Recognizing TH400 failure

The 400 turbo on my '88 5.3 died a couple of weeks ago.This may help another owner avoid the trouble I just went through.
Out for a drive, approaching an intersection, moving around 55mph, I downshifted into second, and the car slowed down severely, as though I had applied the brakes.Being a Jag. owner, I immediately started thinking about over-run valves, and what could be slowing the car so quickly. The red light changed to green, and off I went.
As the car gained speed, it seemed to me that it shifted from first directly to drive, missing second gear, so I stopped (no traffic) and started up again. Sure enough, no second gear.
I have had transmission failures over the years, but nothing quite like this, and if I had recognized what was going on, I might have been able to save my tranny.
Now I’m moving along around 55 and I decided to see if the car would downshift into second gear. That was mistake no.1. There was no response from a forced kickdown.
Then I shifted manually into second to see what would happen. That was mistake no.2.
The car would not accelerate, and it felt like I had one foot on the gas, and the other on the brake.
Turns out that I was destroying my own transmission. This is the reason why.
Above the valve body inside the transmission is an accumulator piston. As far as I remember, the piston was metal, but at some point it changed and was then made of plastic. Doesn’t matter, really.
With age, the plastic piston can disintegrate- one minute everything is fine, the next you’re in trouble.
When the piston fails, you lose second gear, and consequently, no kickdown. If you don’t recognize what is happening (me) and try to find second gear, doing as I did, you will do so much damage to the tranny, that it will need to be rebuilt. If I had limped the car home, avoiding second gear completely,(flatbed even better) I might have got away with dropping the valve body, and putting in an aluminum piston to replace the plastic part. Too late now, though.
If your car has a 400 turbo transmission, make a mental note of this story- better yet change the accumulator piston BEFORE it fails.
The second part of this adventure is about replacing a torque converter.While the trans. was out of the car(and that’s a whole 'nother story) I thought it would be a good opportunity to put in a “Performance” converter. I had all kinds of trouble with vibration, and I removed it finally, but here are my thoughts on how the converter behaved, on its one and only test drive.
The OE Jag converter stalls at 2000rpm. The torque peak for my 5.3 comes at around 2800 rpm.The converter I bought was advertised to stall at 2500 rpm. This means that starting from a standstill, the 2500 converter allows the engine rpm to reach the point where the power comes on, and the car becomes more “lively”. It will accelerate faster, up to a point.This did take place, and my car became less sluggish, and got going faster.
Now the negative part (IMHO!)
At 60 mph, my engine is running at around 2200 rpm. The stock OE converter is about as locked up as it can be, for a non-lockup unit. With the 2500 converter, at the same speed, the converter is still slipping, (generating heat) and I found that as I went up slight hills, the rpm would increase by 300 or so rpm, and fall back again when the road became level.It was like driving a car with a variable speed transmission, the engine rpm would rise and fall constantly. Even my wife commented that the car sounded noisier.
I liked the way the car responded to the “GO” pedal. I did not care for the constant slippage at 60 mph.
I imagine that at higher speeds the 2500 converter would be ok, but I did not test that. You can decide for yourself .There is a trade-off. Less of the grace, but maybe more pace!
Finally, after removing my transmission twice during this fiasco, I jacked up the car again yesterday morning, removed the shields below the bell-housing, and unbolted the six-lug OE converter, after marking flexplate with yellow paint-stick.It took a while, but I thought I should try bolting converter to flexplate in every possible position known to man. I started the engine after each change, and checked for any vibration,writing down the results. I was surprised to find that indeed there was a “sweet spot” at position #2 where there is zero vibration. So much for everything being “balanced” before assembly.
I wonder what the next experience with this very “interesting” car is going to be…
Almost forgot. I applied high-tech thermal-acoustic stick-on insulation to the underside of the car above the transmission. Not impressed! Found some 2inch thick polyester foam sheet at a local craft store, and put a blanket above the tranny as it was raised into position for (hopefully) the last time.
Much quieter!


Thanks for the write-up Dave. I just found out about the plastic accumulator that resides in my TH400. I will be replacing it within the next two weeks! I only have about 50K miles, so it seems to still work. How many miles did you have when yours went out?

From what I’ve read, I don’t think you could have destroyed your transmission so fast? It seems that as it cracks, it uses 2 gears at once, which slowly wears out the clutch packs. So you may have only noticed it once 2nd gear clutch packs totally went out? For all I know, mine is doing that now. Which means I should replace it ASAP.

I have around 76K miles on the car. Actually, your car and mine share the same MY date- 4/87, and my serial no, like yours, I think, begins with 141.
Just to be clear, I did not rebuild the tranny myself.I know a fellow who owns and operates a repair shop, and we go back 45 years or so. We used to do business when I played around with forklifts and such. He knows the weak spots of just about all transmissions, and is super-reliable.
I got the impression from him that the piston doesn’t “wear out”. When it fails it happens just as I described. Instantly!

Wow, thanks for the info. I can’t get to my valve body until next weekend. I think I won’t drive it until then!

Sorry you had to go through it, but that’s what makes this forum so valuable, all mistakes and defects are shared to help the next guy :slight_smile: You may have just saved me a rebuilt TH400!