Speedo / Tach / Gauges: Speedometer pinion question

[53k]

I changed the entire rear end in my Avanti from a 3.54 to a 3.07. Today I finally got a chance to test it. The car started moving, but the speedometer needle didn't. I had bought a new pinion with the correct number of teeth for the 3.07 and my son installed it while he was doing some other under car work. Simple enough I said to myself, he didn't get it in far enough or forgot to tighten the clamp that holds it in the transmission. So, i crawled under the car and took the cable loose. Had the wife watch the speedometer while I turned the pinion. Speedometer reacted OK so I re-installed the cable and made sure it was right. Got in the car, it moved, but again the speedometer didn't. Crawled under the car, removed the cable from the transmission and pulled the pinion out and compared it with the one from the 3.54. Exact same length, design and diameter, just different number of teeth. So, I decided maybe it was pushed too far on to the bottom of the speedometer cable and I put it on, but left a gap between the gear and the end of the speedometer shaft. Had the wife verify speedometer needle movement- OK. Re-installed it, car moved, no speedometer movement.
So, what now? The shop manuals don't say anything about installing the pinion. I really don't want to drive it to South Bend with an inoperative speedometer.

[StudeRich]

I checked the Avanti Parts Catalog Paul to see if the 3.07 Pinion gets a different Drive Gear than the 3.54, but that should not be the problem like I thought it might be. All Ratios from 3.07 to 3.73 get the 8 tooth Drive Gear.

[skyway]

Just from a troubleshooting point of view, I guess I'd
reinstall the old one just to see if it still works. If it does then your issue is with the new pinion, and if not then with the cable. Any chance the square drive inside the new pinion is mis shaped?
Those pinions just snap in, and so should be in the correct position to meet the drive gear. Is the snap ring intact?

[53k]

I checked the Avanti Parts Catalog Paul to see if the 3.07 Pinion gets a different Drive Gear than the 3.54, but that should not be the problem like I thought it might be. All Ratios from 3.07 to 3.73 get the 8 tooth Drive Gear.

Thanks. Yes, I made sure of that before I ever ordered the pinion. I did change a 3.73 to a 3.07 several years ago and found that the speedometer gear was the same for that range and the change worked fine.

[53k]

Just from a troubleshooting point of view, I guess I'd
reinstall the old one just to see if it still works. If it does then your issue is with the new pinion, and if not then with the cable. Any chance the square drive inside the new pinion is mis shaped?
Those pinions just snap in, and so should be in the correct position to meet the drive gear. Is the snap ring intact?

Thanks for the suggestions. I guess I will try the 3.54 pinion and see what happens. I would have thought that turning the pinion and getting a speedometer needle movement would say the cable and pinion are OK, but I will check the square drive. I looked at it with feeble eyes and couldn't see any difference, but I'll get some strong reading glasses and get in good light and check it again. By snap ring are you referring to the small spring around the shank of the pinion or is there another one on the connection that I am missing? After I had the 3.07 pinion off I did have some trouble getting it back in the bottom of the cable housing.
I also thought maybe the gasket got lost off the cable in the handling and was allowing the pinion to go in too far so I went to my parts stock and found a NOS 1544875 gasket which turned out to be an "o" ring. I'm pretty sure it was on the shaft, but I'll check (though I can't imagine it making enough difference to cause the pinion to not line up with the gear.

[pdrnec]

Pop the cable off the back of the speedo and see if it turns there.

[53k]

Problem solved. Probably can be described as a "dumb and dumber" case. Apparently the base of the cable wasn't getting snapped in to the last 1/8 inch or so and the clamp wasn't pulling it in. I decided to try the 19-tooth (3.07) pinion one more time and with better glasses and a strong light, I pushed the cable in all the way. I also decide to jack up the wheels instead of backing off the ramps and up the driveway then moving ahead. When I had the wheels off the ground and gave it gas, the speedometer didn't respond until it was between 5 and 10 mph. So, maybe I wasn't going fast enough either.
Road test validated my choice of 3.07- considerable lower rpm at road speed- quieter and hopefully cooler. I also checked the 3.54 pinion I took out and it proved to be a 22-tooth pinion which is used in a 3.73, not a 3.54. So, I guess I have been driving faster than the speedometer was showing all along.

[skyway]

Glad you fixed it! Yes the small spring around the shank is what I had in mind.