro switch makes contact; but the cable never operates it to the detent to activate the switch.
How do I free this up. The sheath is held affixed by a tiny screw so how does it move anyway. Some one please explain. (Now that I've fixed the major faults I want the other features working...like the kick down.)