Odometer works some times

[akessel]

Okay, so why does the odometer work in spurts?  Like won't move at all then will work for 10 miles and then quit again.

Any thoughts are welcome

Andy Kessel
akessel56@gmail.com
'71 280 SL

[al_lieffring]

This is a common failure on almost all VDO speedometers, including the one in my 66 230sl.

The numbered wheels are on a shaft that rotates one turn per mile (km on metric speedos) at the right end of the shaft is a pot metal gear the same diameter as the digit wheels, a system of cogs on another shaft allow the ones wheel to advance one mile each time the shaft and gear make a revolution, the tens wheel advances one increment each revoloution of the ones wheel, then the hundreds, thousands, etc. the pot metal drive gear is a press fit onto the shaft and over time works itself loose and starts to slip on the shaft.

The fix is to take the speedo completely apart, and using a small very sharp chisel that I made just for this job and make several  parallel staking marks (in line with the axis) on the shaft, exactly where the wheel mates to the shaft.

Getting all the wheels and cogs back into place is a tedious job, one wheel incorrectly aligned will make all the number wheels ahead of it drop to half way between the numeral markings.

And of course, the mileage needs to be written down before dis-assembly so that the mileage is correct once it is all reassembled.

al



I'm riding 50Km. on my Unicycle 18 May, 2008 in the Tour de Cure.
http://main.diabetes.org/goto/al.lieffring
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Al Lieffring

[Mike Hughes]

To follow up on what Al mentions:  

The odometer drive is by means of a spiral gear that drives a cog on one end of a shaft that runs through the number wheel set.  The pot metal drive gear Al mentions is pressed onto the other end and engages the first number wheel.  Every revolution of the drive wheel moves the first number wheel up one number (1/10th of a revolution).
 
The cog rarely loosens on the shaft, but the pot metal drive gear is notorious for loosing its grip so that it slips on the shaft.  It eventually slips along the shaft so that the shaft backs out enough that the cog on the other end looses its engagement with the spiral drive gear.  The shaft may even have backed out of its locating hole in the odometer housing at the pot metal gear end.

What I have done is drive the shaft the rest of the way loose with a long leather-working needle of slightly smaller diameter than the shaft itself.  A drill bit will also do the trick.  Push the needle through the locating hole in the odometer housing, through the set of number wheels, and through the locating hole in the odometer housing on the other side, forcing the drive shaft free of the assembly.  When the shaft is completely free of the instrument the needle has taken its place as a temporary substitute shaft.  All the number wheels should have retained their relative positions.

To keep the pot metal drive gear from spinning on the shaft the shaft needs to have some splines introduced to the area where the pressed gear sits.  You can do as Al suggests or, if you have a pair of model working vise grips with jaws that have small parallel teeth, you can introduce a splined surface by clamping the area near the end of the shaft for a brief time.  (Model working vise grips have jaws that are barely 3/16" wide.)  The surface irregularity required is very slight.  It just needs to be parallel to the length of the shaft and confined to the area where the gear sits when pressed on the shaft.  What you don't want is to introduce a surface irregularity to the section of the shaft on which the number wheels ride or on the tip of the shaft that rides in the locating hole of the odometer housing.

Then just push the shaft back through the number wheel set, displacing the needle or drill bit temporary shaft, until it is ready to be pressed into the pot metal drive gear.  Be sure to support the odometer housing with a small socket so that you do not damage the housing when gently tapping the newly splined portion of the shaft home into the pot metal drive gear and the end of the shaft through to the locating hole in the odometer housing.  Be sure that you do not press it too far.  There should be some "end float" so that the shaft does not bind and the number wheels can move freely on the shaft.

This is a fairly premanent fix.  The pot metal drive gear should not spin on the shaft again.

- Mike Hughes  -ô¿ô-
  1966 230SL Auto P/S
  Havanna Brown (408)
  Light Beige (181)
  Cream M-B Tex (121)

[psmith]

Here's a guy who turned his hobby into a business and makes odometer gears.  http://www.odometergears.com/index.html

Pete S.