Using a constant current source, put 30 to 50mA through the pickup coil (pins 7 and 9 in the amp socket). With the scope on AC coupling, you should get a zero volt signal when measuring from pin 7 (gnd) to pin 9. When the engine is started, you should see a small 'blip' once per crank rev.

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Bob
'82 300D Petrol B-G Metallic

I bought one about a month ago after not having a tach for almost a decade.
Works perfectly; utterly simple installation. Great piece of work!

price????????

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

If a scope is not available, you can use a DVM in the AC volts setting to read the voltage. If you rev the engine and you see a proportional increase in voltage to RPM, it is likely the tach pickup coil is good. If you get no voltage then it is toast.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked