Problem in GM400 Gearbox

Dear all,

After the help of this forum, for which I am eternally grateful, my engine of Jaguar is now working properly. However, unfortunately a new problem has appeared with the gearbox.
Following the recommendations of the forum I changed the filter of the gearbox and put a new atf, but I saw that the oil that I had appreciated totally black and with a burnt smell.

The question is that the car works well with the R, manually putting the 1 and 2. However, it does not work well with the D, the car has no power and does not move.
I understand that it is because the gearbox is bad, does anyone know if changing the torque converter could solve it or should I take the gearbox to fix it all?

Greetings and thank you very much for your help, I am very grateful.

As not transmission expert, I’m guessing that puppy is pooched and i would suggest going to a transmission expert who can do a tear down and let you know what all the issues are.
If you have a transmission with a lot of issues I would recommend just going to the experts and get a quote for a rebuild.

Black fluid means burnt fluid which means burned out clutch packs. Your TH400 is toast, I’m afraid, needs a rebuild.

What happens sometimes when you remove the old burnt fluid and put in new clean fluid, it doesn’t work anymore because the junk in the old fluid was actually helping give some friction to the clutches.

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Remember that the GM400 in the XJ-S is unique and not interchangeable with one from a Chevy. That means it shouldn’t be difficult to find someone who knows how to rebuild it, but it can be challenging finding a replacement.

Rebuilding TH400 is supposedly quite easy. The trick is finding someone with a lot of experience who isn’t retired.

I agree with others assessment that your TH400 is likely toast.

Having just had mine rebuilt, my technique when I was shopping around for a shop to do the work; I told them it was a TH400 but did not mention it was from a Jag until a little later in the conversation. I found a decidedly ‘old school’ shop locally that did great work. Not much to look at on the outside, but years and years of experience on the inside.

As a side note I did the removal and re-installation, delivering and picking up the transmission. I don’t trust anybody to work my XJ-S, but auto transmission are a magic black box (to me anyways!), expert experience required.

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Yes, this is what I’d worry about too…I believe they’d never figure out how to get the transmission mount back together correctly, or would take one look and say “nope, can’t do this job”.

At least the TH400 is easy/cheap. I’ve seen rebuilding go as low as $800. I’m trying to source a rebuilder for my FWD Aisin transmission in my Volvo, and $3000 is about the cheapest I’ve found! Ouch.

Not sure how to get your description. You’ve mentioned dark, smelly oil - have you seen any debris in the sump pan? What’s the mileage? Where are you based?

TH400 is bulk and heavy, it was used in Humvee with much greater fatigue and torque than V12. Don’t write it off just yet.

If TC is trashed - you would hear it, it wouldn’t work with any gear settings.

You’re saying that car has no power in D - please explain. Is it moving or not moving with engine choked? What happens when depressing acc pedal? How hoig the revs are going? Have you tried to raise rear wheels and check if wheels are moving on N and on D?

We need more details.

From here, the evidence is damning. The TH400 has succombed to lack of care.

Rebuild or replace.

It is toast.


You should work as an expert in these days workshops. This way you could benefit every workshops with the excessive bills for the customers.

Josep mentioned that the tranny works in 1,2 and R position. That indicates more likely a number of four things

  1. A-clutch friction plated gone = debris in the pan or possible no debris with extremely high mileage or valve block clogges with crap if oil really dirty. With dirty oil piston seals will go first.
  2. One-way clutch of the 1st gear is damaged (not sure about TH400 having it)
  3. Wrong setting on gear selector or worn mechanism
  4. Oil itself and oil filter. Low level will give you no drive at all.

Worth starting from easiest checks / reanimation like fresh oil and filter, check for debris and slight overfill.


  1. Josep swapped the filter and oil.

I agree. Easy first.

1 & 2 in your list meant R&R. Replace ort rebuild the wise course here.

A pro shop can not afforsd to do the 1 & 2 and stand behind the work. When other parts fail, as they are likely to do, they get the blame.

Kinda like the old tech ring and valve job are seldom done by a ppro shop. Risky.,

My Jeep lost low. a solenoid pack may or may not fix it. I can no longger get down and under. A pro shop would probably not be willing to chance it on my high mileage truck



Th400 is the easiest first transmission rebuild. I’ve done two. The second went much faster, and both are still working.

Get a copy of How to Rebuild & Modify GM Turbo 400 Transmissions by Cliff Ruggles. It’s full color and down the bolt detail level instructions.

Skip the “modify” part and just do the straight up rebuild. Shift kits and like don’t match the personality of the XJS

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Dear, Thank you all for your help.

The first thing I did was change the oil and filter but the oil came out very dark and with a burnt smell.

When I put the car in D, without giving gas the car moves little forward (so I understand that the gear enters). However, when I accelerate, the car does not move from the place even if I put it at 5,000 revolutions.
It only works with R, 1 and 2.
With D, it’s as if the clutch is worn out.
I see you comment that fixing that gearbox is relatively easy. However, I understand that these gearboxes are common in the U.S. but not in Spain, so I’m going to ask a place that repairs boxes to see how much it might cost me.

Thank you very much for your help.

Is there any chance that the cable is mis- adjusted? D & N are close to each other, and it almost sounds like the transmission is N when you believe it is in D. Just a thought, I don’t know anything about adjusting these cables.

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Well, there are a couple of mods worth considering. One is to replace that pesky plastic piston with an aluminum one. And I think there may be some upgraded “sprags” available, supposedly a bit tougher than OEM.

That’s because you’ve changed only quarter of oil in your gearbox. Do it effectively with cheapest ATF available 2-3 tines to see the difference. Run the engine with brake depressed and gearstick in every position for a minute or two - between fluid changes. As that wasn’t obvious - how did you checked fluid level after change? You are aware that level check must be made on hot engine (just touch the pan of your gearbox) with engine running?
Low oil level will give you same symptoms.
If convenient - raise the rear wheels and do what’s above. Your wheels should not turn on neutral, should spin in every other position.

Hi XJSBanger,

Thanks for your help. Does it mean that it may be because there is still bad oil? I mean, if I do a couple more changes is it possible that the problem will be solved once all the fluid has been changed?

Best regards.

Yes, the oil was checked after 5 minutes with the car started, I understand that it is enough? And the level was at the maximum.
It is curious that with the R it works correctly, but with the D it does not even move.

Best regards

A lot of the tranny oil is in the torque converter, and since it doesn’t have a drain plug, it doesn’t get changed in one oil change. If the tranny oil was really burned, it will take several changes to get fresh fluid everywhere.

Also, make sure the filter is properly seated and seals are in place. Be sure to change it again when you change the fluid.

You’re just wasting fluid doing any more flushes. You need a transmission rebuild.

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