Sometimes the tach just goes wild. It may peg at max rpm's, or bounce back and fourth. I've installed a new tach amp (round thing on drivers fender), and replaced the original tach with a used one. The problem is the same. I need some help guys. Any ideas? Thanks in advance.

You have a ground problem or loose wire. You will have to check from top to bottom. Do you have a ETM? (electrical troubleshooting manual?)

They are in the CD we offer in Partsshop.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope :)
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

try removing the instrument panel .clean the ground connection between the plates holding the various meters after removing the screws.
it worked for me.