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Discussion Starter #1 (Edited)
My speedo bombed...screamed, spun around madly and then totally destroyed itself internally, including breaking the needle, unscrewing the face screws and the face spinning, then all the guts threw up on themselves. So it's DEAD DEAD.

Ok, fine...gotta replace the Speedo...well, while taking it off and inspecting the gauge set, the Tach has a broken cable attachment that was discovered as it was held upside down so I could unscrew the back. Which explained why it jumped around a bit. It's an 1983 bike and looks to have been parking lot dropped at least two or three times...so there we are.

Started looking for replacements...Craigslist turned up a similar set for a Magna V30 500. $50 pricetag says go get it and figure it out. Speedo switchout was easy enough, just switched the Bulb Connector & installed, then got to the tach and went...WOOPS!! It's not a cable tach...DUH!

Put speedo on the bike & tested it out, Speedo checks out correct as far as the wife's van is with matched speeds up to 55. So that's ok. Same tire either way...good there.

Currently at a halt...trying to figure out how the heck the Tach is going to get wired up...
Also, it has a few more lights on it than the Original...3 extra idiot lights...Oil, Kickstand, and Temp.

So...are there any helpful hints on how to use a newer tach on an old bike?? At least the Wiring is all Honda ;)
Should I go in search of a Magna to get the wiring harness and figure out how it was originally wired?? Do know where a crashed one is...an hour away...

Ooooorrrrrrr....should I just go find a nice used Honda cable Tach and slap it on fast to keep it simple??
 

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You have a couple options..
You can find a multitude of "inductive tachometers for motorcycles" online. These work by sensing induction off a spark plug wire and do not need to be connected into the ignition circuit.

Most tachs use the interruptions in the signal generator circuit to translate into RPM, and many electronic tachs have either a selector or jumpers to let the tach know how many cylinders so the readout is correct.

Try to find a Magna wiring diagram and see which tach lead connects to the signal generator or a different part of the ignition circuit.

Or the other options you mentioned.
 

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Discussion Starter #4
I found this: http://v4musclebike.com/articles/V30ShopManual.pdf
Chapter 18 is what you need.

From a cursory glance it appears the tach gets its signal from the ICM box.
Not sure if you can work around this with the VT500 ignition system or not.
THANK YOU! That really helps!!
So far I've also found a few helpful Utube vids on the subject as well.
Think I'll dive into it this weekend and see how it turns out...
If it doesn't work, will just go find a cable tach and solve it the easy way,
then sell the Magna tach on ebay or craigslist...
 
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