700r4 speedo port not spinning at all.

[randomTruckKid]

Hey guys,
Recently got me a 79 c10 stepside. It has a 700r4 on it with a 350.
The speedo port does not spin on it. We replaced the cable since it was old, crusty and melted to the exhaust.
When we replaced it a leak started from the port.

How should I go about fixing it. I know almost nofthing about that part of the transmission. I have not opened it yet. I plan to do so when we take off the cab in a few weeks. It is a VERY slow leak.

Here are some pictures.

The one without the liquid is before we put in the new cable when i was checking to see if it spun.

 

[NickTransmissions]

Hey guys,
Recently got me a 79 c10 stepside. It has a 700r4 on it with a 350.
The speedo port does not spin on it. We replaced the cable since it was old, crusty and melted to the exhaust.
When we replaced it a leak started from the port.

How should I go about fixing it. I know almost nofthing about that part of the transmission. I have not opened it yet. I plan to do so when we take off the cab in a few weeks. It is a VERY slow leak.

Here are some pictures.

The one without the liquid is before we put in the new cable when i was checking to see if it spun.

Remove the housing and driven gear. Count the teeth then buy a new gear w/the same amt of teeth if necessary, along with a new stem seal for the gear and o-ring for the housing. There's supposed to be a little "keeper" ring sitting in front of the seal. If it's not there, acquire a replacement.

 

[squaredeal91]

@NickTransmissions , The gears are color coded for gear ratio right? Like pink blue green

 

[NickTransmissions]

@NickTransmissions , The gears are color coded for gear ratio right? Like pink blue green

Not anymore, no color correlation between factory and aftermarket gears. If you count the teeth, you'll purchase the correct gear.

 

[randomTruckKid]

Remove the housing and driven gear. Count the teeth then buy a new gear w/the same amt of teeth if necessary, along with a new stem seal for the gear and o-ring for the housing. There's supposed to be a little "keeper" ring sitting in front of the seal. If it's not there, acquire a replacement.

Ok. Is there some website somewhere where I can figure out what size gear I need? I belive that's how it works??

The previous owner has changes the tire sizes several times so I dont know if it's correct.

 

[PrairieDrifter]

I just did this the other day. There's a small "snap ring" on that orange seal. Then an o-ring on the main part.

That being said if that piece isn't spinning then both gears inside could be garbage which would mean removing the tail shaft housing, and replacing both gears.

 

[NickTransmissions]

Ok. Is there some website somewhere where I can figure out what size gear I need? I belive that's how it works??

The previous owner has changes the tire sizes several times so I dont know if it's correct.

Was your speedo accurate prior to the gear(s) failing? If so, you dont need a website, simply count the teeth on the existing gear then order another with the same amount of teeth.

If not, use TCI's Speedo Calculator (google it) to determine the number of driven gear teeth needed given the number of teeth on the drive gear, overall tire size and final drive ratio.

Like @PrairieDrifter said, check your speedo drive gear to see if it's stripped. While not as common a problem as a stripped driven gear, it can happen.

 

[randomTruckKid]

Was your speedo accurate prior to the gear(s) failing? If so, you dont need a website, simply count the teeth on the existing gear then order another with the same amount of teeth.

If not, use TCI's Speedo Calculator (google it) to determine the number of driven gear teeth needed given the number of teeth on the drive gear, overall tire size and final drive ratio.

Like @PrairieDrifter said, check your speedo drive gear to see if it's stripped. While not as common a problem as a stripped driven gear, it can happen.

It didn't work when we bought it. We will check it out.
But thank you. There's alot of goofy stuff going on with the truck. At least it runs really good.

 

[Matt69olds]

If the cable is melted, then it’s almost a certainty that the speedo gear is stripped. The cable probably locked up, which prevents the speedo gear from turning. No more teeth.

If you know what gear ratio you have in the rearend, you can use this to calculate what gears you need to make the speedo accurate.

Search results for: 'tc speedometer gear calculator' - TCI® Auto

Leading provider of high performance automatic transmissions, torque converters and drivetrain components for race, street/strip, hot rod, street rod and towing

www.tciauto.com

 

[randomTruckKid]

So it said I need 29.4.... drive teeth? I think I did something wrong.

If the cable is melted, then it’s almost a certainty that the speedo gear is stripped. The cable probably locked up, which prevents the speedo gear from turning. No more teeth.

