What transmission do I have? 72 Sierra Grande

[Thatguywithaneye]

Trying to find a replacement tranny, but I’m having a hard time figuring out which one to search for. 72 GMC Sierra Grande with the 350 engine. It’s an automatic and it leaks like crazy! Probably 4 litres of atf fluid on the driveway. The option # on my sticker says M49-Turbo Hydra-Matic, but I know that could mean it’s the TH350 or TH400. Where do I need to look to figure this out?

Thanks,
The new guy

[sick472]

The TH350 has a square or rectangular pan with a bit of one corner missing while the 400 has a funky shaped one. I can't really explain it's shape so look it up. Part of it kinda looks like the lower half of Texas. I believe the vacuum modulator on the 350 is on the right front while the 400 has it on the right rear. The 350 has a kickdown, cable, the 400 does not, it has an electrical swicth.

[Thatguywithaneye]

Originally Posted by sick472

The TH350 has a square or rectangular pan with a bit of one corner missing while the 400 has a funky shaped one. I can't really explain it's shape so look it up. Part of it kinda looks like the lower half of Texas. I believe the vacuum modulator on the 350 is on the right front while the 400 has it on the right rear. The 350 has a kickdown, cable, the 400 does not, it has an electrical swicth.

Thanks! I’ve determined that it’s the TH350. Square with the corner cut off is exactly what it looks like.

On another note have you ever seen an extreme leak from one of these?

[Coley]

As Sick471 mentioned the TH350 has a 45 degree scallop out of the rear corner and it is largely square in shape otherwise.
The TH400 is clearly longer than it is wide and the TH400 has a 'bump' out on one side defeating the rectangular shape in the rear area....basically the TH400 looks a bit like a race car circuit with only a couple of square corners I think.

Coley

[Rich69shortfleet]

They ALL seem to leak from one place or another, but that one looks pretty slimy. If it is just generally tired and leaky why not have it rebuilt instead of buying a new one? Unless you want to upgrade to a more modern overdrive transmission or something. One thing to note, the truck TH350s are different than the car TH350s, the truck versions have one or two more clutch packs than the car versions.

[Thatguywithaneye]

Originally Posted by Rich69shortfleet

They ALL seem to leak from one place or another, but that one looks pretty slimy. If it is just generally tired and leaky why not have it rebuilt instead of buying a new one? Unless you want to upgrade to a more modern overdrive transmission or something. One thing to note, the truck TH350s are different than the car TH350s, the truck versions have one or two more clutch packs than the car versions.

The transmission was rebuilt twice by the previous owner (my grandpa) back in the 90's, but it never fixed it. He has tried everything from new seals and gaskets to a full blown rebuild and it fixes it for a few months and then the leak progressively gets worse. I have a feeling that the metal is warped where the engine meets, but that doesn't seem to be the only location. I will take it in to a transmission specialist and see what they say, but I don't want to fix it again. I found a brand new factory tranny for $1400.

Thank you for the information on the differences between the car and truck though, I will definitely use that.

[demian5]

Looks like the dipstick o-ring side leaking. Also the pans can get bent up and leak. I had to use 2 gaskets and "the right stuff" to seal up one once.

[MARKDTN]

A good shop should be able to fix that. The TH350 is a pretty simple transmission.

[BCOWANWHEELS]

yep, pull it and take to trans shop