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John Kirchhoff
08-23-2006, 02:59 PM
I came across a VDO electric tachometer that looks pretty good in my Hawk I'm rebuilding. It uses a signal from either a signal generator or from an alternator. I'm using a factory Motorola alternator which of course doesn't have a tach connection. I've read where you can tap into one of the three field to diode wires to use as the signal. Has anyone tried this before?

Dick Steinkamp
08-23-2006, 03:59 PM
quote:Originally posted by John Kirchhoff

I came across a VDO electric tachometer that looks pretty good in my Hawk I'm rebuilding. It uses a signal from either a signal generator or from an alternator. I'm using a factory Motorola alternator which of course doesn't have a tach connection. I've read where you can tap into one of the three field to diode wires to use as the signal. Has anyone tried this before?


I had never heard of a tach getting its signal from an alternator until you posted this. Here are some downsides of the system...

http://www.amplepower.com/primer/tach/

Sounds like it might ba a can of worms.

I do remember the early Corvettes with the tach mechanically driven off the back of the generator. Worked fine until the belt started slipping [:0]


http://thenobot.org/images/s2d/s2d_01.jpg

Sudsy
08-23-2006, 05:47 PM
A lot of *** diesels run the tach off the alternator but i think if you start using different pulley sizes it could alter your reading

Regards Bob

John Kirchhoff
08-23-2006, 10:47 PM
Thanks for the web site, Dick. They sure make it sound impractical but I know the system works. I have a diesel tractor that has an electric tach getting its signal from an internally regulated GM alternator. I don't know about the no load-no signal problem mentioned because the only electical device on the tractor that can't be shut off is the fuel solenoid on the pump which draws very little juice. In fact, most all of those type GM alternators have a place on the casting where a hole is drilled for the signal tap. Sudsy is correct in that pully size effects the number of signals the tach receives for a given revolution, but you'll find an adjustment on the back of the tach which allows it to be calibrated to correspond with the true engine speed as shown by a diagnostic type tach. Thanks guys, I'll keep you posted if I get it figured out. john