Speedo Issue
Thread Starter
Intermediate
Joined: Nov 2012
Posts: 48
Likes: 14
From: Rochester Hills MI
Speedo Issue
My 74 speedo is ok up to 40 mph. Above that speed the needdle starts to jump around a bit and then starts to squeal from the speedo area and pins to 160 mph. The odometer starts racking up miles very quickly.
Any suggestions? I've only owned this car for a few months and it needed a lot of work just to get it on the road so I haven't driven it much yet.
Any suggestions? I've only owned this car for a few months and it needed a lot of work just to get it on the road so I haven't driven it much yet.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
The first worm bushing is on the fritz...
This is a common problem and so you understand what is going on. The magnet (1) and worm ride in a bushing (labeled first worm bushing below). When the bushing wears out it will tilt the magnet and then it will start hitting the the speed cup. This is why the speedo needle jumps wildly.
The fix is something you'd have to send in to us to have done. I have instructions on how to remove the driver side dash pad at this link.
Corvette Left Hand Dash Pad Removal Made Easy 1968-1977 Just click on supporting document!
What causes this? Age of the bushing for one.. but people putting oil or lubricant's on the speedo cable is the biggest cause. The speedo cable is reverse wound and the bushing is made from Oylite which doesn't take to kindly to oil. While oil may quite a noisy cable it also causes the bushing to fail prematurely.
This is a common problem and so you understand what is going on. The magnet (1) and worm ride in a bushing (labeled first worm bushing below). When the bushing wears out it will tilt the magnet and then it will start hitting the the speed cup. This is why the speedo needle jumps wildly.
The fix is something you'd have to send in to us to have done. I have instructions on how to remove the driver side dash pad at this link.
Corvette Left Hand Dash Pad Removal Made Easy 1968-1977 Just click on supporting document!
What causes this? Age of the bushing for one.. but people putting oil or lubricant's on the speedo cable is the biggest cause. The speedo cable is reverse wound and the bushing is made from Oylite which doesn't take to kindly to oil. While oil may quite a noisy cable it also causes the bushing to fail prematurely.
Last edited by Willcox Corvette; Nov 8, 2012 at 09:43 PM.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Not me.. 
About 10 years ago I would agree... But as our fiberglass, arm itching toys become older... The odometers are failing at a faster rate.. toss in some speed cup stimulation at an erratic pace combined with a failing odometer and you'll find it.
If the tenths inner gear in the odometer is stuck.. it will click off wildly.... Then put it in direct drive with the cable (magnet hitting the cup) and who knows what will happen. Really the only way to know what is going on is to have it in my hand on the tester machine.
Willcox
About 10 years ago I would agree... But as our fiberglass, arm itching toys become older... The odometers are failing at a faster rate.. toss in some speed cup stimulation at an erratic pace combined with a failing odometer and you'll find it. If the tenths inner gear in the odometer is stuck.. it will click off wildly.... Then put it in direct drive with the cable (magnet hitting the cup) and who knows what will happen. Really the only way to know what is going on is to have it in my hand on the tester machine.
Willcox
Melting Slicks
Joined: Sep 2010
Posts: 2,413
Likes: 59
From: Emerald Isle NC
Not trying to hijack here, but I am experiencing speedo wierdness as well. I reset my trip meter to zero and since then my odometer is racking up miles at a great rate. The odo started at 820 mi. and now shows 1000 mi. Meanwhile, the trip meter indicates 140 mi. traveled, for an approx. 40 mi. dispairity. Other than this, my speedo is kinda "swoopy" in the 40-60 mph range only, during acceleration, and steadies out above that. Am I experiencing the same issues as the OP? This prob is making it tough to track mileage, and I need to do this in order to change diff and trans. lubes after diff rebuild and new TKO-600 install.
Thanks Vetteman and Mr. Willcox Dude!
Carter
Thanks Vetteman and Mr. Willcox Dude!
Carter
Thread Starter
Intermediate
Joined: Nov 2012
Posts: 48
Likes: 14
From: Rochester Hills MI
Update. The speedo problem seems to have resolved itself. The needle still wavers a little bit but not bad. Hopefully it's not an intermittent problem. Maybe just needed exercise.
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