Speedo Cable Question!!!

[exmonkeypunk]

72' GMC C/2500.
Speedo didn't work when i got the truck, i looked under and saw i didn't have a speedo cable. There's a bolt and washer plugging the hole in the trans.
I'm not really sure why someone would want to eliminate a working speedometer, but who knows. Could there be some sort of issue that would cause someone to block off the hole and not use a cable?
I ordered a cable not knowing which size to get. There were 3 choices. I don't remember the sizes, but i went with the longest one thinking better too long than too short. Does anyone know off hand how to tell how long i need? If it matters?

[eightbanger]

To tell how long your speedo cable should be would usually be determined by measuring your excisting...but since that's not an option i'd say ordering the longest one would be the right way to go. Maybe someone else who knows better will let you know.

[exmonkeypunk]

i am planning on installing the new cable tomorrow (saturday) since the weather is supposed to be 50.
Hope the speedometer works.

[Andy4639]

The length want matter except you will have to keep it out of the drive shaft and anything hot.
If there is a bolt into the side of the transmission I would bet there is going to be more to it than a cable. The plastic gear is right inside the cap and if that screw or bolt was tightened up to stay put the gear will be of no use.

[exmonkeypunk]

So i might need to take off the tail shaft to inspect the gear. The hole isn't big enough to see the gear with a flashlight, is it?
I guess pulling the driveshaft would be a good time to inspect and replace the u-joints as needed. So, it's win win. As long as the bolts co-operate during removal.

[exmonkeypunk]

well, i tried to install the cable today, no dice.
The bolt was actually just a rubber plug with an adjusting bolt head. After pulling it out, there's nothing to thread the cable into. There's no gear inside either. I know where to get the gear, but what about the threading?

[tim21391]

tap and die set would be my guess.. im thinking that it leaked tranny fluid and they just plugged it and when they tried plugging it is when they messed up the threads...

[Jim Osborne]

Sounds like you will need the part that the gear goes in and it is held in by a hold down that looks like a smaller verson of a distributor hold down with a 1/4 bolt then your cable screws to it

[exmonkeypunk]

LMC should carry what i need, right? kinda doubting the parts stores.

[Jim Osborne]

I looked in LMC and could not find it not sure where you can find it but I will research my books and get back to you

[spinning wheels]

What kind of trans are you using?

These are photos of a tail section from a 700r4. Top is outside and the speedo connection with the hold down tab and screw.
Second is inside the tail section, it shows what you need inside.

Hope this helps.

[exmonkeypunk]

I have a Turbo Hydra-matic trans.
I am wanting to have the trans rebuilt, maybe i should have the shop add what i'm missing while they're in there.

[spinning wheels]

The tranny shop, would need to know what, the rear axle gear ratios the truck has installed.
That way, they can install the correct gear in the tranny's tail section.

Isn't it funny how you start fixing one small thing, and it turns out to be a major overhaul?

[exmonkeypunk]

Yeah, go figure. I don't know what gears i have in the truck. Any way i can tell without pulling the cover? If not, the chrome cover could use a cleaning anyways.

[spinning wheels]

The way I was taught to tell what gears you have is somewhat simple.

Jack up the rear of the truck/car, so that the rear tires can spin easily.

Rotate the tires one full rotation, and count how many times the driveshaft and rear yoke turn. Example: four turns and a little more, you can figure it has 4:10 gears. If it spins less than four turns, calculate if it went past 3 1/2 turns or more. You might have 3:55 or 3:73 or something close to that.

By turning the tire, you can also tell if you have a limited slip or an open rear axle. The open rear axle, (the most common), one wheel will spin in the opposite direction.

Oh yeah, you will need to have your park brake off and the tranny in neutral

I hope this makes sense. Be sure to use good jackstands. Nothing worse than having a truck smack your melon as you are counting the diff's yoke

[exmonkeypunk]

I'll have to wait to put it on stands, all my stands are under the Corvair while i fix the floors. I'm working on that today and next weekend to hopefully finish up.

[Jim Osborne]

got any pics of the Corvair

[Green Machine]

If you don't have posi, it will be hard to get both wheels to turn together, likely the other side will turn backwards when you side is turned forward. Just leave one tire on the ground and turn the free wheel 2 times and count the drive shaft. If you want to get real precise, turn the wheel 20 times, if the driveshaft turns 41 times even you have a 4.10, 45 1/2 turns would be 4.56 etc. Actually this would be a good time to pull the rear cover and put in new lube, then you can find the tooth count stamped on the ring gear, should see something like 10 41 which would be a 4.10 rear.

[exmonkeypunk]

I posted a few pics of the vair here a few months back. I'll have to dig some up.