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Old 11-08-2004, 10:49 PM
Registered User
Join Date: Nov 2004
Posts: 2
rpm gauge needle jumping

1983 300sd turbo Mercedes with over 245,000 miles on it with a rebuilt engine at 198,000 or so miles and I am having trouble with the rpm gauge.
My rpm gauge needle suddenly jumps back and forth as high as 55rpm and back to zero. It does this a few times then rests at zero. sometimes it comes back to life in the course of an hour drive and other times it does not.
When I start the car after resting an hour or so the needle seems fine, but after a time the needle jumps again and rests at zero or returns to normal.
Does it have anything to do with the cruise control, because I am using that when it happens some of the time, other times not.
Any ideas????
Thanks from a new poster
and a new Mercedes owner!!!

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Old 11-09-2004, 05:42 AM
SL Owner
Join Date: Sep 2004
Location: East Coast U.S.
Posts: 131

I am assuming the rpm of the car is not actually jumping but instead the gauge is giving a false reading. This can be caused by a bad ground behind the instrument cluster or from corroded contacts. It may also be caused by a failing rpm sensor. What you need is the electrical manual from MB for your car either in print or on CD (fastlane sells them on cd, also ebay I think). You may have to remove the instrument cluster on your car, not necessarily a big deal (search on this site, see mb manual, it slides out). I do not know if on your car whether or not the steering wheel needs to come off to get to the cluster. If so, also search on this site for help on that.
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Old 11-09-2004, 06:31 AM
Posts: n/a
psycho tach

Do a search (in diesel discussion or tech help) for ...

"how I fixed my tach with no tools" By Mr. Goofahrt.

read about the rubber bands... that's what I did.

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Old 11-09-2004, 03:44 PM
Wes Bender's Avatar
Retired User
Join Date: Mar 2001
Location: Alpine, AZ / Green Valley, AZ
Posts: 733
Beerman -

Ditto what joer2001 said. I'll bet that you don't need to buy an expensive replacement tach amplifier. Most of these can be fixed.


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