Transmission leak

[johnny]

have a look at the modulator and make sure it has the correct O-ring fitted. Jack the coach and crib it safely, then proceed underneath with a roll
of paper towels. Wipe the pan and edge of the castings clean and then return every few hours looking for trans fluid. A leak at the modulator will
run down the edge of the pan and appear on the head of all the pan bolts in the back passenger side of the trans. Tuck a piece of clean paper towel
under the modulator and above the pan lip. If it turns oily red that's where your leak is - either the modulator or the chain cover. A leaking chain
cover gasket normally requires removal of the transmission to replace... but they aren't usually leak sources. Look at the modulator, the
fill/dipstick tube and the pan itself.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased

 

[bob heller]

In the mid 70's my first car, a '65 Mustang, needed an engine replacement due to the idiot owner (me) not checking to see if the cooling system had
the proper concentration of antifreeze. It froze up and cracked the block. A couple of days after installing another 289 the transmission would dump a
substantial quantity of fluid onto the driveway when the car was parked. Overnight wasn't too bad but when left for a few days I would need about a
quart to make it drive again.

Since I had a part time job at a gas station and using their lift, removing the C4 trans was pretty easy. I replaced the front pump seal, reinstalled
the trans, and the leak was fixed. I had the car a few months after that, rust eventually did it in (like most every other older car in New Jersey at
the time).

My guess was that moving the torque converter back and forth just that little bit when replacing the engine buggered up the old front seal and caused
it to leak. If I were to do it all over again I would have replaced the trans front seal while the engine was out. It's not the cost of the seal, it's
the cost of labor if I was to pay someone (I've always done all my own repair work so the labor has been free). That's also why I love the GMC, I can
work on it.

I'm not saying that this is the problem you are having with your GMC, maybe it is and maybe it isn't. This is just what happened to me.
--
Bob Heller
1974 X-Canyonlands 26ft
Original 455 exc for timing chain,
Rockwell intake, valve covers. 145k miles.
Winter Springs FL

 

[david1]

I agree with Johnny you need to identify where the leak is coming from without those facts you'll be chasing several rabbits down their holes. clean
everything off inspect regularly you'll be able to determine the source of the leak and from there your diagnosis will be easier.

-Dave
--
'78 Palm Beach car nut & new to RV'ing
26 ft. 403 OEM wheels, 2 ACs, Pod,Brady bunch stock

 

[LQQKatJon]

I am not sure what sirum did, or how well he did the work.

These old transmissions can have interal parts wearing and that can put fluid/pressure where it does not belong.

The main leak points are the dip stick tube(easy). Modulator(easy). Take pan gasket down to seal that along with the orings on kickdown and shifter
shaft.

If it is not those item, you will
Most probably need a rebuild.

--
Jon Roche
75 palm beach
St. Cloud, MN
http://lqqkatjon.blogspot.com/

 

[jim kanomata1]

Rebuilding a transmission does not assure no leak.
A person that is familier with these old units and have been working around
will know where the leaks are coming from.
I can tell you where those places are as I use to crawell around these old
units.
Majority of trans shops are not aware and replace only the parts that come
in the Kit.
I know that Jeff Sirum knows and he convays that to his techs. We do the
same, but there is so much one can do to avoid leaks.

> I am not sure what sirum did, or how well he did the work.
>
> These old transmissions can have interal parts wearing and that can put
> fluid/pressure where it does not belong.
>
> The main leak points are the dip stick tube(easy). Modulator(easy). Take
> pan gasket down to seal that along with the orings on kickdown and shifter
> shaft.
>
> If it is not those item, you will
> Most probably need a rebuild.
>
>
> --
> Jon Roche
> 75 palm beach
> St. Cloud, MN
> http://lqqkatjon.blogspot.com/
>
> _______________________________________________
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>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502

 

[LQQKatJon]

I dont care if it is engine, tranny or interior. I can agree with Jim on the need of someone that knows.

https://youtu.be/T3DAPmb0UnU

Video of my th425 that drove just fine. But obviosly was going to have some issues. And leaked.

I know of 4 transmissions that had the same issue. Drove fine, but leaked because of this bearing/input shaft issue. One of those 4, he had spent a
whole summer trying to figure out why it leaked. It only have him problems on long hot runs. Never leaked around town.

--
Jon Roche
75 palm beach
St. Cloud, MN
http://lqqkatjon.blogspot.com/

 

[Ken H.]

In 2000, returning from Alaska, the GMC developed a terrible ATF leak 6
miles west of Sheridan, WY. Eventually, I wound up in touch with Darrel
Winterfeld in Longmont, CO. After I arranged a tow from Cheyenne to his
shop (his home driveway), he told me to pick up an input shaft seal ($5.28)
since that "had" to be the problem.

After he dropped the transmission and pulled the seal, he said it looked
OK. So I hit the output shaft of the torque converter with a mallet -- a
ring of ATF appeared ALL THE WAY around where that shaft was welded into
the torus! Only the jagged edges of the crack meshing together enabled the
coach to move while trailing a cloud of red mist.

A new torque converter and that new seal got us back on the road with no
more problems.

Darrel said if he'd had decent help (me) it would have taken less than
those 5 hours to do the job.

Not a frequent failure, but it can happen!

Ken H.

> I dont care if it is engine, tranny or interior. I can agree with Jim
> on the need of someone that knows.
>
> https://youtu.be/T3DAPmb0UnU
>
> Video of my th425 that drove just fine. But obviosly was going to have
> some issues. And leaked.
>
> I know of 4 transmissions that had the same issue. Drove fine, but leaked
> because of this bearing/input shaft issue. One of those 4, he had spent a
> whole summer trying to figure out why it leaked. It only have him
> problems on long hot runs. Never leaked around town.
>
>
>
>
> --
> Jon Roche
> 75 palm beach
> St. Cloud, MN
> http://lqqkatjon.blogspot.com/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[LarryW]

I wrestled with a leak for about a year. Main pooling spot was in the valley below where the cooler lines go into the trans. As the pics in the
following link show, a spot right next to one of the trans line inputs was what looked like the end of a bolt hole. Turns out it is a locator pin for
a valve body under the large chain cover pan. What I could see was the end of the pin. I checked two other GMC transmissions that I have and neither
have that locator pin showing through. Anyway, I cleaned the hole with brake clean, roughed it up with a diamond bit on a rotary tool, and applied
some JB Weld. Leak gone!! So here's some pics to show the odd hole.

Oddest thing I've encountered. Haven't seen it on any other 425 trans.

http://www.gmcmhphotos.com/photos/g7115-transmission-leak.html

Maybe on yours ??
--
Larry
78 Royale w/500 Caddy
Menomonie, WI.

 

[Digger]

How about the vent on the top of the tranny, is it clear, My new transmission from Manny didnt have the cap removed after installation and kept losing
transmission fluid out the fill tube, massive amounts. Called Manny and he asked me if we removed the cap he puts on them for shipping purposes.
Duh. We also put a new seal on the final drive only to find out that the other seal's vent hole was covered up and not venting properly. So for me,
it was a couple of issues. I'm not very mechanical but I hope this helps.