80s TH400 modifications

Was going thru my notes on my transmission, and thought I’d share these, as questions come up time to time.

  • If you have a 1981-1988 XJS with the TH400 transmission (actually manufactured 80 up to 87, which may include 88 XJS) GM unfortunately switched to a plastic Accumulator Piston. These WILL fail, and render your transmission needing a rebuild. The aluminum one was used before 1981 and after 1988 XJSs. It is only $20. You’ll have to drop the valve body to replace it. Here’s a link to my job:
    TH400 Plastic Accumulator Piston Valve Body

  • While you’re dropping the valve body, why not put in a B&M shift kit? They are only $50. There are two options within the kit, make sure to go for the LESS aggressive one. Shifts are much firmer, especially when accelerating hard. But nothing too hard. Make sure to get the right version, they changed around 1987/88. Above link also shows what I did for that.

  • Want a sportier shifting transmission? Simply put in a B&M 20234 Vacuum Modulator. Only $40 and easy to install. You can adjust it depending on how sporty you want it. This will simply hold lower gears a bit longer, and downshift a bit earlier. All dependent on vacuum setting you choose.

  • This isn’t transmission related, but worth mentioning. I replaced my Vacuum Full Throttle Switch (EAC 4371) with this: Stewart Warner 77343-04. It triggers at 4 InHg vacuum. A little higher vacuum than the OEM one. So you’ll get fuel enrichment a bit earlier when flooring it. :slight_smile:

  • And I only use Valvoline Dex/Merc ATF, NOT the full synthetic. I was told by a TH400 rebuilder that these transmissions do not like full synthetic.


Also make sure you’re full-throttle kickdown switch is working. Or…modify the kickdown circuit to make the kickdown occur a bit before full throttle. There are various ways of doing this.

I also add a compressor cut-out to the kickdown circuit.



That is an excellent list of recommendations! Not that I am ever going to do any work on a 400, but it’s good to know these kind of information

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Don’t put in a shift kit. Shift kits are for tow trucks and RVs. You want shifts between gears to be as seamless as possible, and a shift kit eliminates all overlap and other shift-softening that was designed into the transmission.

Imagine someone who doesn’t really know how to drive a manual transmission… thats what you make you automatic feel like.

Manual transmissions are fun. A shift kit just makes you automatic worse. It’s not a Camaro, you’re not at the strip.


Speaking for myself and not for others, I’m a big advocate of shift kits. Both my Jag V12/TH400 cars have them, as well as many of my other cars over the years.

You can go from mild to wild with these kits. True enough, if you pick the wrong kit you’ll regret it. Not pleasant.

The mild shift kits just slightly firm-up the shifting. I like it, personally, and the reduction in slippage is better for the transmission.



Agreed, I believe thats where you get into stage one and stage two differences. I think by tweaking the modulator you can set your preferences a little more.
I also understand the more of the softer shifting as in an old mans.Cadillac is actually harder on the tranny more slush means more filings etc.
Great list Gregg, when are you coming up to Canada to do mine.
I think there’s one more though. Something about the line replacement to stop the whining noise when cold.

More wear on the clutches.

The TH400 was universally known for being buttery smooth…but not universally known for being long-lived. The slippage/overlap wears out the clutches. Decades ago, when there were still many TH400 cars on the road, it was common for them to come in with a “transmission slipping” complaint. That was in my early days at GM dealerships. A “soft parts overhaul” at 80k-100k miles was common. If left unattended it could easily turn into a “hard parts overhaul” at much higher expense.



Agree. The th400 shift kit i put in is the least of them all. Just firmed up the shifting to feel like a normal trans. And yes, you can go way too far with th400 shift kits. Only good for whiplash. You can also bypass safety, like putting it in R while driving, and going past redline with no forced shifts. No thanks.

My grandfather was a transmission rebuilder and at least for purposes of longevity, he noted that a quick snappy shift made the clutches last longer. So I agree w/ doing a shift kit. It doesn’t mean that the shift will be harsh (depending on how it’s done) but quick. And the more gently one drives, the more buttery the shift feels, which is about perfect for a compromise. I also added a hold-in-gear feature so I could manually shift up at 6500 if I wanted to.

Like you I rebuilt my trans back in the 90’s and had, besides the standard rebuild manuals, the benefit of reading one of the best books (still highly sought after) on the subject:


“How to Work with and Modify the Turbo Hydra-Matic 400 Transmission”
by Ron Sessions
ISBN: 0879382678
ISBN13: 9780879382674

There are a fair number of good and not terribly expensive upgrades to be done when one is apart. Changing plastic thrust bearings into needle ones. Putting in a better sprag. Adding more clutches by using different thickness steels (keeping clearances the same). Using a kevlar rear band. And then of course there are hard part upgrades that are nice like a steel front drum (but that’s expensive) or a hardened input shaft. -and probably a lot more from the book I did but forgot about…

The front drum is what failed on mine. The retaining ring teeth broke and let the front clutch pack spill forward.

The NEXT best upgrade that can be done is to figure out WHERE to add a cooler for the transmission. Fluid overheating is a trans killer. I added a Derale cooling pan w/ cooling tubes in it instead of trying to tap into the transmission lines and add a cooler up front somewhere. Made the trans a bit lower, so it more easily caught high speed bumps, but it was also thick steel, so tough. The new style ones come with a drain plug.

And I used Redline ATF (possibly the only synthetic available back then that improved trans engagement like ford type F.) Now there are a lot of reasonable synthetics to choose from.

~Paul K.


This is all good info, as my next plan of attack is to service the tranny and somehow do some upgrades. I’m totally resisting yarding it out for a 5 Speed.
But if I can tighten things up I’m all in. Just need to get some tight crisp shifting and take some of the laziness out of the slush-bucket. (I’m not getting any younger).
I think the Jaguar engineers back in the day knew which transmission to hook up a V-12 to based on torque numbers and reputation.
I’m not quote prepared to throw the TH400 under the bus just yet.

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Well, there was the smoothness thing. And, strength-wise, the 400 was way more than enough to handle the amount of power a V12 would produce.

My understanding (FWIW) is an element in the decision was the the 400 internals could handle the 6500 RPM redline of the V12…for the few who would take it that high. More precisely built, better internal balancing.


My link at beginning explains a bit of what i went through. All relatively easy, and quite inexpensive. Pulling the valve body felt like brain surgery (as it looks like a brain) but being methodical and taking photos saved the day for me.

Kirby was surprised when i realized the ball bearings weren’t the same size. Even the shift kit didn’t mention that.

In the end, I’m quite happy with the improvement it made. No slush box, but no whiplash.