Strange speedometer issue

[filozof97]

Hello;

I've got really, really strange issue with speedometer in my '53 Mercedes - Benz 300.

 

When I'm accelerating, speedometer stands at zero. When I press the clutch down, it goes up to actual speed. Same happens when I rapidly take my foot off the gas pedal. When I'm driving at steady speed, it's random - sometimes zero, sometimes actual speed, sometimes slighly reduced speed, sometimes something like 20 km/h at 100 km/h.

 

I've disconnected cable from the speedo, to see if the problem is in speedometer or in cable. When accelerating etc. cable does not move, so speedo is O.K.

 

I've changed speedometer cable for a new, custom made one. Nothing has changed. Put some rubber hose over it, as I thought it's bending in where it passes in beetween frame and body, while engine moves on its mounts.  Nothing has changed.

 

I have absolutely no idea what to do next. Any ideas?

[Oldtech]

Not a MB tech but I assume it runs off trans tailshaft.  Could there be a loose bearing combined with worn gears such that on coast, it engages the gear, and on acceleration moves away from it?   All I can think of.

[m-mman]

Yeah, a speedometer is a fairly simple Rube Goldberg type device. 

It takes the spinning motion of a part (typically the transmission output shaft) and uses a meshed gear to spin a cable to eventually spin the mechanism the speedo instrument. 

 

If your cable isnt turning, just keep following it back further and further to discover where the spinning forces are no longer being transmitted to the cable. 

 

50s-60s American cars used plastic gears (on the cable) that contacted the metal gear on the transmission output shaft. Long lasting, but eventually the plastic gets worn. 

[joe_padavano]

1 hour ago, m-mman said:

50s-60s American cars used plastic gears (on the cable) that contacted the metal gear on the transmission output shaft. Long lasting, but eventually the plastic gets worn. 

 

In fact, I had a similar issue with the speedo gears in a Muncie trans. When the gear oil was cold, the speedo would read fine. Once the oil warmed up and thinned out, the speedo would read slower and slower and eventually not work at all. Turns out the plastic gears were worn. The cold viscous oil provided a fluid coupling, but that went away as the oil warmed.

 

Of course, the actual cause of the worn gears was a worn speedo cable sheath that increased friction in the speedo cable and thus increased the load on those plastic gears.

Edited June 19, 2021 by joe_padavano (see edit history)

[filozof97]

5 hours ago, Oldtech said:

Not a MB tech but I assume it runs off trans tailshaft.  Could there be a loose bearing combined with worn gears such that on coast, it engages the gear, and on acceleration moves away from it?   All I can think of.

That's what I thought after posting problem here; I'll look tomorrow how hard is to get gear removed, and then will check it for wear.