SlickGTP
Full Access Member
- Joined
- Feb 24, 2011
- Posts
- 770
- Reaction score
- 55
- Location
- Houston, TX
- First Name
- John
- Truck Year
- 1973
- Truck Model
- C10
- Engine Size
- 454
I did this with my TH-400, but most automatic transmissions will be similar. I also went the long way around and replaced the drive gear. Some will be able to just replace the driven gear which is much simpler as it is one bolt for the retainer on the Speedometer Pinion.
1) Jack up the back of the truck and support with jack stands so that most of the fluid is towards the front of the transmission.
2) Remove the driveshaft.
This is what you'll be looking at:
I have a ratio adapter from the factory. With this fix I can correct the speedo and get rid of the adapter.
3) Remove the tail housing. To do this I had to use a piece of wood to spread the load across the transmission pan and a jack to lift the transmission so I could remove the rear mount and clear the crossmember for the lower two bolts.
Result:
4) Remove the Speedometer Pinion. There's one bolt that holds the retainer and then the pinion pulls straight out.
You'll notice the governor gear behind the speedometer drive gear. During all this you want to prevent any foreign objects from getting into the transmission.
5) Remove the drive gear. This is actually a lot easier than I had first imagined. If your output shaft has an o-ring, remove it first. Then press down on the metal clip and slide the gear backwards. Be careful as the clip will fall as you slide the gear off.
New gear vs old gear...
Off to the left you can see the speedometer pinion with the driven gear.
6) Install the new drive gear. I found it helpful to put the clip in place and then rock it backwards so that the gear would more easily slide over it and then slide the gear over it allowing the clip to locate in it's place on the output shaft until everything clicked in place.
It's a little fuzzy, but you can see the clip at the bottom of the shaft at the edge of the gear.
7) Replace the o-ring and seal on the speedometer pinion and reinstall. For the OEM pinions there is only one way for them to go on so make sure that it's rotated so that the retainer fits into it's indents on the pinion. Bolt in the retainer and call it good.
8) Clean off the old gasket and replace the seal and bushing in the tailhousing. Unfortunately I don't have pictures of what I did here. For the Seal there's a small hole that you can put a punch and hit the seal from behind. Hell of a lot easier than any other method. Sadly I didn't realize this until fighting with it from the outside for awhile.
The bushing I ending up carefully using a chisel to cave it in and down until it dropped out. Then I used a piece of flat steel that fit over the entire bushing to help drive it in as I didn't have a bushing driver. Just go slow and it'll go in just fine.
That's all... Lube up the rear seal and reinstall the driveshaft, drop the truck back on the ground, take her for a spin and verify that the speedometer reads correctly!
1) Jack up the back of the truck and support with jack stands so that most of the fluid is towards the front of the transmission.
2) Remove the driveshaft.
This is what you'll be looking at:
You must be registered for see images
I have a ratio adapter from the factory. With this fix I can correct the speedo and get rid of the adapter.
3) Remove the tail housing. To do this I had to use a piece of wood to spread the load across the transmission pan and a jack to lift the transmission so I could remove the rear mount and clear the crossmember for the lower two bolts.
Result:
You must be registered for see images
4) Remove the Speedometer Pinion. There's one bolt that holds the retainer and then the pinion pulls straight out.
You must be registered for see images
You'll notice the governor gear behind the speedometer drive gear. During all this you want to prevent any foreign objects from getting into the transmission.
5) Remove the drive gear. This is actually a lot easier than I had first imagined. If your output shaft has an o-ring, remove it first. Then press down on the metal clip and slide the gear backwards. Be careful as the clip will fall as you slide the gear off.
New gear vs old gear...
You must be registered for see images
Off to the left you can see the speedometer pinion with the driven gear.
6) Install the new drive gear. I found it helpful to put the clip in place and then rock it backwards so that the gear would more easily slide over it and then slide the gear over it allowing the clip to locate in it's place on the output shaft until everything clicked in place.
You must be registered for see images
It's a little fuzzy, but you can see the clip at the bottom of the shaft at the edge of the gear.
7) Replace the o-ring and seal on the speedometer pinion and reinstall. For the OEM pinions there is only one way for them to go on so make sure that it's rotated so that the retainer fits into it's indents on the pinion. Bolt in the retainer and call it good.
8) Clean off the old gasket and replace the seal and bushing in the tailhousing. Unfortunately I don't have pictures of what I did here. For the Seal there's a small hole that you can put a punch and hit the seal from behind. Hell of a lot easier than any other method. Sadly I didn't realize this until fighting with it from the outside for awhile.
The bushing I ending up carefully using a chisel to cave it in and down until it dropped out. Then I used a piece of flat steel that fit over the entire bushing to help drive it in as I didn't have a bushing driver. Just go slow and it'll go in just fine.
That's all... Lube up the rear seal and reinstall the driveshaft, drop the truck back on the ground, take her for a spin and verify that the speedometer reads correctly!
