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Old 03-27-2012, 05:47 PM
bustedbenz's Avatar
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Location: Lexington, NC or Boone, NC
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300SDL Dead Speedometer

Hey, folks.

A few weeks ago, my 86 300SDL started exhibiting the relatively common odometer failure -- the speedometer needle registered the correct speed but the odo gears didn't turn at all.

So, we took the cluster apart, removed the speedo/odo unit, discovered a broken gear, and replaced the gear. We reassembled everything.

For the first 10 miles after the replacement, the symptoms were now REVERSED. The odometer was clocking the correct number of miles at the correct speed, reliably and accurately -- but the speedometer was reading severely under-speed. As in, I would be driving 30 mph and it would read about 8 mph. I'd accelerate my actual speed to 45 mph, the instrument registered 15 mph or so. At 65 mph actual speed, the needle never moved higher than 25-30 mph.

After 10 or 15 miles of driving this way, the needle suddenly fell to zero, and it hasn't budged again since. The car has since been driven about 500 miles or more, and the odometer has continued to register the correct, accurate number of miles. The odometer is running at full speed -- but the speedometer needle has been at zero without even a wiggle or a flicker or a jump since then.

Any ideas? I'm already pretty much committed to having to either replace it, or else just live with it and use a GPS speedometer. What did we probably damage while the unit was removed, and does it sound fixable?

I'd love for this to be something as simple as a loose wire behind the dash.

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1986 M-B 300SDL, retired due to rust and electrical problems. Donated engine to:
1987 M-B 300SDL, odo dead. New project.
1982 M-B 240D, odo stopped at 308,000
1982 M-B 300SD, 175,000
1989 Dodge Ramcharger, 87,000 - 4wd, 318
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Old 03-27-2012, 08:35 PM
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Since the odometer is reading properly we can assume that the gauge cluster is getting a proper signal. I would guess that it would be the gauge itself that is the problem. They are pretty sensitive so it was probably damaged inadvertently during the gear change. You should be able to just swap out the speedometer module itself with a JY unit.

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