74 manual speedometer drive gear replacement.
Thread Starter
Instructor
Joined: Nov 2021
Posts: 101
Likes: 5
From: Warwick, NY
74 manual speedometer drive gear replacement.
Hello,
My speedometer started to jump around then finally quit working on a drive a couple of weekends ago. I pulled the cable from the tranny and with a drill in reverse confirmed the speedo is working. The end of the cable is square. I then removed the 90 degree angle drive off the tranny as it wiggled when looked to see if it was screwed in tight to the tranny. After cleaning and confirming the shaft in the 90 degree drive gear was square. I reinstalled. The nut on the hold down was tight keeping the end drive piece sticking out from the inside of the tranny. Can I assume I need to replace the drive gear inside the tranny? If so, can this be done without removing the tranny and how difficult?
Regards,
Eric
My speedometer started to jump around then finally quit working on a drive a couple of weekends ago. I pulled the cable from the tranny and with a drill in reverse confirmed the speedo is working. The end of the cable is square. I then removed the 90 degree angle drive off the tranny as it wiggled when looked to see if it was screwed in tight to the tranny. After cleaning and confirming the shaft in the 90 degree drive gear was square. I reinstalled. The nut on the hold down was tight keeping the end drive piece sticking out from the inside of the tranny. Can I assume I need to replace the drive gear inside the tranny? If so, can this be done without removing the tranny and how difficult?
Regards,
Eric
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Removing the bullet and inspecting the plastic gear is really easy. Remove the bullet retainer bolt and retainer. Twist and pull the bullet straight out of the transmission tailshaft housing. Inspect the plastic gear. If it's tore up, (most likely) yes then replace with correct color gear. But also ask why is it tore up. Too much friction in the cable?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
H-m-m-m-m-m. Me thinks we just saw this topic a couple weeks ago.
But anyway.
In the event you must change the "Drive" gear, the tranny does not have to come out.
You have to remove the driveshaft, and the tail-shaft housing of the trans.
This is accomplished by placing a jack under the trans pan with a large piece of plywood to distribute the weight evenly.
Then, the trans rear rubber mount can be loosened from the crossmember.
There are four bolts holding the tail-shaft housing. Using a 3/8 drive ratchet and a foot long extension, you can reach them.
It's not fun, but doable. Very little fluid will come out.
Once the housing is removed you will see the color-coded Drive Gear. It's held on to the tail-shaft by a special clip.
While the housing is removed and sitting on your workbench, great time to swap out the tail-shaft seal. Don't forget a new gasket (O-Ring) for the housing also.
But anyway.
In the event you must change the "Drive" gear, the tranny does not have to come out.
You have to remove the driveshaft, and the tail-shaft housing of the trans.
This is accomplished by placing a jack under the trans pan with a large piece of plywood to distribute the weight evenly.
Then, the trans rear rubber mount can be loosened from the crossmember.
There are four bolts holding the tail-shaft housing. Using a 3/8 drive ratchet and a foot long extension, you can reach them.
It's not fun, but doable. Very little fluid will come out.
Once the housing is removed you will see the color-coded Drive Gear. It's held on to the tail-shaft by a special clip.
While the housing is removed and sitting on your workbench, great time to swap out the tail-shaft seal. Don't forget a new gasket (O-Ring) for the housing also.
Thread Starter
Instructor
Joined: Nov 2021
Posts: 101
Likes: 5
From: Warwick, NY
Thanks! Back up on the quick lift this week. The cable to the speedometer is free from any sticking and friction as well as the angle gear drive. Could it be damaged by the metal drive gear?
Thread Starter
Instructor
Joined: Nov 2021
Posts: 101
Likes: 5
From: Warwick, NY
Thanks for the info. I hope its the driven gear that needs replacing. I just finished the suspension rebuilt, installed a new radiator, and restored some of the motor accessories and started to drive the car a few weeks ago. I would like to drive this car after almost 3 years of it up on stands.
Regards,
Eric
Regards,
Eric
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
The Drive Gear on the tail-shaft is plastic or nylon just like the Driven Gear.
The whole idea with the speedo is that the cheapest and easiest to get at hopefully breaks first.
Some speedo cables had a spring on the end designed to snap if something was not lubed after 40+ yrs of neglect. Anything is better than breaking the speedometer inside the dash cluster.
There is a web site with lots of GM speedo parts. I believe its PATC? Or speedo-world? Something like that.
They have driven, drive gears, clips, reducers, cables, 90-degree adaptors, and calculators for speedo tuning.
