Odometer not working

There is a plastic gear in the speedometer that eventually breaks on mechanical speedometers called an odometer drive gear. This will involve removing the speedometer from the instrument cluster in the dash.

SPEEDOMETER TROUBLESHOOTING

Can be loose speedo wiring connectors under instrument panel or its issue with speed sensor on transmission.

I suggest you to go through help links mentioned below with appropriate solutions to confirm the problem :------

How to trouble shoot speedometer?

http://schematicsdiagram.blogspot.com/2011/06/speedometer-not-working.html

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1996 fiat bravo speedometer not working?

http://technoanswers.blogspot.com/2012/01/1996-fiat-bravo-speedometer-not-working.html
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Speedometer works but odometer not working?

http://technoanswers.blogspot.in/2012/01/speedometer-works-but-odometer-not.html

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Speedometer runs backward?

http://technoanswers.blogspot.com/2012/01/speedometer-runs-backward.html

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Warning lights on and speedometer died?

http://howtobyme.blogspot.com/2011/08/warning-lights-on-and-speedometer-died.ht

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Speedometer not working on 1993 Ford F350?

http://repairhelpcenter.blogspot.in/2012/02/speedometer-not-working-on-1993-ford.html

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ABS & Brake lights warning indicator steady on dash and speedometer not working for Dodge Ram?

http://howtobyme.blogspot.com/2011/11/abs-brake-lights-warning-indicator.html

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Vehicles Cluster panel Not reading Miles?

http://schematicsdiagram.blogspot.com/2011/12/vehicles-cluster-panel-not-reading.html

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Odometer works properly but speedometer works intermittently?

http://technoanswers.blogspot.com/2012/01/odometer-works-properly-but-speedometer.html

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These will help.

Thanks.

if the speedo is not sending a speed the trans will go into back up mode and the trans will not shift normally check the speed sensor

could be the VSS - vehicle speed sensor .
Speedometer and Odometer
The Vehicle Speed Sensor (VSS) is a gear driven permanent magnet generator. The vehicle transaxle houses the VSS. The VSS generates a sine wave output with a frequency proportional to the vehicle speed.
The Powertrain Control Module (PCM) converts the CKT 389 signal to an output that switches to ground at a frequency of 4000 pulses per mile. The CKT 389 signal drives the speedometer and the odometer. Various other vehicle components use the CKT 389 signal. Refer to Engine Controls.

neoprene gear in transmission were the speedometer cable goes into transmission should not have a dished appearance if you pulll it out of transmission look at it from the side it should appear flat from the start of the gear to end of gear no dish down in center

Is the odo electrical or mechanical? If mechanical then little gears inside speedometer/odometer are worn or broken. If electrical then circuit faulty.

You need to replace the speedometer gear that drives the sensor.

If the speedometer is working but not the odometer it's in the speedometer it's self. replacing the speedometer cluster is the easiest their available on ebay or your local salvage yard.
what happens is the trip odometer wears and sticks, theres a small motor on a circuit board with a plastic sun gear, inside the speedometer housing. it drives both the odometer and the trip odo
the teeth strip off the sun gear and odometer stops.

The problem is going to be a worn or broken odometer gear. You can purchase the gear at odometergears.com for $29.00 with the shipping. Follow their how to repair on the Mustang to help in the repair. I believe you will have just the worm gear to replace.
http://www.odometergears.com/subpages/MustangHowTo.html

Suggest you visit this website... http://www.hyperocity.com/volvo240/volvoodometer.htm
Odometer is turned by a mechanical gear wheel, which is the likely culprit.