car is running really good now, I'm just speedometerless but I use my gps... I'm wondering what the normal rpm shift point is and if my rpms might be off? Car shifts at 4500 when wot. If I use the shifter and put it in first it gets to 4500 or so and bucks like it hit redline... same in 2nd gear. Hit 4500 and bucked right at 85mph. What is the max speed per gear in the xjs th400? And if I'm not able to hit redline is that seem like a centrifugal advance or vacuum advance issue? Cause I think both might be in so so condition on mine. Thanks

 

Common issue/complaint....early shifts.

If *manually* shifted at redline the shifts will be at about 65 and 110 mph.

In '89 a change was made to increase shift points at WOT...but I can't remember the details

On my '88 I had a shift kit and adjustable modulator and the WOT shifts were at about 6000 rpm. In reality not much is gained, in terms of power and acceleration, by going right up to 6500 rpm. Prior to the mods it shifted at about 5000 RPM as I recall

The trans on my present '88 V12 (sedan, not XJS) shifts at about 5500 at WOT

Your bucking could be an ignition fault

Cheers
DD

 

6000rpm shifts

1st 102km/h
2nd 172km/h
3rd 254km/h

My 89 shifted about 5700rpm

 

i have heard the complaints about the early shifting. Car still feels fast and love the tq! I guess the bucking was more worrying than the early shifting. Would the ecu just @know" to shift so early? Because I might have an ignition issue? Car feels solid. Pulls nice

Thank you

 

No communication capability between the ECU and the transmission. Neither knows what the other is doing


Cheers
DD

 

On A TH400 the shift point is controlled by the governor weights and springs. I have a Daimler limo with the TH400 and it shifted at 2700 rpm! Just as the engine started making power, it shifted.
I took the governor from an older Suburban and that was just about right, I think shifts are ~5300 rpm now.

 

So is it just coincidental that it's shifting at 4500 and if I try to hold it past that it bucks?

So if I changed the gov to shift at 6k will it effect the shifting at say 1/4 throttle? Cause it is buttery smooth now once up to temp.

 

The part throttle shift points are controlled by the vacuum modulator, max rpm shifts by the governor.

 

oh cool, thank you

 

Until we know what's causing the bucking it's hard to say what is or isn't coincidental.


But the ECU and transmission are not tied together in any way.


Cheers
DD

 

On jag-lovers it describes that if the governor won't even allow you to hold past it's rpm if you toss it in 1st etc? Will it shift up to 2nd if it's in 1st? Or will it just hit the governor and buck like hitting the rev limit?

 

It will shift. The governor is purely hydraulic, unlike a rev limiter than can cut fuel.

I have not tried a 1-2 shift, but I know for a 2-3 shift if the lever is left in 2 it will shift to 3 if the governor rpm limit is hit. I assume 1-2 shifts are the same.

 

ok thanks... wonder why my gov would be so low

 

Could simply be weak springs. Or it's been replaced in the course of the cars life with an incorrect part. All the TH400 governors from any GM product will drop right in. It's behind the square cover on the right side of the transmission.

There are quite a few combinations of weights and springs that have been made over the years. I pulled junkyard ones until I had one that I liked the shift points. I think it was from an early 80's Suburban, but it might have been a Cadillac.

 

I see 170-180 km/h on the speedo, when mine shifts to 3rd.
2nd gear RPM before shifting is indicated to be around 5200-5300, but that does not fit with the above info.

I guess I will have to check both instruments for deviation. Í have done it for the speedo, but only at more sensible speeds, where it (except from fluctuations) is pretty accurate. I actually hope it's my rev counter, that is off

The rear end of the car does by the way get very loose, when it shifts to 3rd at WOT on wet roads. Scary stuff, but it's great to know the beast has some power...

 

interesting.. I learn new stuff everyday with this car. I forgot that the trans in my car does not look right and Im thinking now the whole transmission was changed and who knows what's in it!

 

Do you mean a constant bucking or one single buck? I'm wondering whether the terminology is getting people confused here, because I would describe the 1st to 2nd shift as a buck if you manually select first, it will spin the wheels and "buck" when it shifts.

 

not sure it shifts or not because once it bucks I let off the gas

 

ahh okay, so by bucks do you mean it act like its out of gas?

 

yeah feels like it hits a rev limiter on a new car