Speedo Problem
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
Remove the cable and see if the needle falls to zero.
If the needle falls to zero there may be something broken off in the head (a tip from a plastic cable) not allowing the new cable to seat all the way. When you screw the cable all the way on, it will put pressure on the first worm and magnet and hold the needle.
The only other idea I have would be the cable tip is to long and binding.
If you have part of an old plastic tip stuck in the back find some welding wire and heat it up until it glows red. Then push it in the back of the speedometer head and allow it to cool. Most of the time if there is an old plastic part still in there when the wire cools you can pull it out.
The cable screws in to what is called the first worm and magnet. If the cable is pushing on this to hard it will drive the first worm magnet in to the speed cup and hold the needle. Even a little bit of pressure inside the two can cause this. The speed cup is attached to a tiny spring that winds tighter as the speedometer needle moves upward. The less wound the less tension the spring puts on the speed cup to return to zero.
For a better understanding of what I'm talking about go to this link. It's not about your problem but the second page will show you the inner workings and help you see what I'm talking about.
http://willcoxcorvette.com/repairand...lp.php?hID=145
Willcox
If the needle falls to zero there may be something broken off in the head (a tip from a plastic cable) not allowing the new cable to seat all the way. When you screw the cable all the way on, it will put pressure on the first worm and magnet and hold the needle.
The only other idea I have would be the cable tip is to long and binding.
If you have part of an old plastic tip stuck in the back find some welding wire and heat it up until it glows red. Then push it in the back of the speedometer head and allow it to cool. Most of the time if there is an old plastic part still in there when the wire cools you can pull it out.
The cable screws in to what is called the first worm and magnet. If the cable is pushing on this to hard it will drive the first worm magnet in to the speed cup and hold the needle. Even a little bit of pressure inside the two can cause this. The speed cup is attached to a tiny spring that winds tighter as the speedometer needle moves upward. The less wound the less tension the spring puts on the speed cup to return to zero.
For a better understanding of what I'm talking about go to this link. It's not about your problem but the second page will show you the inner workings and help you see what I'm talking about.
http://willcoxcorvette.com/repairand...lp.php?hID=145
Willcox
Last edited by Willcox Corvette; Apr 15, 2008 at 12:07 PM.
Pro
Joined: May 2004
Posts: 702
Likes: 1
From: Sealy TX
St. Jude Donor 06-'07
On my 76, the cable does not screw into the head, it pushes in and is held in place with the spring clip on the back of the speedo. (maybe mine's not original to the car?)
Last edited by ccs96; Apr 15, 2008 at 06:43 PM.
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
You are correct. In 1969 GM changed the W series speedometer and tachometer heads to a clip on design. Same parts, just a different way of holding the cable on to it. (PITA getting them to un-clip too).
Either style can cause this problem though!
If you use the clip on cable you still need to push the cable in to the back of the speedo hard enough for the clip to catch. If there is something in there it will push on the first worm and could. . . cause this.
I’m only speculating here as to what the problem could be! It could be a needle issue, but the only thing that would cause it to do this would be if the needle was bent and hitting the face or the first worm and magnet is worn out and physically hit the speed cup in the head. When this happens it will spin the needle in a one to one ratio and slam it! This will usually result in a broken needle, speed cup or other parts in the head and not allow the speedo to function at all.
There are still other things that could cause this problem internally in the speedometer, but I would check the simple cause first.
Willcox
Either style can cause this problem though!
If you use the clip on cable you still need to push the cable in to the back of the speedo hard enough for the clip to catch. If there is something in there it will push on the first worm and could. . . cause this.
I’m only speculating here as to what the problem could be! It could be a needle issue, but the only thing that would cause it to do this would be if the needle was bent and hitting the face or the first worm and magnet is worn out and physically hit the speed cup in the head. When this happens it will spin the needle in a one to one ratio and slam it! This will usually result in a broken needle, speed cup or other parts in the head and not allow the speedo to function at all.
There are still other things that could cause this problem internally in the speedometer, but I would check the simple cause first.
Willcox
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