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VDO Tach Questions
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dgsaz
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Joined: November 04, 2008
Posts: 241
Location: Phoenix / San Diego
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PostPosted: Sun Aug 06, 2017 11:43 am    Post subject: VDO Tach Questions Reply with quote

I bought a NOS VDO Tachometer here on the Samba. After reading the instructions I'm wondering if this tach will work for me at all.

It appears that this tach was intended for use with a diesel engine and receives it's "signal" from a "W" terminal on the alternator.

A search of the internet yields instructions for other VDO tachs for wiring to the coil. Nothing so far says you can wire either way. It looks, to me, that this unit is exclusive to diesel and wires to the W terminal.

I don't know if my alternator even has a W terminal. Anyone have any experience with this subject? Attached are the instructions from the package and a download form the net.

It would be great to find out it's either or........Thanks.

dgsaz

Original instructions from the package:
Image may have been reduced in size. Click image to view fullscreen.


Other VDO instructions purloined from the web:

Image may have been reduced in size. Click image to view fullscreen.
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mikedjames
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Joined: July 02, 2012
Posts: 1215
Location: Hamble, Hampshire, UK
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PostPosted: Sun Aug 06, 2017 11:57 am    Post subject: Re: VDO Tach Questions Reply with quote

As it clearly shows in the schematic the W terminal is a direct connection to one of the windings of the alternator.
It would need pulling the alternator to get at the rectifier pack and take a connection off there.
But having done that put a fuse in the wire as it can deliver maybe 30 amps shorted and run it to the front. Your alternator AC fed rev counter will work more reliably with that connection than the type connected to the coil.
Then all it needs is calibrating as unlike coil driven rev counters (4 cyl = 2 pulses per rotation of the engine) it cannot know the ratio between engine and alternator rpm.
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