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Speedo??

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Old 09-13-2017, 02:46 PM
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Ben1
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Default Speedo??

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.
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Old 09-13-2017, 03:51 PM
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MelWff
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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
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Old 09-13-2017, 04:04 PM
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Ben1
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Where does it connect on the tranny?? Top left or??? Thanks for the help!
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Old 09-13-2017, 04:46 PM
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TimAT
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Left side, rear, 3-4 inches forward of the driveshaft. Does your car have cruise control?
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Old 09-13-2017, 08:37 PM
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Ben1
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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.
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Old 09-14-2017, 07:17 AM
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gdm78vette
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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.
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Old 09-15-2017, 07:57 AM
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LenWoodruff
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Originally Posted by gdm78vette View Post
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.
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.

Last edited by LenWoodruff; 09-15-2017 at 07:57 AM.
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