changing transmission fluid, filter, gasket, etc

[Cape Cod Fleetwood]

This is part of my planned mntx for the car since I received no records at all with it.
Change out the tranny fluid, new filter, new gasket, clean the pan, etc.

Its on my 'list' of mechanical items to be done in the shop... that I showed to 'dad' today. 'Dad' is Gary Amster, Mike's father, who owns Hyannis Vintage Auto... and has been restoring cars professionally for 50 years.

"Why do you want to do that?" Because I have no mntx records with the car, I don't know when it was done last.
"Is it shifting well?" I only drove it twice, it seems fine.
"Is the fluid burnt?" No, its pink, smells sweet.
"Leave it alone". But -
"And stop biting your nails." But <laugh>- I don't know when it was done last.
"Sometimes when you do that to an old tranny that's working fine, you dislodge some gunk and suddenly you
could have shift problems..." But - its a mntx thing, its a Naval Aviation thing, I really want to change the fluid,
and the filter, see the pan, new gasket, see if there's any signs of metal in the pan...
"Its your car, I don't recommend it." <sigh>

How can I even breath the air in the same room with this guy... he's been doing this forever, I've owned a
classic for 90 days.

I really want to change the fluid, the fluid is in such good condition I have to believe this service was done in the last few years. I just don't know when. And the pan was pretty weepy last time I saw it on a lift. I really want to do this. But I've got a bona fide Car God, someone who prepares cars for Mecum, Barret-Jackson, Concours, who does work for guys named Chambers and Leno, telling me not to.

What to do?
Where are my 1970 Caddy folks here?
\m/
Laurie?

[TJ Hopland]

Changing the fluid on a auto trans that is in good condition isn't going to cause it to fail, it will actually extend its life. 

Changing the fluid on a neglected, abused, or tired auto trans may make it fail within miles of the change but it would be failing soon anyway even if you didn't change it.   I have had that happen more than once.   Both times I ended up at a different shop than did the service for the rebuild and both times they said the trans was just wore out and if it was working normal recently I had just been lucky.   

I would do it.   When is the question.   Are you prepared and able to spend the money on a rebuild now if it comes to that?   Would 6 months from now be better?   Maybe by then you will have a better idea how much you like the car and will feel better about spending that much money?    If 6 months will put you in the middle of your cross country tour that may not work.   

A good thing about your transmission THM400 is its still well supported with parts and people that know how to build em.  Its also not known for self destruction of hard parts from normal driving.   Usually when they fail it doesn't take much more than a standard parts kit to get em running again.   Same can't be said for earlier and later models.  There are things in the earlier models that can self destruct and can be very difficult / expensive find replacements as well as someone to install them.   

[Cape Cod Fleetwood]

Great points, thanks TJ.
\m/
Laurie

[Jeff Rosansky CLC #28373]

I am in the same boat as you. Fluid looked good but have no idea when it was done. If it makes you feel any better (and without jinxing myself) we have put about 9K on the 70. It always appears a little low on the dipstick but it leaks if I top it off.
For what it's worth anyway.
Jeff

[35-709]

A good transmission shop can do a complete fluid change, without dropping the pan, through the transmission oil cooler lines.  The shop that is doing your work may or may not have the equipment to do this service unless they do a lot of tranny work.  As I remember, the trans. shop I go to charges about $100-$125 for this service.

What it will NOT get you is a new trans. gasket seal to replace the "weepy" seal or a new filter, which, if the fluid is pink, you probably don't need anyway and it won't get you a look-see at the inside of the pan --- you WILL find some residue in there.  Having said all that, if it were my special car, I'd be dropping that pan just for that feel good feeling in a heartbeat.   

[TJ Hopland]

Having a look in the pan can be a very valuable tool to guessing the condition of things.  Someone who sees them all the time can likely guess when it was last apart by what the filter and gasket look like and the brands. 

Also a chance to make sure the filter is properly installed.   I don't remember if the 400 is one of them but sometimes there are gaskets or O rings that get missed or doubled up and cause air to get sucked in which isn't a good thing long term. 

[Scot Minesinger]

The transmission casing is metal so the only place it can leak is the seals, and you can easily reseal the trans in a night.  The trans seal kit is $50 or less.  In order of what always leaks (my experience); Reseal inner and outer drive shaft seals (have to remove drive shaft and tail), reseal electrical connector, reseal pan, replace speedo cable o-ring, reseal turbo hydramatic modulator, reseal governor, reseal dip stick, and reseal trans shifter link.  While your at it replace trans mount.

The pan may have suffered bolts over tighten previously (to stop leaks - never works), so you will probably need to straighten it before installing.  Carefully follow torque instructions in shop manual, when re-installing.

You can use a vacuum canister to remove majority of fluid and then drop the pan - makes cleaning easier.

I have to do this job o a 1965 Cadillac next week.

