how do i find out what speed my 1977 GMC automatic transmission?

Question

i bought a 1977 GM C15 1/2 ton off of my uncle it has a 307 motor and an automatic transmission i need the rear seal on ther transmision but dont know what speed it is how do i figure that out? i do not have owners manual

Answer 1

If it says P R N D 2 1, it's a 3 speed. (Park, Reverse, Neutral, and 3 forward gears, including Drive)



If it has a D inside an O between the N and the D, or between the N and a 3, it's a 4 speed.



OD is short for Over-Drive. That means the top gear is less than a 1:1 ratio.



That is, the driveshaft spins faster than the engine when in overdrive.



3 speed automatics are all 1:1 ratio for the highest gear.



In the case of your truck, almost certainly a 3 speed "Turbo" or "TH" (or more correctly, THM, short for Turbo-Hydramatic) 350 or 200, which is a lightweight version of the THM 350. Should be the same rear seal with either of those.



Turbo 400's were only used in trucks of a heavier weight class and/or behind bigger engines.



GM didn't produce any OD transmissions until the 1981 model year.



Fixed the missing "R". Tired today. LOL

Answer 2

Your truck almost certainly has 1 of 2 transmissions in it. If the trans pan looks like a square with 1 corner snipped off, its a turbo 350. If on the other hand, the pan looks more like the state of Illinois, its a turbo 400. I seriously doubt that the truck has an overdrive trans in it. On the rare chance that it does, the pan will be noticeably rectangular (turbo 700). Good Luck.

Answer 3

90% of 1977 vehicles equipped with auto transmission had a 3 speed auto trans





so most likely it is a 3 speed...





if it says overdrive or has symbols for over drive on the shifter it is probably a 4 speed....if it doesn't its a 3 speed....