78 Speedometer not working

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Jul 13, 2011 | 12:33 PM

Valentine

Thread Starter

Cruising

Joined: Jul 2011

Posts: 12

Likes: 0

From: Clinton Township Michigan

78 Speedometer not working

Okay, I hate asking but, I cannot learn if I do not ask.

I replaced a melted cable on 78 Corvette and it fixed nothing. Yes, the old one was bad. I replaced the fuse for the gauges even though it mechanical. I pulled the speedometer off and the I can make the needle move when I spin the connector. I'm going to pull the bottom next and hook it to a drill to make sure im working from there.

Can anyone tell me what is my next move from there?

Jul 13, 2011 | 01:15 PM

Lon Wayne

Drifting

Joined: May 2006

Posts: 1,480

Likes: 2

From: Circleville Ohio

Vader Vette

Quote:

Originally Posted by Valentine

My 75 Has a break in the middle(of the cable) that just unscrews and then, one at the tranny pull the one from the tranny be careful there is a gear there then look at the gear and try spinning it(the cable) from the tranny connection.

Jul 13, 2011 | 01:56 PM

Valentine

Thread Starter

Cruising

Joined: Jul 2011

Posts: 12

Likes: 0

From: Clinton Township Michigan

pulled the trans side of the cable and I can spin that and the speedometer moves. So, im guessing the next step is to pull the cone off the transmission?

Jul 13, 2011 | 02:17 PM

TimAT

Le Mans Master

Joined: Oct 2006

Posts: 7,123

Likes: 433

From: Gladstone MO

C3 of Year Finalist (appearance mods) 2019

Actually, you can pull the speedo gear adapter out- look in the hole and make sure the speedo drive gear is centered. Check that the square in the driven gear is actually still square and that it is getting the cable. I've seen a couple of inner cables that were too short and didn't quite make the speedo turn.

Jul 13, 2011 | 02:26 PM

Valentine

Thread Starter

Cruising

Joined: Jul 2011

Posts: 12

Likes: 0

From: Clinton Township Michigan

The cable appears to be long enough and it appears that the everything is centered. Do you or anyone have a write up or something depicting the removal of this gear for inspection?

Jul 13, 2011 | 04:19 PM

szinkel

Cruising

Joined: Jun 2011

Posts: 13

Likes: 0

On an 82, there is an upper and lower speedometer cable. Both have to be working. I don't know if the 78s are the same though, or if you have checked both cables. Just a thought.

Jul 16, 2011 | 12:33 PM

Valentine

Thread Starter

Cruising

Joined: Jul 2011

Posts: 12

Likes: 0

From: Clinton Township Michigan

Okay, I have replaced the speedometer cable cuase it was bad.

I replaced the the drive gear cause it was worn.

I have tested from the drive gear all the way to the speedometer and it works.

I am thinking my next step is inside the transmission cone. Unless someone has another idea? Anyone know what I am looking for when I get it off?

Jul 16, 2011 | 12:38 PM

pawpaws toy

Burning Brakes

Joined: Jun 2011

Posts: 816

Likes: 11

From: lewisburg wv

Quote:

Originally Posted by Valentine

Check this.http://www.partshp.com/speedometer_gears.htm

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Jul 16, 2011 | 04:15 PM

terry burkland

3rd Gear

Joined: Jul 2011

Posts: 3

Likes: 0

From: Sparks Nevada

Speedo fix

The speedo on my '78 has never worked the way it should. After 15 years of half hearted try's here is what worked. First spin tested cable from cruise control transducer, was good, then same for lower cable from trans. end, good, then changed driven gear on lower cable end, good. Still no speedo function except for needle jumping. The only thing left was the drive gear in the trans. IT is not that hard to do. Car up on stands, drop drive shaft out of the way, pull out spline, take out 4 bolts from tail housing and wiggle out of the way and you will see the plastic gear. Mine was very warn. Next release the clip holding it on. I would recommend getting a new clip with your new gear as mine broke getting the gear off. Then just put it all back in reverse.
Took car on freeway with my GPS and 70 mph was right on the money and for a bonus my cruise control was working.
Hope this helps......

Jul 17, 2011 | 10:44 AM

#10

Valentine

Thread Starter

Cruising

Joined: Jul 2011

Posts: 12

Likes: 0

From: Clinton Township Michigan

Thanks for all the help on this. I have a new update.

First, although the car has cruise, it looks like someone bypassed it and went direct to the trans. Not sure what affect, if any, this will have on the car.

I want to thank you all for the help on this. The insights and links were awesome.

Jul 17, 2011 | 08:19 PM

#11

terry burkland

3rd Gear

Joined: Jul 2011

Posts: 3

Likes: 0

From: Sparks Nevada

If you bypass the cruise and the speedo still is not working and the cable is OK than the problem is most likely one of the two gears, driven or drive.

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