HD Turbo 400
 

I have acquired several parts vehicles for a 1 ton 4x4 I'm building. I have one Turbo 400 from from a 1990 Suburban mated to a NP241 t-case, and another from a 1985 military 1 ton M1008 mated to a NP208. The one from the military truck says "HD" on it. Is there such a thing as a "Heavy Duty" Turbo 400? What is the difference from a regulary Turbo 400?

Re: HD Turbo 400
 

Yes there is a difference and I can't remember 100% but I think it has atleast one more clutch in the forward drum, and I think the output shaft is bigger but I'm not 100% on that on the 4wd's.

Re: HD Turbo 400
 

there is a good chance that the military t400 will not have a parking pawl inside the trans pan

Re: HD Turbo 400
 

Thanks for the replies. What is a parking pawl?

I did a search and found this from an article in Peterson 4 Wheel and Offroad:

"Two cases were available. 4x4 models have the "HD" case (stamped on the housing) with a cast-aluminum flywheel cover. Car applications got the "K"-stamped case and a plastic or sheet-steel cover. If you're sourcing a junkyard core, make sure it's an (uncracked) HD model."


The article seems to imply that any Turbo 400 that came in a 4x4 truck says HD. I don't think the one I have from the 4x4 Suburban says HD. However, I don't know for sure that it's the original transmission. Also, the article said that in 1987 the nomenclature was changed and the burb is a 1990.

Re: HD Turbo 400
 

There was a heavier TH400 that had straight cut gears instead of helical as well as an Allison torque converter. They may have actually been called TH475 and I think were limited to RV chassis (P-chassis).

Re: HD Turbo 400
 

I think I also remember the HD ones havind a 6 lug convertor instead of 4 but I can't remember if all TH400's had 6 or not.

Re: HD Turbo 400
 

4X4s had HD cases. Not sure if they all did, but some did for sure. The HDs did have an aluminum flywheel cover with a couple of brace rods hooking to that cover and going up to the engine.

There's a huge wealth of aftermarket and good used parts available for Th400s and you can build (or have built) whatever you want. The heavy duty version was called a Th475 and basically the only difference was that it had straight cut planetaries, which was supposedly stronger, but also noisier. There have been several clutch piston styles and depths over the years. Bigger clutch stacks were usually done by using lower profile pistons. There are aftermarket, deeper drums available too. I expect there were quite a few different clutch stack setups from the factory since they made them for 40 years and put them in virtually everything.

Converters from the small Allisons will fit the 400. The basic difference is the Allison converter has steel internals vs. aluminum internals for the 400. There may a stall speed difference too (not sure about that though). And as mentioned, the Allison converter has six bolt pads, vs. three for the 400. I don't *think* any Allison converters were factory fitted to Th400s.

Some heavy duty ones did not have a parking pawl. My dad owned an Airstream motorhome so equipped (or non-equipped) that way once. It was a one-ton van chassis. It also had a mechanical cable installed in place of the vacuum modulator.
BTW, the parking pawl is a small hook inside the tranny that latches into splines on the output drum, providing the "Park" shift position. Allisons don't have the park pawl, and you have to put them in neutral and set the parking brake to park them.

Re: HD Turbo 400
 

Bret, thanks for the info. It's been a while since I have messed with a Th400 and couldn't remember for sure.