ALVETTE00

My speedometer needle was bouncing for a while then stopped working entirely. I replaced the speedometer cable (what a pain in the butt that was) and it's still not working. The needle does move so I'm pretty confident it's not an instrument issue.

Now I'm wondering what this is and if it could be the issue:


Any input is appreciated!
Thanks

HeadsU.P.

It appears to be an old transducer. When different gear ratios are used, quite often simply changing the "driven" gear in the tranny will recalculate the speedometer. But in extreme ratio changes, say from a 2.73 to a 3.73 the "drive" gear has to be removed. Now we are talking removing the driveshaft, removing the tailshaft housing and swapping out the gear, not fun but do-able.
Along comes the transducer. It compensates the new gear ratio with a lot less labor. The input cable spins at a different rate than the output, thereby getting the speedometer pretty accurate. They are available at Speedo-World and some math is involved for proper selection.

MelWff

remove it and the plastic driven gear and check if the driven gear has broken or missing teeth and check if you turn the input to the transducer that the output turns.

ALVETTE00

Ok, I will try removing that piece and go from there.

robert.sperry

I have a 74' and the adapter shown in the picture is a normal production part for a 74' . For troubleshooting unscrew the new cable from the adapter, attach it to a battery operated drill set to run backwards (unscrew direction) and run the drill up and down in speed. If the speedometer needs moves smoothly up and down in speed then you know the problem is with the adapter or the plastic gear on the other end of the adapter which is inside the gearbox or there is a problem with the gearbox itself.
To remove the adapter remove the single bolt on the flat plate that holds the adapter to the gearbox. Pull the adapter out and away from the gearbox. On the end of the adapter will be a plastic gear. My guess this gear is worn. Get a correct replacement gear from any of the major corvette parts suppliers (be sure to get the same color gear as the one you remove) and reinstall the adapter. That will likely fix the problem but if not then you know the issue is with the adapter or with the mating gear inside the gearbox.

wer2xu

My speedo was doing exactly what you described...can you hear any noise coming from it? I had to get the gauge rebuilt....

ALVETTE00

Nope, there was no noise...just stopped working. I haven't had any time to continue the troubleshooting process.