The whole idea with the speedo is that the cheapest and easiest to get at hopefully breaks first.
Some speedo cables had a spring on the end designed to snap if something was not lubed after 40+ yrs of neglect. Anything is better than breaking the speedometer inside the dash cluster.
There is a web site with lots of GM speedo parts. I believe its PATC? Or speedo-world? Something like that.
They have driven, drive gears, clips, reducers, cables, 90-degree adaptors, and calculators for speedo tuning.
Thread Starter
Instructor
Joined: Nov 2021
Posts: 101
Likes: 5
From: Warwick, NY
Up date on my speedometer issue.
1) I pulled the cable, connected the drill, the little lady stated see saw the middle move up to 100 MPH.
2) checked the cable end. It's square.
3) Removed the 90 degree angle drive. both the input and the output shaft are square. I cleaned the unit.
4) Removed the speedometer gear sleeve and gear. The gear was not damaged. cleaned both parts reassembled and confirmed the "O" ring on the sleeve was in good shape.
5) Reached until the trans tail to see if I could turn the drive gear. I'm not sure it turned, because I had my rubber gloves on. I think it turned.
6) Reinstalled the speedometer sleeve. Placed the 90 degree angle drive shaft into the sleeve and tried to turn the drive gear. I could not.
7) Reinstalled the angle drive and cable.
8) Took the car out for a drive. The needle intermittently bounces up to 25-30 MPH and the odometer and trip meter are turning slowly. not accurately representing the mileage.
Where do I go from here? I'm not sure if it's the drive gear or, the speedo. Any guidance is appreciated!
PS)
The power steering pump gave up the ghost on this test drive. Any recommendations on a company that rebuilds the original pumps. I found, Turn One in the web but am looking for recommendations.
Regards,
Eric
1) I pulled the cable, connected the drill, the little lady stated see saw the middle move up to 100 MPH.
2) checked the cable end. It's square.
3) Removed the 90 degree angle drive. both the input and the output shaft are square. I cleaned the unit.
4) Removed the speedometer gear sleeve and gear. The gear was not damaged. cleaned both parts reassembled and confirmed the "O" ring on the sleeve was in good shape.
5) Reached until the trans tail to see if I could turn the drive gear. I'm not sure it turned, because I had my rubber gloves on. I think it turned.
6) Reinstalled the speedometer sleeve. Placed the 90 degree angle drive shaft into the sleeve and tried to turn the drive gear. I could not.
7) Reinstalled the angle drive and cable.
8) Took the car out for a drive. The needle intermittently bounces up to 25-30 MPH and the odometer and trip meter are turning slowly. not accurately representing the mileage.
Where do I go from here? I'm not sure if it's the drive gear or, the speedo. Any guidance is appreciated!
PS)
The power steering pump gave up the ghost on this test drive. Any recommendations on a company that rebuilds the original pumps. I found, Turn One in the web but am looking for recommendations.
Regards,
Eric
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
As stated, the Drive Gear is clipped to the tail-shaft. The only way it would move with your finger, is the whole driveshaft would have to turn & the rear axle.
If that Drive Gear truly did move, I would say the steel clip holding the gear is missing, or damaged.
A jumpy speedo needle is 90% of the time, just plain-bone-dry. Lots of friction there inside the sheath and around two foot in length.
Do not use oil to lube it. The design of the cable is a spiral. It will "wick-up" thin lube and eventually be introduced into the back of the dash cluster. Not good.
Either a form of graphite or Hoppes Gun Grease works well and handles the heat from the exhaust pipe.
And last, how do you know the PS pump is shot? There are six more parts.
As far as a replacement, I just went to the local NAPA for a rebuilt. Then a machine shop to swap the pulley over for me.
If that Drive Gear truly did move, I would say the steel clip holding the gear is missing, or damaged.
A jumpy speedo needle is 90% of the time, just plain-bone-dry. Lots of friction there inside the sheath and around two foot in length.
Do not use oil to lube it. The design of the cable is a spiral. It will "wick-up" thin lube and eventually be introduced into the back of the dash cluster. Not good.
Either a form of graphite or Hoppes Gun Grease works well and handles the heat from the exhaust pipe.
And last, how do you know the PS pump is shot? There are six more parts.
As far as a replacement, I just went to the local NAPA for a rebuilt. Then a machine shop to swap the pulley over for me.
Last edited by HeadsU.P.; Aug 13, 2024 at 08:55 PM.
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