Speedometer needle bouncing

[roger97338]

I'm not the first to have this problem, and I know I won't be the last.

The problem is that my speedometer needle bounces when I drive. It also seems to wind up, like a spring. Once I'm at speed, then slow down, the needle remains at speed and gradually lowers, independent of vehicle speed.

The problem started when I replaced the original Saginaw 3-speed with a TH-400. (1975 Chevy, 350, 2-wheel drive) I'm not sure what combo of keys, adapters, gears and such I might need to fix this problem.

The first thing I thought was a dirty or kinked cable, so I replaced them. (It has the two piece speedo cable.) That wasn't the problem.

Any advice would be appreciated.

[zieg85]

You may have a drive or driven gear in the transmission causing this since it sounds like the issue started when the transmission was changed?

[roger97338]

It did start after the tranny swap, yes. I totally agree that the problem lies with the speedometer interface hardware in the tranny that I installed. I thought it was equally likely that I damaged the speedo cables while I was installing the Turbo 400, but I ruled that out by replacing the cables.

Do you happen to have a link to how to replace the speedometer drive gear and driven gear? The last time I looked into this, I couldn't find any details about those two gears in any of my reference material.

[bd]

The 1974 Overhaul Manual outlines speedometer driven gear removal, drive gear replacement, and driven gear installation.

Also see, Common causes of "bouncy" speedometer needles.  The driven gear support bushing, or sleeve, is manufactured from aluminum and the gear is made of plastic.  Excessive wear of the driven gear shaft and the support bushing hole wearing oblong are common problems.  If you need to replace the aluminum sleeve, be aware that the sleeve part number must be matched to the number of teeth on the driven gear to compensate for different gear diameters and offsets.

[roger97338]

The 1974 Overhaul Manual outlines speedometer driven gear removal, drive gear replacement, and driven gear installation.

Also see, Common causes of "bouncy" speedometer needles.  The driven gear support bushing, or sleeve, is manufactured from aluminum and the gear is made of plastic.  Excessive wear of the driven gear shaft and the support bushing hole wearing oblong are common problems.  If you need to replace the aluminum sleeve, be aware that the sleeve part number must be matched to the number of teeth on the driven gear to compensate for different gear diameters and offsets.

Thanks bd. I didn't remember which forum I'd seen that in.