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Discussion Starter · #1 ·
My speedometer runs at 5mph when I'm idling(10 at fast idle), and i'm assuming that the speedometer cable is the culprit. Does this sound right?

Is there any way to lube the cable to correct this, or should I replace it?

Thanks!
 

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Interesing problem. The speedo and tach are both mechanical. If the speedo is showing 5 mph in park, is the tach reading the correct numbers? Maybe someone swapped the cables under the dash. If not, I doubt the speedo cable is at fault. More likely the speedo head needs some work.

:cheers:
 

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Interesing problem. The speedo and tach are both mechanical. If the speedo is showing 5 mph in park, is the tach reading the correct numbers? Maybe someone swapped the cables under the dash. If not, I doubt the speedo cable is at fault. More likely the speedo head needs some work.

:cheers:
... or are you perhaps mistaking the tach for the speedo .... since it changes with engine speed.
Should be interesting ... first time I've heard this problem.
:buhbye:
 

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Discussion Starter · #4 ·
I'm definitely not mistaking the Tach and Speedo...give me a little credit! :D

The tach seems fine, reading about 800rpm at idle and 1500rpm at fast idle, but the speedo shows me moving and even the odometer turns with it. The speedo doesn't jump up like the tach does when I rev it, it just runs at 5-10 mph.

Oh, and this is a manual tranny, and the speedo reads accurately when driving(It's not 5-10mph off).
 

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Does the speed-0 work at highway speeds? Do you think the cable could be broke and the engine vibration is causing the cable to turn? I've seen stranger things happen. (Just can't see any other way the cable would turn all by itself.)
 

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Both the speedo and tach use a "flying magnet" to drive the pointer(s). I'd have to say that the tach is driving the speedo. Time to pull and get a rebuild. Someone may have another idea, but that's my shot..

:cheers:
 

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Discussion Starter · #7 ·
Ok, so if the "flying magnet" theory is correct, I should be able to disconnect my speedo cable at the tranny, and it would still be reading 5-10 mph... If it just quits reading, then the issue would be with the cable or tranny connection. Sound right? I'll try this test tonight and see what happens.
 

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Ok, so if the "flying magnet" theory is correct, I should be able to disconnect my speedo cable at the tranny, and it would still be reading 5-10 mph... If it just quits reading, then the issue would be with the cable or tranny connection. Sound right? I'll try this test tonight and see what happens.
Sorry for not giving correct amount of credit.
I just don't know you, Doug ... so first things first.

If the unit works well at driving speeds (and correct),
then the speedo itself must be getting hung up at the lower end of the range.
I'm afraid it is not the cable, gear, or tach. You likely need another speedo,
or I have seen replacement "guts" to rebuild a speedo.
:buhbye:
 

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Speedo and tach are not related and do not run together. Each has a separate cable and neither is dependent upon the other.

Check your speedo gear in the trans.

Engine idle speed should not affect the reading of the speedo since the tranny is not turning in Park.

:thumbsup:
 

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The speedo runs directly off the output shaft of the tranny, so there's no way the speedo cable can actually be turning unless the prop shaft and rear wheels are. Are you sure the needle isn't just not returning to zero properly? That sounds like the most obvious cause to me. The return spring inside the speedo head's maybe not returning the needle back all the way when it's "idle". The spring is the only thing that determines the needle's position when there's no magnetic drag from the speedo cable rotating. (well I guess the spring and the zero stop, to be more precise) Might need your speedo going over by an instrument guy.

:thumbsup:
 

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The speedo runs directly off the output shaft of the tranny, so there's no way the speedo cable can actually be turning unless the prop shaft and rear wheels are. Are you sure the needle isn't just not returning to zero properly? That sounds like the most obvious cause to me. The return spring inside the speedo head's maybe not returning the needle back all the way when it's "idle". The spring is the only thing that determines the needle's position when there's no magnetic drag from the speedo cable rotating. (well I guess the spring and the zero stop, to be more precise) Might need your speedo going over by an instrument guy.

:thumbsup:
:agree:
 

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I have only ever encountered this when the speedo is electronic, and instead of a cable, it has a transducer. A bad diode in an alternator can then induce a "phantom" pulse that tricks the elctronic speedo.:D
 

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The speedo runs directly off the output shaft of the tranny, so there's no way the speedo cable can actually be turning unless the prop shaft and rear wheels are. Are you sure the needle isn't just not returning to zero properly? That sounds like the most obvious cause to me. The return spring inside the speedo head's maybe not returning the needle back all the way when it's "idle". The spring is the only thing that determines the needle's position when there's no magnetic drag from the speedo cable rotating. (well I guess the spring and the zero stop, to be more precise) Might need your speedo going over by an instrument guy.

:thumbsup:
I would agree except for:

... but the speedo shows me moving and even the odometer turns with it.
I have no explanation for that at all. :huh:
 

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Yes, that is strange ... very strange.
Did you name your vette .... Christine ?
:buhbye:
I did, for similar reasons, but that is another story.

I like the theory of the tach's magnetic field influencing the speedo. It seems to be the only thing that would explain the action of the odometer as well as the speedo. The speedo and odometer are obviously turning while the car is at a dead stop. The only thing that is hard to grasp is that the strength of the magnets used on the speedo and tach are not that great.

What else could be causing a magnetic pulse that would influence the speedo?

A bad diode in the alternator creating spikes?

A flaky voltage regulator?

an electric fuel pump?

Again, I am employing the theory of SWAG here.
 

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I did, for similar reasons, but that is another story.

I like the theory of the tach's magnetic field influencing the speedo. It seems to be the only thing that would explain the action of the odometer as well as the speedo.
A mechanical speedo has a gear driven odometer. All the magnetism you can muster won't move it at all.
 

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I agree with Tim on the magnetic field,sometimes strange things happen :crazy:

Take for instance ever since my motor swap if you turn the key off the motor continues to run,but pull up on e-brake handle and it dies :WTF now go figger :crazy:



Kevin
 

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I agree with Tim on the magnetic field,sometimes strange things happen :crazy:

Take for instance ever since my motor swap if you turn the key off the motor continues to run,but pull up on e-brake handle and it dies :WTF now go figger :crazy:



Kevin
Probably a bad ground. Pulling on the e-brake activates a light.
 
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