Re: Reprogramed GM400

["pcsolutions" <pcsolutions@mindspring.com>]

From: armsco@primenet.com To: jag-lovers@sn.no

> I have read several vague referances to reprogramed
> GM400 transmissions for the XJ-s. The latest was in
> referance to the 6.0L engine. Does anyone know the
> specifics? What exactly was done, and what were the
> benefits?  Michael 

The Turbo 400 in a Jag is just like any other GM TurboHydraMatic 400
made, except for the case the yoke on the output shaft and torque
converter. Those are the big pieces and represent about 3% of what a
Turbo 400 is comprised of.  Parts and kits are pretty easy to come
by.  You can go to any local speed shop and get some pretty serious
parts to do just about any modification you might want, even adding a
trans brake... which BTW I am not recommending.

A shift kit from B&M will do the trick.  Very easy to install,
well... relatively easy... well ok it can be a downright pain in the
butt.  The main thing that helps is putting in the red front pump
spring.  This raises line pressure and firms up the shifts some. 
Most kits come with parts and instructions for different stages which
is to say you can get the shift from mushy to downright firm... tire
chirping firm.  I'd avoid the tire chirping firm, it's hard on the
rest of the drivetrain... especially u-joints.  And you won't attract
the local policemans attention with a crisp 1-2 shift that breaks the
tires loose. 

If your tranny is in need of an overhaul then the mods are very easy
to do while the tranny is out.  To install the stiffer red front pump
spring the tranny must be out and at the very least you must pull the
front pump out from the front of the case to install it.  The next
step would be to install the new seperator plate between the valve
body and the case that came with the kit or drill the appropriate
holes to the correct size as per the instructions that came with the
kit.  Some kits ask you to leave out or add some additional check
balls other than those that were originally in the tranny.  Exactly
what is done and in what order will be determined by 

1)  The condition of the tranny at the time the modifications are
made.
2)  Your goals in modifying the transmission

The one thing that a kit sometimes comes with I almost never use is
the pieces to modify the governor.  Those are usually thrown in the
trash by me.  It's the one of the parts a tranny I am content to
leave alone.  The govenor can be a real finnicky pain in the ass to
get right. If alot of other mods are done then in my opinion it's too
much to do all at once.  Unless you have built several transmissions
and know for sure what those mods will accomplish, then this is one
item best left alone.  In my opinion a transmission is an item that
can be tuned just like an engine. And in fact can be tuned to the car
it is in.  There are lots of variables to consider and it's usually
best to follow the old addage of doing things one step at a time.

If you just want a little extra performance on of the easiest things
to do is to restore the shift points to what they would be in just
about any other application.  A 15 mph 1-2 shift is a good place to
have the shift point.  To do this pull the vacuum line from the shift
modulator and remove the one bolt that holds the clamp that holds it
in place, remove the modulator and throw it in the trash.  Go to a
local parts house and buy an adjustable replacement.  The adjustment
is inside the hole in the short pipe where the vacuum line fits to. 
To adjust it simply install the new modulator and drive the car.  Pay
close attention to the MPH where the 1st to 2nd shift occurs, be sure
that you put your foot on the gas just as you would in normal
driving... in other words don't put your foot all the way to the
firewall and don't take off like you were in a school zone.  About 10
to 20 percent of the pedals travel will do nicely.  When you have
determined the shift point then return to the place where you left
the jack and raise the car again. Remove the vacuum line and insert a
slender flat blade screwdriver into the aforementioned hole and turn
the screw 1/4 turn in the appropriate direction.  If you lower the
shift point it or raise it too much then you'll have to play with the
setting a time or two.  It won't take you long to get the hang of
adjusting the modulator... it really is an easy thing to do, the
frustrating part will be raiseing the Jag with the jack till you get
it right.  And PLEASE be sure to use jack stands, I know its tempting
to raise the car and think that you'll only be under there for a
second or two and just not want to bother with the blocks/stands, but
it only takes a second or two for the seals to blow on your floor
jack or some other mishap to happen.  Jackstands are the way to go.

If your tranny is working ok and not in need of repairs the modulator
would be the only modification I'd do.  Ok here is another opinion,
it's best not to fix something too much that isn't broke.

I hope this helps and was a little more clear than mud.

Tim

----------------------------------------------------------
"Cogito Ergo Lpomoea Batatas"
I think therefore I yam
----------------------------------------------------------

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