TH400 speedometer drive gear replacement

[53Rick]

Looking for a step by step on how to replace a speedometer DRIVE gear. '72 GMC pickup with a TH400. What is involved in pulling the output shaft and then the drive gear? Can it be done with the transmission still in the truck? What holds the output shaft in place?

[LockDoc]

Originally Posted by 53Rick

Looking for a step by step on how to replace a speedometer DRIVE gear. '72 GMC pickup with a TH400. What is involved in pulling the output shaft and then the drive gear? Can it be done with the transmission still in the truck? What holds the output shaft in place?

You might have better luck with this question over in the "Engine and Drivetrain" Forum....

LockDoc

[old Chevy guy]

You don't have to pull the output shaft. If it's a two wheel drive you should be able to pull off the Tail housing ithe gear slides on the shaft. I forget exactly but there may be a clip or something. That gear Is nylon. I'm assuming that's what your after,there's two gears one on the shaft and one on the end of the speedometer drive. The drive one you can get to by just pulling the drive out. One bolt and a small metal braket hold it in. The gears are color coded to determine gearing to match the speed depending on rear axle ratio. Why do you want to change it? To get the one on the shaft you will have to jack up the unit remove the mount and driveshaft then 4 bolts on the tail housing and I'm not sure but I think you have to pull out the speedometer drive. You may want to pull the crossmember to make it more accessible. Have a drain pan read as you will lose lome ATF when you pull the shaft and more when you pull the tail. You can't remove the output shaft without dismantling the unit,but if it's just the gear you don't have to. Post more info on what your end goal is here so we can answer better.

[53Rick]

Here's some additional information. Vehicle is a '72 GMC 1/2 ton 2 wheel drive with a TH400 transmission. I'm trying to get some advance information in case I have to replace the speedometer drive gear. The driven gear was just replaced 3 years ago. I doubt that it is already shot, but it is possible. Speedometer stopped working. The cable is good, The speedometer adapter functions properly. I jacked up the rear end and put the truck in drive with the speedometer cable and adapter removed. I can see the square end of the speedometer driven gear and it does not turn when the drive shaft is turning. At this point I believe the problem has to be the driven gear which is relatively new or the drive gear. I will, of course, pull the driven gear first and look inside to examine the condition of the drive gear. If that gear is no good, I am trying to find out the procedure for replacing it in order to determine whether it is something I can do of if I am getting into something more complicated and best done by a mechanic. I'm frustrated by the fact that neither the shop manual nor a manual on TH400 transmissions shows the procedure. Hope this additional information helps.

[53Rick]

Disregard. I found what I'm looking for in the Overhaul manual.

[old Chevy guy]

It's not hard either way as you must be seeing now. Post what you find as the problem when you get it apart. I would be curious to know.