[bcroe]

I am in the camp that wants to get the pan off occasionally and
see what is in it.  And get a new filter on, a plugged up filter has
caused me grief.  A drain plug really eases the job, a TH400 does
not come with one, but all of mine have a drain. 

Reworking the external seals might help keep your floor clean, but
it will do nothing to preserve your trans.  The seals on the pistons
that engage the clutches are the ones that really matter, but the
trans must come completely apart to change them.  I recommend
changing the seals every couple decades, low miles may allow
using the same (expensive) hard parts a very long time.  Easy
for me to say, who does his own transmissions.  Bruce Roe

[Cape Cod Fleetwood]

Thanks guys, I appreciate all the input.

I'm changing the fluid, dropping the pan, already have the new fluid here, new gasket and new filter, all
AC Delco.

I remember how pretty and green the radiator looked when I took off the cap. Then I drained it and flushed it, all brown under the green and tons of crud. And let's not forget the mouse in the reservoir.

Tighten pan to 12ftlbs and check for warpage.

Just read up in the shop manual - seems the intake pipe should be changed as well? This part didn't come with  the filter, just the O ring. Should I order one specific to the car or does the one Autozone sells work?

Bruce? Scot?


You guys RAWK!
\m/
Laurie?

[bcroe]

Just read up in the shop manual - seems the intake pipe should be changed as well? This part didn't come with  the filter, just the O ring. Should I order one specific to the car or does the one Autozone sells work?    Bruce? Scot?
You guys RAWK!
\m/
Laurie?   

The tube to the filter is metal on earlier TH400s, which never needs
replacement.  Later ones were plastic, I would reuse it if there is no
obvious fault.  However I don't have direct experience with the
plastic over all this time, managed to find enough metal tubes in the
junk yard for my cars.  Bruce Roe

[Cape Cod Fleetwood]

The tube to the filter is metal on earlier TH400s, which never needs
replacement.  Later ones were plastic, I would reuse it if there is no
obvious fault.  However I don't have direct experience with the
plastic over all this time, managed to find enough metal tubes in the
junk yard for my cars.  Bruce Roe

Thanks Bruce
\m/
Laurie

[Jeff Rosansky CLC #28373]

Easy
for me to say, who does his own transmissions.

I am envious of that. I have done nothing with a transmission (never even changed fluid) and I know less than what I have done.
Jeff

[bcroe]

I am envious of that. I have done nothing with a transmission (never
even changed fluid) and I know less than what I have done.  Jeff   

Jeff, The TH400 is probably one of the easiest automatics to work on, partly
because of the straight forward design, and partly because its so rugged
that very few parts will require service.  The bean counters must have
been on vacation during the design.  There is none of that engine black
magic, like boring, balancing, break in procedure, etc, etc.  I wonder why
everyone is happy working on an engine but not the trans?   Gorilla tactics
won't go well, and like any job a few special tools and tricks help a lot.  A
lift makes it pretty simple, but I did transmissions for decades before
getting a lift.  Bruce Roe

[cadillacmike68]

The other beauty of the TH400 is that it requires so little in regular servicing. Fluid and seal changes are all that's ever really needed, plus the occasional filter.

I wish they had made the TH400 in a 4 speed overdrive instead off the 700R4. That would solved / prevented a LOT of problems back in the late 70s - early 80s.

As far as changing I would do a fluid exchange using the transmission's pumps and disconnecting the fittings at the radiator cooling tank, then drop the pad, keeping the fluid in something clean, change the filter and pan gasket, put it back together and refill the pan with what was just drained from the pan.

That way you get all the old fluid out and filtered through the old filter which will be getting tossed and you don't lose any fresh fluid except maybe a few ounces due to spillage.

You should be able to use the newest Dexron XI but if you feel better you can use the old Dexron III. I'm going to convert mine over to Dex XI this spring.

[TJ Hopland]

They did come out with the 4L80e in 91.  I don't know if it went into any cars.   There was the 4T80e that went into the supercharged 3800 cars and I believe the Northstar Cads.    Those seemed to be pretty durable units unless you compared them to the Allison.     

[bcroe]

They did come out with the 4L80e in 91.  I don't know if it went into any cars.   There was the 4T80e that went into the supercharged 3800 cars and I believe the Northstar Cads.    Those seemed to be pretty durable units unless you compared them to the Allison.   

That trans is directly based on TH400 parts for ruggedness.  But its
not a bolt in, quite a conversion process for a 3 speed vehicle. 
Bruce Roe

[Jeff Rosansky CLC #28373]

When I finally get tired of the drips, I know would to call for help. I think both the 70 and 55 need a reseal. But, that's what cardboard is for.
Jeff

[cadillacmike68]

When I finally get tired of the drips, I know would to call for help. I think both the 70 and 55 need a reseal. But, that's what cardboard is for.
Jeff

I keep a stack of cardboard on the side of the garage. 

Actually, i think all the leaks are now fixed - finally.