Speedometer problems

I just installed a new speedometer cable from the transmission to the speedometer hookup (push on - clip type) on the back of the dash bezel in a 1969 Valiant. While driving, the speedometer needle goes crazy! It wildly bounces back and forth and is registering 80 to 110 mph when I know I'm going 30 mph. There are no kinks or sharp bends in the cable at all. Anyone ever have this problem?

4 new speedometer threads in about a week. Might be a record.

Just a thought, I know it's a new cable, but you can try lubricating the cable with some powered graphite to see if that helps, or maybe you need to rebuild the speedometer. Was this happening before you changed the cable?

Sometimes when that happens it's the part of the mechanism that the cable drives on the back of the speedo itself making contact with the bell that moves the needle.

This is the deal, the cable turns all the time when the car is rolling and it turns a part on the back of the speedo.
That part spins next to a magnetic part so it drives the cable via magnetic resistance.
When those two parts come in contact with each other the needle jumps up because the needle is now moved directly by the cable instead of by the resistance of the magnetic part.

I other words, the cable and needle normally never come into direct contact with each other and when they do the needle jumps up way higher than the car is actually going.

This problem is ALWAYS either the cable binding and winding up so when it releases it spins way faster for a second causing the needle to jump up (this one us usually accompanied by some pretty definite noises from the cable)

Or the two magnetic pieces are coming into direct contact with each other (usually doesn't make much noise)

The cable binding is usually a routing an/or cable lubrication problem.
The second one is usually caused by physical wear of the speedo unit (head) and is best solved by sending it to a speedo repair business or replacement.

The old cable was frayed, split and broke off the end of the connector at the speedometer. Prior to that it was working fine for a couple years when I purchased the car. I put about 5 miles on the car yesterday after installing and the odometer read an additional 25!

Thanks TrailBeast. That makes sense. I sure hope that's not the case as I've just had everything taken apart, re-chromed,dash painted, steering wheel off, etc. Now it's all back together as of yesterday and encountered the speedo issue when taking the car out for a much needed drive. I'm going to try and disconnect the cable at the speedometer from underneath-behind the dash and take a look at the end of the cable to see what it looks like. Will also powder graphite it real well...Thanks for the info!

TrailBeast said:

This problem is ALWAYS either the cable binding and winding up so when it releases it spins way faster for a second causing the needle to jump up (this one us usually accompanied by some pretty definite noises from the cable)

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nitronut said:

Thanks TrailBeast. That makes sense. I sure hope that's not the case as I've just had everything taken apart, re-chromed,dash painted, steering wheel off, etc. Now it's all back together as of yesterday and encountered the speedo issue when taking the car out for a much needed drive. I'm going to try and disconnect the cable at the speedometer from underneath-behind the dash and take a look at the end of the cable to see what it looks like. Will also powder graphite it real well...Thanks for the info!

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What Greg described could very well be what's happening.

When you installed the cable, did you pull the cable and lubricate it with the powdered graphite? Mine came with a tube of it when I bought it. I pulled the cable completely out of the sheath and put the powdered graphite in the sheath and re-installed the cable.

I put the new cable in and still had a problem, until I figured out that I had an excessive bend in it right behind the speedo head. Once I fixed that, it works flawlessly.

My problem was slightly different, it would read fairly accurately, but when I stopped the speedo would remain on about 30 MPH. Once the cable released it would drop to zero.

JUST IN CASE.

Every once in a great while I run across a speedo that jumps around like that and pulling the cable back from the speedo head a little while it's still connected fixed it.
The cable was pushing the little (thing) whatever it's called that it turns in the speedometer against something else in the speedometer and making the needle erratic and way higher than the actual speed.

Just reach up under there and pull it towards the fire wall a little.

Might make it work right again.

Accumulation of debris in the female socket where the cable attaches can cause unwanted pressure on the instrument. Yes you would need to remove the panel again to properly clean that.

And it can do this if the new cable core's square end does not actually engage the socket in the speedo, but just rides on the surface of the socket; it jumps around and engages the speedo erratically. Sometimes, you have to pull the core out a bit at the speedo end, rotate it to perfectly align with the square socket hole, and then insert it into the speedo to get proper engagement. It may work better to connect the speedo end first, and then the trans end.

They can be fussy, 'specially if not the perfect length in the core.

I cleaned that socket out real good, along with all the gauges, etc. when everything was apart. Thanks!