C3 Turbo 400 Transmission leak

[Harmon C.]

I drove my 73 to Galvistion convention in 2001, 6000 miles total and had just a mist on the underbody and needed to add no fluid. I wiped and cleaned everything and found the hose clamps at the radiator were seeping so I took off the orginals and put on new ones. The front seal shows no leak with the front inspection pan removed. I replaced the pan seal and left it on the lift 2 weeks and it seemed to be ok. In storage for 3-4 months it leaks about 1 Quart.Cleaned it up again refilled fluid seemed ok for 2 weeks and big leak again after a couple months. When I spray it down and check for leaks I get 1 drip in a week but long term sitting it leaks alot. Any ideas or simlar cases?

Lyle

[Jim T.]

Re: C3 Turbo 400 Transmission leak

I don't know what pan is on your turbo 400. I changed the fluid on my pan a few times since it was new and about 15 years ago I ordered a new pan from GM to replace my original. I was really impressed with the new prelacement pan, the metal was thicker and using the torque of 11-12 inch pounds on the attachment bolts like I had always done with the previous pan has given me a leak free transmission between the pan and the transmission. This could help yours, for sure it would eliminate one source of leak if you installed a new pan like the replacement I installed.
The other places to check is the front seal, transmission selector shaft seal, speedometer connection, filler tube, rear seal, and vacuum modulator. The vacuum modulator can leak in two places, externally and internally.
I removed my transmission and replaced the front seal. I replaced the transmission selector seal and the speedometer seal with the transmission in the car.

[Harmon C.]

Re: C3 Turbo 400 Transmission leak

I have thought that as it sets for long periods maybe the fluid rises in the transmission and shift lever or dipstick tube could be the problem. I don't know how high the fluid level is when full on the dipstick is it above the dip stick tube where it enters the transmission? I have checked the pan area and feel sure it is ok. Also the front seal.

[Terry M.]

Re: C3 Turbo 400 Transmission leak *TL*

Lyle,
TH400 has a ball that acts as a check valve to hold fluid in the torque convertor - that fluid is the highest point in the transmission. It is common for that ball to fail, or the seat to fail. The result is the fluid drains from the TC, overfills the pan and leaks from the filler tube connection to the case, gear selector input shaft or pan seal.
I have that problem with the "pumpkin" Suburban. I believe that ball is in the valve body, so it might not take a complete R & R to fix it. Want me to ask a real transmission expert?

[Harmon C.]

Re: C3 Turbo 400 Transmission leak

Terry Yes ask the expert. That sounds like it could be my problem because it don't leak when cleaned up and watched for a few days but let it set a month or two and you have a big puddle.

[Terry M.]

Re: C3 Turbo 400 Transmission leak *TL*

It will take a day or two for me to get to him. in the mean time maybe someone here is real familiar with the TH 400.

[Jim T.]

Re: C3 Turbo 400 Transmission leak

If you think this could be the problem, why not try changing the fluid? Do you know when the fluid was last changed?

[Harmon C.]

Re: C3 Turbo 400 Transmission leak

The trans was removed before the restoration for Galiveston Duntov but it was not rebuilt but the fluid was changed twice trying to find the leak which is hard to find cause it doesen't start to leak for a week or two after being cleaned up. As Terry said it may be a drain dow problem.

[Terry M.]

Re: C3 Turbo 400 Transmission leak *TL*

Good suggestion Jim, but it didn't work for my Suburban. Since it only leaks a quart or so, and it is outside on a paver brick driveway - the trans fluid controls the weed growth - I haven't bothered with a fix. I'll ask the transmission class instructor at school where the ball is at, and get back to Lysle. Next term is trans class.....maybe I’ll get around to it, especially now that I have all this time in my retirement.

[Patrick H.]

Re: C3 Turbo 400 Transmission leak

Retirement???