Speedometer jumps from 30 to 70

Andyw · Aug 5, 2014

Here is a strange one. Replaced the speedometer gear and the cable last month. Been driving the car fairly regularly (put about 200 miles on it) On my way to work this morning speedometer was working just fine and silent up to about 30 mph then it jumps to 70, bouncing wildly and consistently increasing with my speed and I can hear whining in the cluster. I come to a stoplight and start out again and the same thing, 30 to 70 in less than a second, I know the car is fast but that's insane!

But seriously, any ideas?

72 Roadrunner 150MPH cluster

mrsnicks · Aug 6, 2014

Did you lube the wire inside the cable before you installed? Most speedometer cables come with a tube of lube and you need to pull the spring/wire/cable out, lube it and stick it back before installation.

Patrick

2059 · Aug 6, 2014

sounds like the drive wire is binding. lube and make sure no hard bends have developed in the cable.

Andyw · Aug 6, 2014

The odd part is that the speedometer stays at 70 tho. It does not bounce between 30 and 70. I will check my routing tho, maybe something is pinching. And yes, I did lube it first as well.

I dont see how a bound up wire would spin twice as fast consistently mile after mile tho

khryslerkid · Aug 6, 2014

Sometimes the speedometer "head" itself can become dry inside, causing the needle to jump. You have to remove the speedometer cable from under the dash and try to get some light oil into the back of the unit.
I'm not sure when they might have sealed up the speedometer unit that you wouldn't be able to do this but it's worth a try. While you have this end of the cable disconnected is when you would run some light oil in there also.

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Andyw said:
I dont see how a bound up wire would spin twice as fast consistently mile after mile tho

The cable will bind, then let go, bind, then let go. When it lets go it does with more force than normal, which makes the speedometer register 30 to 70mph.

Reading your symptoms again, seems to me it might be in your speedometer itself. Having a noise inside. Sometimes when the odometer is turning a lot of numbers this will happen too. Causing more pressure on the internal gears. Once they all turn then it fixes itself. (Not always)
Anyway I would see if you can get some light lube into the rear of the unit. If not it might be time for a replacement or rebuild. (Sorry for all the info at one time)

Photon440 · Aug 6, 2014

As khrysler kid said, the speedo itself may need lube. I lubed mine with a very light synthetic oil meant for record turntables and now it's quite smooth.

Andyw · Aug 6, 2014

khryslerkid said:
The cable will bind, then let go, bind, then let go. When it lets go it does with more force than normal, which makes the speedometer register 70 mph.

Yes, I understand this, I actualy drove about 5 miles at what it said was 75mph give or take 8 mph when I was actualy only doing 40. Gummed up speedometer is sounding right... looks like I will just go thru the whole run trying to loosen it all up. Had I been thinking I should have snapped a video of it due to it being such an odd occurrence.

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Thanks for all the info guys!!

kdtjb · Aug 6, 2014

Did you put it in the trany at the correct mark for the amount of teeth on the drive wheel? I bought a RR that did that, they had it out of phase.

Sonny Black · Aug 6, 2014

every once in a while min will do a similar "jump"....but then if I drive it more often it does not do it!..?
...go figure.

69roadr · Aug 6, 2014

kdtjb said:
Did you put it in the trany at the correct mark for the amount of teeth on the drive wheel? I bought a RR that did that, they had it out of phase.

i think this is what's wrong with mine. thanks kdtjb!

khryslerkid · Aug 6, 2014

kdtjb said:
Did you put it in the trany at the correct mark for the amount of teeth on the drive wheel? I bought a RR that did that, they had it out of phase.

So the gear is mounted on an eccentric, (egg shape) you have to rotate it to make the gear ingage?

Andyw · Aug 7, 2014

All fixed, lubed the back of the speedometer and it smoothed all out. Thanks guys!

UKPlymouth · Aug 10, 2014

Glad you fixed it. Mine was so noisy it made me want to hit it with a hammer,
Instead I lubed the back of the speedo and it was fixed immediately. Don't you love cheap easy fixes!?

mrsnicks · Aug 10, 2014

I know where/how to lube the cable. Pics of what you're lubing on the back of the speedo please.

Patrick

Speedometer jumps from 30 to 70

Andyw

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mrsnicks

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2059

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Andyw

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khryslerkid

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Photon440

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Andyw

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kdtjb

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Sonny Black

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