Downshift TH400 into 2nd for downhill compression braking?

[GOLDWINGER2]

Originally Posted by 370xl

Gary, Buy a pan gasket and filter kit for your turbo 400. Drop the pan and drop the filter. The servo has a cover over it, drop it and the servo will drop out by just pulling it down. This is the servo piston that activates the band with fluid pressure. Check the seals on the servo piston. They are made of metal not rubber like a engine piston. If they are worn and passing fluid then this is why your 2nd gear does not downshift. Turbo 350 transmissions use to wear the case out. Also check the spring at the same time. You have the part numbers and list in this board so you should be able to get the parts as needed. This is not an expensive procedure. You will also be able to look in the bottom of the pan for band flakes, if you find the pan to be particle free then most likely the band is good.

The servo that you see when you drop the pan is the rear band. (low and reverse)
The servo for the front band is accessed by dropping the valve body. You really need an instruction manual is you want to go forward. And even then you might not want to do the job. Thrust washers and torrington bearings must go back like they came out. All clutch packs must fully engage, as well as the torque converter. Have you ever replaced a trans yourself? The manual second gear piston has a lot of spring pressure behind, the low band servo does not.
J

[Gary - K7GLD]

Originally Posted by 370xl

Gary, Buy a pan gasket and filter kit for your turbo 400. Drop the pan and drop the filter. The servo has a cover over it, drop it and the servo will drop out by just pulling it down. This is the servo piston that activates the band with fluid pressure. Check the seals on the servo piston. They are made of metal not rubber like a engine piston. If they are worn and passing fluid then this is why your 2nd gear does not downshift. Turbo 350 transmissions use to wear the case out. Also check the spring at the same time. You have the part numbers and list in this board so you should be able to get the parts as needed. This is not an expensive procedure. You will also be able to look in the bottom of the pan for band flakes, if you find the pan to be particle free then most likely the band is good.

Sounds like potentially good info - I had the tranny serviced right after getting it home from buying it late last summer - I told the guys at the garage that since the MH had just been bought, I was especially interested in the condition of the fluid and residue in the pan - and was told by them after the tranny service that all looked great inside the tranny and pan.

Yesterday, I bought and added a pint of Seafoam auto tranny treatment to the AT lube, and will drive it a few times to see if there's a remote chance that a stuck servo or check valve might loosen up and return the tranny to normal - might sound like wishful thinking, but I *have* had good results with this approach in another problem GM tranny in the past.

[TitanicPilot]

Originally Posted by Bagger97

Just keep in mind you will need a very large bucket to contain all the trans fluid that will come out if you decide to drop the pan. It's a major OH SH*T if you try to use an oil drain pan.

If you have a stock pan and an oil cooler, the total system holds about 12-13 quarts; but the torque converter won't drain so you'll end up with maybe 6-7 quarts in the drain pan... I think....

If you like gauges, this would be a good time to install a temperature sender in the pan.

[JimElliott]

Originally Posted by TitanicPilot

If you have a stock pan and an oil cooler, the total system holds about 12-13 quarts; but the torque converter won't drain so you'll end up with maybe 6-7 quarts in the drain pan... I think....

If you like gauges, this would be a good time to install a temperature sender in the pan.

Yep, It is a perfect time for the "PAN SENDER"......
Just order the bung and find out which position in the pan has the most clearance and have it welded in.....WELD not braze.....Both sides.
Next time the pan needs to be drained just remove the sender for a complete and clean drain....

Jim

[Gary - K7GLD]

Originally Posted by JimElliott

Yep, It is a perfect time for the "PAN SENDER"......
Just order the bung and find out which position in the pan has the most clearance and have it welded in.....WELD not braze.....Both sides.
Next time the pan needs to be drained just remove the sender for a complete and clean drain....

Jim

YUP - I have done the same to the differential cover on my '02 Dodge/Cummins - seems stupid that the maker was so cheap that they didn't provide it from the factory - just like the tranny pan and torque converters...

[TitanicPilot]

Originally Posted by JimElliott

Yep, It is a perfect time for the "PAN SENDER"......
Just order the bung and find out which position in the pan has the most clearance and have it welded in.....WELD not braze.....Both sides.
Next time the pan needs to be drained just remove the sender for a complete and clean drain....

Jim

Or, if you want to do it yourself and don't want to weld, you can go down to your local hardware store and buy a bulkhead fitting that threads together with a gasket on both sides of the hole you drill in the pan. Works well as long as you choose a flat section of the pan.

If you use a mechanical gauge you'll also have to plan for the (usually) six foot capillary tubing length from the sending unit to the gauge. Be sure to look at the "stuff" inside the transmission that might interfere with your placement. This is a view into the left (driver's) side of the trans..

[TitanicPilot]

Originally Posted by TitanicPilot

Hi Gary,
....If I wasn't working on the brakes I'd take it out and see if I've got second gear compression braking; the trans has just been completely rebuilt during our engine swap.

I got the brakes done so I took Titanic out for a test drive and tried downshifting into 2cnd gear. It didn't freewheel but there wasn't a huge amount of compression braking; certainly not like downshifting a stick shift. I've only got a few miles on the engine and transmission so I didn't try it at a very high rpm.

[Nick-B]

Originally Posted by TitanicPilot

It didn't freewheel but there wasn't a huge amount of compression braking; certainly not like downshifting a stick shift. I've only got a few miles on the engine and transmission so I didn't try it at a very high rpm.

Yup, sounds about normal. There is no way it will compare to the compression braking you get in a stick shift especially when you consider the torque converter "slippage". Basically it is just enough to keep you from freewheeling.

[Gary - K7GLD]

Originally Posted by TitanicPilot

I got the brakes done so I took Titanic out for a test drive and tried downshifting into 2cnd gear. It didn't freewheel but there wasn't a huge amount of compression braking; certainly not like downshifting a stick shift. I've only got a few miles on the engine and transmission so I didn't try it at a very high rpm.

Thanks for the update Dave - we might get ours out this week to "exercise" it a bit now that I've added the "magic liquid transmission fixer"...

Actually, with the 4:88 differential gears, high range in the MH isn't all that far from what 2nd range would be in a vehicle with 3:55 gearing - so even with only high range, engine braking is pretty decent...

[GOLDWINGER2]

Originally Posted by Gary - K7GLD

we might get ours out this week to "exercise" it a bit now that I've added the "magic liquid transmission fixer"...

Actually, with the 4:88 differential gears, high range in the MH isn't all that far from what 2nd range would be in a vehicle with 3:55 gearing - so even with only high range, engine braking is pretty decent...

What brand of "magic" did you buy?
J

[Gary - K7GLD]

Originally Posted by GOLDWINGER2

What brand of "magic" did you buy?
J

The stuff I mentioned back in post #30....

That seemed the best locally available - and hope it works as well as a similar concoction about 30 years ago in another GM auto tranny with a similar problem...