Replaced the Sending Unit for the tach today, old one broke about 6 months ago and tach was working up until then. Now at Idle the tach shows 0 rpm as soon as the accelerator is pressed in the tach jumps up and shows 1200 and seems to be working up through 2000 and 3000. When the accelerator is released it falls back down to 0 at idle. I thought about alternator voltage first and shows 13.7 at idle and 13.7 at 2000 rpm with the battery connected. Any Ideas?

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1983 Mercedes Benz 300D Turbo (Daily Driver)
1979 Mercedes Benz 300CD (Needs Lotsa Work)
1963 Chevrolet C10 Stepside Shortbed (Not Stock)
1999 Chevrolet Silverado Z-71 (Fun to Drive)
2007 Nissan 350Z (Wifes Car - Lucky!)

No difference whatsoever!

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1983 Mercedes Benz 300D Turbo (Daily Driver)
1979 Mercedes Benz 300CD (Needs Lotsa Work)
1963 Chevrolet C10 Stepside Shortbed (Not Stock)
1999 Chevrolet Silverado Z-71 (Fun to Drive)
2007 Nissan 350Z (Wifes Car - Lucky!)

New tach amp?

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83 SD

84 CD

Sound like it could be too large a gap in the pickup. Try adjusting it.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked