Ben1

Just bought a 1978 c3 and the Speedo doesn't work, at all. I realize there is a cable that comes from the back of the Speedo and follows down to the tranny and connects to a hard plastic gear, I guess my question is, how do I know what to replace?? In addition, if anyone has a diagram to the vacuum lines for headlights that would be awesome. Mine turn on but dont come up. Thanks in advance.

MelWff

disconnect the cable at the trani and use an electric drill in reverse to spin the cable while checking the speedometer.
if the gauge moves continue with pulling out the plastic gear and check the teeth.
this link is for the vacuum diagrams
http://repairs.willcoxcorvette.com/w...schematics.pdf

Ben1

Where does it connect on the tranny?? Top left or??? Thanks for the help!

TimAT

Left side, rear, 3-4 inches forward of the driveshaft. Does your car have cruise control?

Ben1

Yes it does but it's a mess, I'm not sure who did what to it but the wires are everywhere and not connected, the actual control box under the hood is missing.

gdm78vette

With cruise control there are 2 cables, one from the transmission to the cruise control box and one from the box to the speedometer. If the control box is missing then whoever removed it probably didn't replace the 2 cables with the longer non-cruise single cable directly from the transmission to the speedometer.

LenWoodruff

I agree. You need to have the cruise module for the speedo unless you change to the longer non cruise speedo cable.

I just replaced my cruise module and now the speedo works fine.