TH425 Problem

[Mario0804]

Hello all.
I am new to the forum so sorry if something goes wrong.
     I am asking for help because my Caddy 71 with TH425 transmission only goes backwards. There is no reaction to any forward gear. Do I need to regenerate my TH425? Is the Master repair kit enough? Maybe it's some other simple matter?
Thank you all in advance for helping me with my problem.
Greetings to all.

PS I apologize for mistakes with the help of an interpreter

[Cadman-iac]

I had that problem with a 69 Deville convertible I had years ago. It turned out to be the accumulator piston in the front pump assembly was stuck.
It started out as just a extremely soft shift from second to third if memory serves correctly.  Or maybe it was first to second, but regardless,  it soon stopped shifting altogether and would only move in reverse.
Being a novice at transmission repair at the time,  I swapped it out for one out of another 69 parts car I had.
But a friend of mine had told me that was the issue after looking at it and pulling the pan for inspection.
I had driven it to his shop, and afterwards half way home before it quit on me. I wasn't going to leave it sitting on the side of the road at midnight,  so I put it in reverse and drove the rest of the way home. Thankfully that was only a few miles on country roads.
However, yours may not have the same problem. If you're not inclined to do your own rebuild,  you might want to have a reputable shop look into it for you. Being an Eldorado,  I'm not sure just how hard it will be to remove the transmission. Mine was very straight forward.

For what it's worth,

Rick

[79 Eldorado]

Witam Mariusz,
I'm sure someone with more direct TH425 experience will also answer. I have the TH325. Yours is generally thought of as being better than mine. That said there are a few things you may want to check:
1) Fluid level! - That should be done when fully warmed up but if you check and it's very low after just starting it's still low. I don't know however whether or not it would still go into reverse.
2) Fluid pressures. Normally there is a small port with a solid plug. I think it's normally 1/8" NPT. That's a port where you can check all of the pressures in each gear.
3) Linkage position. Is the linkage actually indexing through the proper positions according to your gear selector. You don't need to have the vehicle running to check this.
4) Did you recently service the transmission? I started having delayed response into gear with my TH325. I serviced it and changed the filter. I used a FRAM filter and not the original. A long story short after 3 times pulling the pan my neighbor figured out that the FRAM check valve was a lot stiffer than the original. I bought a GM original and the check valve easily opened with light suction. The car would go into gear with the original and the GM replacement but not the FRAM filter.
5) If you changed the filter recently did you make certain you removed the original o-ring and did you remember to install the replacement?

Generally speaking Transgo shift improver kits are well respected for older transmissions. They generally try to address common issues. They are not the typical shift kits which people use to give harsher shifts. They are more about improvement according to what the factory probably should have done originally (my impression at least).

One other caution: Make certain you use the proper transmission fluid. Some of the new ones are not reverse compatible as I understand.

Pozdrawiam,
Scott
PS: Assumed that you are likely from Poland by your first name. Your second name is also the same as one I remember from the City I was living in Poland.

[TJ Hopland]

What is the recent history of this car?  Is it new to you and you don't know any history?  And it was like this when you got it?

If you don't know its history and you are willing and able to get it rebuilt you might as well do it.  Also a good time to pull the engine too and do the timing chain and some new gaskets and seals if its likely all original too.

If you just had a service done then I would take the pan off and make sure its the correct filter and it was installed correctly.

[bcroe]

Have said so many times before, every couple decades a trans
needs to come apart and get a (cheap) internal seal kit refresh,
cause rubber ages.  If it is low miles that may be all it needs. 
Drive it with a fault, and risk major (expensive) damage. 

You can get a TH425 rebuild kit, but as with a TH400, it will
not contain everything needed.  I would be inclined to check
the damage first, then get what is needed.  I usually leave all
the original hard parts, which are so tough as to infrequently
need replacement, and may be superior to replacements.  All
seals need replacement and the Torrington thrust bearing set. 
Do not be surprised that a kit has only 10 smaller clutch
frictions, but your car will need 11.  Finally, do not try to use
an original cork gasket to replace the chain cover (is it already
leaking?).  Some owners glue them back on with VALCO for
leak free. 

I figure that a trans either needs to come out or not, find out the
details on the bench.  However that can mean pulling the engine
as well for a TH425 (not such a big deal).  Good time to do the
stuff others recommended above. 

Just parked my TH425 which works great, having been refreshed.
good luck, Bruce Roe

[Mario0804]

Thank you everyone for the advice. Yes, the car is in my hands recently, I do not know its history. There are many empty oil bottles in the trunk, and after a few days of standing there is a large stain under the car. I unscrew the crates and I will definitely seal it by replacing the tape and discs. But before I do it, I will check everything according to your instructions, maybe even the first to seal the pans and change the filter and new oil, then see if it helps. Yes I am from Poland, as a colleague noted well. But in Poland it is worth having a piece of America, I hope that I can fix it and enjoy the ride. I am fascinated by American motorization but I am dealing with Cadillac for the first time. Until now, like most Europeans, I have been to Mopar. Now, despite everything that I still have Chrysler 300 68 'I had the opportunity to buy this Caddy and I will have the opportunity to learn it. The repair book has already arrived from the US.
Best regards from Poland

[Mario0804]

Welcome back.
If I have to disassemble Th425 gearboxes, do I have to pull out the engine? Will I be able to disassemble without pulling out the engine?
Greetings

[The Tassie Devil(le)]

G'day Mariusz,

The Transmission can be removed and this is described in the Shop Manual, and therefore the Hood (Bonnet) can be left in place.

But, I have found that it is easier to remove the engine and transmission as a unit, and then separate the Trans Final Drive from the engine.   There are a couple of tricks in performing the task which involve removing the left side exhaust manifold, which allows the whole unit to slightly twist, and clear the Windscreen Wiper motor.

One of the really difficult parts of removing the engine, or transmission on their own, is removal of the front cover plate that hides access to the torque Converter fastenings.   This piece of tin plate is very awkward to get to, and involves getting ones' fingers into a very tight space to get to a small bolt.   There is a part of the piece that has to be hinged downwards, or it will never come out.

I have lots of pictures

Bruce.

[bcroe]

A shop may be able to remove the trans from underneath, which
involves suspending and moving forward the engine and the final
drive.  I prefer to do the job from the top.  Rather than risk
damage trying to maneuver out the very heavy engine/trans/final
drive package, I pull the engine out first.  Then it is not difficult
to pull the trans out, with the final drive, or support the final drive
and leave it in place.  your choice

There are a couple tricks to disconnecting the flex plate, read the
factory manual.  good luck, Bruce Roe

[Mario0804]

Thanks a lot, I will try