Need Help With Speedometer Weirdness

[DC3]

Last fall, I sent the speedometer from my '73 out for repair of the trip odometer and check of the calibration. While I had everything apart I also replaced both the upper and lower speedometer cables.

The new lower cable that was sent to me was about 12" longer than the old lower cable (~24" vs ~36"). With the extra length, the cables do not hug the firewall as they did previously and there is a fairly large loop below the brake booster before the cable turns into the firewall.

The speedometer now hangs at times and then releases. When this happens the needle first dips to a lower speed and then abruptly increases to a much faster speed and then dips again before settling to the normal speed. I can actually hear it happening. Sounds like a whirring type noise coming from behind the dash and is loud enough to get my attention. It seems to happen as often as every 15 seconds.

Example: When traveling 60 MPH, the speedometer is accurate. When the problem occurs, the needle dips hard to about 50 MPH and then slings up to around 90 MPH. Then the needle swings back pretty hard to about 50 MPH and then returns to 60 MPH which is the normal speed.

It is much more noticeable the faster I drive. It is barely noticeable at 40 MPH. At that speed, the needle only swings about 1 MPH in either direction. Below 40 MPH, the needle just wiggles a bit.

Is the problem with the speedometer or does the cable loop due to the extra length in the lower cable have something to do with it?

I can't put the old lower cable back in to test as the reason I replaced it was that it was broken. I really hope I don't have to pull the speedometer again. The car is a '73 w/ SBC & TH400.

Thanks,

DC

 

[TimAT]

I would suspect the extra length is your problem. Cables will take a wide radius bend with no problem, but a loop is a different thing. Look at all the extra friction the cable has to overcome. You had your speedo rebuilt, so it's unlikely that is where the trouble is.

 

[DC3]

As Tim suggested, the extra length was the problem. I replaced the too long lower cable with a correct new one and the problem went away. Before I received the new cable, I played around with the existing cable and tried various routes to make the thing work but in the end there was no route that was suitable to prevent the binding. It was simply too long.

For future record, C3s with automatic transmissions and no cruise control came originally with a 24" lower speedometer cable. Somewhere along the way GM superseded the original design with a 23" cable.

DC