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Old 05-02-2022, 09:42 PM   #1
ThreeQuarter
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TH400 Questions

1. If I pull the slip yoke out of my TH400's 2wd tail housing, should I lose a little fluid?
- I'm parked on a level slab and my pickup is at factory height & stance.
- I pulled my slip yoke and only got a few drips from the yoke, nothing flowed over the rear seal.
- Dipstick has been consistently reading full when hot. I think I am using the right dipstick and tube combo, but how can I know for sure?
- The transmission seems to have been driving fine.

2. My slip yoke was new & shiny when I installed it, and after 2000 miles in my transmission it now has rust around the end. I think that's not normal--what could be happening here?
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Old 05-03-2022, 08:05 AM   #2
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Re: TH400 Questions

That doesn't seem right. I would question the dip stick too although while driving there should be fluid flying around everywhere. For peace of mind I would change the trans fluid and check the position of the dipstick.
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Old 05-03-2022, 06:49 PM   #3
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Re: TH400 Questions

I am at loss, on the rust.... agree with what cadillac Al said, but will add to the mix. I would polish the stub with a crokus cloth & replace the rear seal....(the rust could have torn that seal on the pull out. I dont know why there was rust on that slip yolk......did the truck sit a lot? Longhorn
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Old 05-03-2022, 08:05 PM   #4
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Re: TH400 Questions

Thanks, Al. I’ve found plenty of people of the internet that discuss dipstick length (heh) but that is dependent on having the right tube length to go with it. Would you happen to know the correct reference point inside the pan/case where the crosshatch should land?

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...did the truck sit a lot? Longhorn
It does sit for a month at a time in the winter, but my shop is very dry all year round—I don’t open it up much and try to manage down the humidity. My welding table has hardly started to rust yet and it’s 5 years old. Slip yoke is 2yrs old.
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Old 05-03-2022, 10:32 PM   #5
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Re: TH400 Questions

What do the splines on the trans look like? Same for the yoke? If it is running dry enough to rust, might be getting some wear on the splines.

One way to check the dipstick calibration is to do a fluid/filter change. Add in one quart less then the amount needed. See where the level is on the stick with things warm. Should show low. Add in 1/2 quart. Should be close to the bottom of the hash marks on the dip stick. Add the other 1/2 quart. Should show full.
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Old 05-03-2022, 11:01 PM   #6
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Re: TH400 Questions

Good idea on the seal replacement. I don’t have any crocus cloth, how does that compare to the abrasive pads I use to clean copper pipe before I solder it? I think those are the least coarse option I have on hand. I don’t mind buying something locally, but it looks like I’d have to order crocus cloth online and wait for it to arrive.

The spline and yoke mate up very nicely—no play or wear at all.

I think I will change the trans fluid. Seems like there must be a level inside the case where all TH400s have to be filled to—I wonder what that is so I can see if my dipstick is at the same level.
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Old 05-04-2022, 07:31 AM   #7
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Re: TH400 Questions

The best way to check your dip stick length is to pull the pan off, then mark your stick even with, or just slightly above the pan rail on the case. You'd be surprised how many are off. When my trans guy re did the 350 in my camaro we checked it, was about half a quart off.
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Old 05-04-2022, 01:08 PM   #8
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Re: TH400 Questions

Quote:
Originally Posted by ThreeQuarter View Post
Good idea on the seal replacement. I don’t have any crocus cloth, how does that compare to the abrasive pads I use to clean copper pipe before I solder it? I think those are the least coarse option I have on hand. I don’t mind buying something locally, but it looks like I’d have to order crocus cloth online and wait for it to arrive.

The spline and yoke mate up very nicely—no play or wear at all.

I think I will change the trans fluid. Seems like there must be a level inside the case where all TH400s have to be filled to—I wonder what that is so I can see if my dipstick is at the same level.
If you are talking about scotch brite pads, that should do the trick.....or I have also used a 400 grit wet /dry paper , followed with a 600 grit . As long as the rust is off & smooth, you will be fine Longhorn
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Old 05-04-2022, 05:43 PM   #9
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Re: TH400 Questions

I'm about 95% positive that the full line on the dipstick is very close to the pan rail/gasket area. If you ever park uphill that yoke should be practically submerged in oil. The rusty yoke must be your warning.
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Old 05-18-2022, 09:21 AM   #10
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Re: TH400 Questions

Quote:
Originally Posted by cadillac_al View Post
I'm about 95% positive that the full line on the dipstick is very close to the pan rail/gasket area. If you ever park uphill that yoke should be practically submerged in oil. The rusty yoke must be your warning.
You are correct that's where the level should be when running.

If I pull the slip yoke out of my TH400's 2wd tail housing, should I lose a little fluid?
You should be good. Typically very little fluid comes out.

I have had more transmissions out than I can count and the rust on the drive shaft is concerning me. If you have that much water in the transmission it will not last long. Water is bad antifreeze is catastrophic. Antifreeze attacks the glue holding the linings to the steels and it will flake off. Its usual source is a defective cooler in the radiator. Other water ingress is typically from driving in deep water. I would immediately change out the fluid.
You will have to change it in the pan and then unhook the cooler line and pump out the torque converter.
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Old 05-21-2022, 10:43 PM   #11
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Re: TH400 Questions

Keep the suggestions coming—thank you!

I dropped the pan and changed the filter today. The dipstick looked right. I got 5 qts out of it.

But I do see some grime and moisture in the pan. Given that this transmission was supposedly in storage for some years before I bought it and had it rebuilt, is it possible this grime/moisture was inside the transmission and just now getting loosened up? I ask because my few experiences with leaks in the trans cooler showed up more as trans fluid in the coolant, and less as coolant in the trans. I don’t see any signs of trans fluid in the coolant. I’m not trying to say I don’t have a problem, but I want to make my best guess at the source of the problem before I do something like remove and re-test my radiator. (It was tested 2500 miles ago but of course it could’ve developed a leak 2499 miles ago.)
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Old 05-22-2022, 01:57 AM   #12
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Re: TH400 Questions

Nice to have a picture of the pan.
It really looks pretty good.
In my experience the coolant is still pressurized with the engine off and it migrates to the transmission at that time.
If you have had the radiator checked and have changed the fluid and confirmed the level. Does the transmission work as designed?
Proof is in the function, guess it's time to drive it and see if it works and continues to.
That's the bottom line. Unless their is a question I missed.
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Old 05-22-2022, 11:42 AM   #13
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Re: TH400 Questions

What I saw in the pan was actually a little concerning to me. At 2500 miles after a rebuild I would normally expect to see a squeaky clean pan. But there was a considerable amount of black grime. And by the way the trans fluid beaded up you could tell there was water in the mix as well.

That said, the trans has functioned fine this entire time. I would not have suspected anything if I hadn’t pulled the slip yoke in the process of getting a new driveshaft built.
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