If you know what gear ratio you have in the rearend, you can use this to calculate what gears you need to make the speedo accurate.

Search results for: 'tc speedometer gear calculator' - TCI® Auto

Leading provider of high performance automatic transmissions, torque converters and drivetrain components for race, street/strip, hot rod, street rod and towing

www.tciauto.com

When we took it out it did spin. But I di think it was stripped.

 

[Matt69olds]

Obviously, you can’t have a gear with 29.4 teeth, your choice is a 29 or 30.

I’d go with a 29, it will probably be close enough.

 

[BadBowtie]

I've only seen it one time in my 52 year life, but the driven gear inside and on the tailshaft has a small metal clip. The clip had broken on this truck that kept that inside gear locked on the transmission shaft. Then the gear was push back down the shaft so the drive and driven gear didn't line up together.


Crazy I know. But I seen it once....

 

[59840Surfer]

Remove the housing and driven gear. Count the teeth then buy a new gear w/the same amt of teeth if necessary, along with a new stem seal for the gear and o-ring for the housing. There's supposed to be a little "keeper" ring sitting in front of the seal. If it's not there, acquire a replacement.

When we replaced the seals on these speedo-driven gear holders, we could most time stay with the old plastic gear (some are impossible to buy last time I looked) and we just put a second - new - seal on top of the old one.

Be sure to put the split ring on top of the new seal though.

You must be registered for see images attach

The shaft is long enough to hold two seals and it also helps to support the plastic gear shaft a little better.

OMG! This is an Amazon listing ---> PS - the driven gears are color-coded if you don't like counting teeth.

You must be registered for see images attach

LA Speedometer Gear 700r4 and BOP TH350 clip plus 40-45 tooth driven gears and 15 17 18 19 drive gears

There is an o-ring on the outer side of the holder - and it's an open-stock partshouse item so you don't have to buy one from AC Delco ($$$) and I think I still have a half-hundred of them left in my shop yet.

La Speedometer Gear 25512340 34-39 Tooth Speedometer Gear Housing 700r4/TH350

La Speedometer Gear 25512340 34-39 Tooth Speedometer Gear Housing 700r4/TH350

• GM 1361618
  Part Description
  RING, Speedometer Driven Gear
• Manufacturer
  General Motors
• Manufacturer's Notes:
  This part is discontinued according to GM.

• (GM 326561)
  Part Description
  SLEEVE, Speedometer Driven Gear
• Manufacturer
  General Motors
• Manufacturer's Notes:
  This part is discontinued according to GM
• 
  
  
  
  

 

[59840Surfer]

Addenda --->

You must be registered for see images attach

GM's # 25512339 & 1362049
AMAZON LINK HERE

 

[NickTransmissions]

When we replaced the seals on these speedo-driven gear holders, we could most time stay with the old plastic gear (some are impossible to buy last time I looked) and we just put a second - new - seal on top of the old one.

Be sure to put the split ring on top of the new seal though.

You must be registered for see images attach

The shaft is long enough to hold two seals and it also helps to support the plastic gear shaft a little better.

OMG! This is an Amazon listing ---> PS - the driven gears are color-coded if you don't like counting teeth.

You must be registered for see images attach

LA Speedometer Gear 700r4 and BOP TH350 clip plus 40-45 tooth driven gears and 15 17 18 19 drive gears

There is an o-ring on the outer side of the holder - and it's an open-stock partshouse item so you don't have to buy one from AC Delco ($$$) and I think I still have a half-hundred of them left in my shop yet.

La Speedometer Gear 25512340 34-39 Tooth Speedometer Gear Housing 700r4/TH350

La Speedometer Gear 25512340 34-39 Tooth Speedometer Gear Housing 700r4/TH350

• GM 1361618
  Part Description
  RING, Speedometer Driven Gear
• Manufacturer
  General Motors
• Manufacturer's Notes:
  This part is discontinued according to GM.

• (GM 326561)
  Part Description
  SLEEVE, Speedometer Driven Gear
• Manufacturer
  General Motors
• Manufacturer's Notes:
  This part is discontinued according to GM
• 
  

Yea, the gears available in the aftermarket are color coded but you still need to know what color to buy which means you still need to count teeth on your originals than look up the tooth count by color on the replacement drive/driven gears so you purchase the right set for your vehicle. Also you dont need to stack seals, just replace the old one with a new one then the little retainer on top of the new seal.