'78 Speedometer or Cable?
Thread Starter
5th Gear
Joined: Apr 2012
Posts: 5
Likes: 0
'78 Speedometer or Cable?
I have a 1978 Pace car (manual) with a jumpy speedometer (friction based). Most the time, it doesn't work (it appears to be deteriorating). It will often work when I am slowing down (with foot off the throttle) to about 20 mph. But once I start going again it stops working.
Sometimes when I hit the gas hard, it will jump and *occasionally* work for a few miles.
Overall, it works about 40 miles for about 20 gallons of gas (about 15% of the time assuming mediocre millage)
My real question is: Is this a problem with the speedometer or the cable?
I have checked the connection at the transmission and have ruled that out
Thanks for the help
Sometimes when I hit the gas hard, it will jump and *occasionally* work for a few miles.
Overall, it works about 40 miles for about 20 gallons of gas (about 15% of the time assuming mediocre millage)
My real question is: Is this a problem with the speedometer or the cable?
I have checked the connection at the transmission and have ruled that out
Thanks for the help
Burning Brakes
Joined: Apr 2009
Posts: 807
Likes: 2
From: VIRGINIA BEACH VIRGINIA
I would say a bad cable. Pull the cable and you can pull the inside out and check it. They are cheap at advance auto. cant hurt to change it. One of the ends may be rounded out. Another possibility is it is not sitting all the way into the speedo (there is a clamp that holds it in) and it is jumping out.
Instructor
Joined: Dec 2009
Posts: 195
Likes: 0
From: Chesterfield Virginia
I had the same symptoms you describe, had my speedo rebuilt by Wilcox and it works fine now. I changed the cable first, but that did not solve the problem. Check your cable first because it's the easiest. You really don't want to take apart your driver’s side dash unless you really have to, real PITA. Good luck
Steve
Steve
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,850
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I have a 1978 Pace car (manual) with a jumpy speedometer (friction based). Most the time, it doesn't work (it appears to be deteriorating). It will often work when I am slowing down (with foot off the throttle) to about 20 mph. But once I start going again it stops working.
Sometimes when I hit the gas hard, it will jump and *occasionally* work for a few miles.
Overall, it works about 40 miles for about 20 gallons of gas (about 15% of the time assuming mediocre millage)
My real question is: Is this a problem with the speedometer or the cable?
I have checked the connection at the transmission and have ruled that out
Thanks for the help
Sometimes when I hit the gas hard, it will jump and *occasionally* work for a few miles.
Overall, it works about 40 miles for about 20 gallons of gas (about 15% of the time assuming mediocre millage)
My real question is: Is this a problem with the speedometer or the cable?
I have checked the connection at the transmission and have ruled that out
Thanks for the help
If the problem persists with the upper cable drill test then I would then remove the housing.. Why? Because its easy to remover. Corvette Speedometer and Tach Housing Removal Instructions 1978-1982 http://willcoxcorvette.com/repairand...lp.php?hID=146
and then I would check the upper cable first before assuming the issue is the speedometer.
Failure in the 1978-1982 speedometer is very far and few between. The head style changed in 1978 and the newer design seemed to solve an age old issue with the failing speedometers first worm and magnet issues.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,850
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Jessie.. We can rebuild the tach.. but you can do it yourself too! Here is the thread I just bumped on the video... and below are some pages that may help you determine what is wrong, or how to get it out.
http://forums.corvetteforum.com/c3-t...post1580584947
Corvette Tachometer Testing, Removal, Board Installation, Video Link 1978-1982 <--- best one to use
Corvette Tachometer Board Installation Instructions 1978-1982
Corvette Speedometer and Tach Housing Removal Instructions 1978-1982
Corvette Tach Filter Installation Instruction Sheet Revised 3-11
http://forums.corvetteforum.com/c3-t...post1580584947
Corvette Tachometer Testing, Removal, Board Installation, Video Link 1978-1982 <--- best one to use
Corvette Tachometer Board Installation Instructions 1978-1982
Corvette Speedometer and Tach Housing Removal Instructions 1978-1982
Corvette Tach Filter Installation Instruction Sheet Revised 3-11
Last edited by Willcox Corvette; May 6, 2012 at 11:20 AM.
Intermediate
Joined: Apr 2008
Posts: 43
Likes: 1
From: Dodgeville Wisconsin
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Pro
Joined: Aug 2007
Posts: 707
Likes: 31
From: Pitman NJ
If you have cruise, pull the cable loose at the cruise transducer and hook up a drill to it and see if your speedo is still jumpy. If it is not jumpy then I'd take a hard look at the lower cable and/or the transducer.
If the problem persists with the upper cable drill test then I would then remove the housing.. Why? Because its easy to remover. Corvette Speedometer and Tach Housing Removal Instructions 1978-1982 http://willcoxcorvette.com/repairand...lp.php?hID=146
and then I would check the upper cable first before assuming the issue is the speedometer.
Failure in the 1978-1982 speedometer is very far and few between. The head style changed in 1978 and the newer design seemed to solve an age old issue with the failing speedometers first worm and magnet issues.
If the problem persists with the upper cable drill test then I would then remove the housing.. Why? Because its easy to remover. Corvette Speedometer and Tach Housing Removal Instructions 1978-1982 http://willcoxcorvette.com/repairand...lp.php?hID=146
and then I would check the upper cable first before assuming the issue is the speedometer.
Failure in the 1978-1982 speedometer is very far and few between. The head style changed in 1978 and the newer design seemed to solve an age old issue with the failing speedometers first worm and magnet issues.
Because of the transducer, the lower cable has more tension on it, and is more likely to break before the upper cable.
1978, 78, c3, change, chevy, corvette, cruise, speedomenter, speedometer, transducer, troubleshooting, vette, willcox, wire, wont, work
