I recently bought a 1975 Corvette with the base 350 and a turbo 400 transmission. When I bought the car the guy who sold it to me told me that he thinks that the transmission needs rebuilt. I drove it home and did not notice anything wrong with the transmission on the drive, it shifts hard, doesn't grind or make any strange noises, and seems to be working great to me. The issue is, after sitting for two weeks it has literally leaked all of the transmission fluid out. There are not any visible punctures in the pan...that I could see with a quick once over anyways. I talked to the guy who sold me the car again and he told me that the leaking is why is thinks it needs to be rebuilt. My question is, since I am new to this whole thing and am not too familiar with automatic trannys, does it sound like a problem that requires the transmission to be rebuilt, or could it be something like the pan gasket is completely trashed or there is a hole in the pan somewhere?

Sorry if this seems like a newb question, but I just do not know much about this stuff yet.

 

Could you see where it looked like the fluid was coming out? Pan gasket's usually go out on these.

 

First, you need to find the leak. You probably don't need to rebuild a transmission for a simple leak.

Some problem areas can be the transmission pan gasket and the front pump seal. You can determine if it is the pan gasket by cleaning it real good and then watching it to see if it is leaking out.

The front pump seal cannot be changed without pulling either the engine or the transmission because it is behind the torque converter.

 

Mine is leaking out the speedo cable. I replaced the seal, but the plastic shaft has a groove worn into it so it needs to be replaced.

Check the pan, and the vacuum modulator.

 

Wow, thanks for the quick responses. At least I have a few things to look at. If I have to pull the engine to find the leak it will have to wait a while. I was having a hard time believing that a great working tranny needs to be rebuilt to fix a leak.

 

Cheak the real seal also. you will not have to pull the tranny for this. Just disconnecting the driveshaft. All the seals are cheap, just a pain to replace. My bet it is the pan gasket. Put it on jacks, clean the everything under the tranny then start by adding small amounts of fluid until you find the spot where it is leaking. good luck

 

Since it didn't puke out everywhere while driving it,,front seal is probably OK.

Look closely at selector shaft seal on left side of trans, the speedo adapter..it has an O ring under it, the governor cover, the vacuum modulator (pull vacuum line and make sure it's not sucking fluid into engine..it should not have any fluid in the line), the electrical plug for the downshift wire, the dipstick and of course the pan and front and rear seals.


JIM

 

The TH400 (aka M40) tranny is a really stout unit, and if kept full of fluid, will easily run several hundred thousand miles before a rebuild is needed. I taught the M40 rebuild class at the GM Training Center, and I traded my 4-spd M21 '69 GTO for an M40 GTO once I "got into" the 400 tranny.

Pull the tranny dipstick and smell it. If the fluid is brown and smells burnt & sour, the tranny will need a rebuild. Often, leaky trannys have been run low on fluid and will burn the clutches.

If the fluid is bright red and does not have a nasty burnt smell to it, you just need to replace some gasket and seals.

Problem with tranny fluid is that it's like blood: It just smears everywhere, and it's difficult to tell where it's coming from. You need to get under the car and wipe the tranny down really well. Once you have it wiped down, get a roll of paper towell and some tape. Tape the paper towel around every seal, gasket, and penetration in the tranny. i.e., Run a string of towell around the pan gasket. Then, wad up and tape some towell around the speedo cable outlet. Do the same at the modulator, governor cover, etc. Jam one in under the front pump/front seal. Wrap one around the kickdown switch and the shift lever. Whichever one turns red first is the leaky area. Simply replace that seal, and you're good to go!

 

Mine leaks there too.

 

...don't forget too this car being an automatic it has transmission cooler lines going to the radiator, which, on the metal part of the lines they can be starting to rust out or pinholes developing. Also, the short rubber line connections at the radiator(top cooler line & bottom cooler line)are prone to leaking, especially if the car has the original "D" style side screw clamps.

 

Jim and Lars are right. These are tough transmissions if kept full. Leaks are the death of them. The best way to nail down the leak is to get the car in the air (safely!) and go after the transmission case with several cans of Brakleen or equivalent (Autozone's stuff works great for me). Make sure you are in a well ventilated area when spraying the transmission case clean and cover the floor with something to catch the mess.

When I first got my '74, my transmission would leak better than a quart of fluid out in 24 hours. I jacked it up, degreased it with Brakleen, let it dry off for an hour and came back to look it over. I had fluid coming from the selector shaft, the speedometer housing seal (there are two of 'em), the vacuum modulator valve o-ring, and the transmission pan gasket (very common - cork sealing a sheet metal pan - jeez Chevy, even I saw that one coming). It took me a few hours after gathering the parts (an independent trans shop can be your friend) to pull the items apart and replace the seals and I've never lost another drop. Of course, the car hasn't moved yet.....

I suggest pushing two seals into the speedometer gear shaft seal (the smaller of the two), since the gears are plastic and typically the seal will wear a groove into the shaft. Two seals means that one of them will ride on a good part of the gear.

The selector shaft can be intimidating if you've never done one. You're likely going to replace the pan gasket anyway, and you'll have the pan off to do it, so I'd go ahead and replace this one since it's the easiest time to get it done. They will leak, it's just a question of when.

Properly set up, a THM-400 is a great transmission.

Steve

 

It is leaking out directly under the transmission. It does not appear to leak hardly at all until it sits and cools and then all hell breaks loose. I drove it 100 miles and the fluid level looked good the entire trip, and it was still red and happy. The person that I bought it from said it did not leak until he let it sit in his garage for about a year, and then one day he went out to the car and there was all of the tranny fluid on the ground. He drove the car maybe 500 miles since he noticed the tranny fluid leak (I can actually confirm that since I know the guy very well), and he filled it up every time before he left (the case of ATF in the back confirms that). The odd thing to me is that it is leaking itself completely empty, to the point that the dipstick maybe has a tiny drop of fluid on the very tip. Unfortunately I have college finals all week, but I am going to try the paper towel thing this weekend, and see if I am able to find the mystery leak.

While on the subject of leaks, is it pretty common for the rear main seal to leak on these cars? Mine drops a pool about the size of a quarter (or about 1 inch in diameter) on the ground after two weeks of sitting. Is this a cause for concern, ie. replace the rear main seal, or is this just a common thing with these engines?

 

When a th400 sits a long time, the fluid in the converter will dump back into the pan, speedo cable and dip stick are the most likely culprits for leaks. I bet dip stick o-ring.

 

The rear main will need to be done eventually as it will get worse just not as big of a problem as your tranny right now. Just did mine. You're not leaking much fluid if there's still some on the dipstick. I can't remember how many quarts these things hold, but when hot, it takes a pint to raise it from empty on dipstick to full.

 

3 to 3 1/2 gallons to fill it from bone dry

 

I just went thru this with my '75. I replaced the modulator "o" ring and pan gasket. I found that a cork gasket would squeeze out if it was over torqued. I think the proper torq is about 12 inch pounds. I bought a rebber or neoprene gasket and torqued it to 12 inch pounds and that solved the leak. It could be any or all of the answers you received so far but being that the pan gasket is so big, the odds lean toward a bad pan gasket in my opinion. Good Luck

 

My 400 leaks out of the front seal if I let it sit for over a week. If I crank it up every week or so, it does't leak.. much anyways.

 

Mine too!