tach

chevyguyjay · Post by **chevyguyjay** » Thu Feb 26, 2009 1:58 pm

so i was at the junk yard today and scored on a 2" mini tach. now, my question is, can this be hooked up to a scooter? im thinking no. all it has is a switch for 4-6-8 cylinder. didnt cost me anything. shure would look pretty on my aero though.....lol

TURN1 · Post by **TURN1** » Thu Mar 05, 2009 7:44 pm

You should be able to hook it right up. Here's mine:

chevyguyjay · Post by **chevyguyjay** » Thu Mar 05, 2009 8:45 pm

well i thought about it, if it does work, it prolly will max out pretty quickly. i was hoping to get some more info from people in here. guess no ones tried it.

mousewheels · Post by **mousewheels** » Fri Mar 06, 2009 12:17 am

Chevyguy - interesting idea - I was looking for a tach to try out before saying anything, but have been thinking about it this week.

Turn1 - where did you tie your tach input wire to? Does yours read the true rpm? (See below)
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Old time tachs would tie to the coil and points and thus would handle 12v switched to gnd. I looked at Sun and Auto Meter sites and found the new tachs can be hooked up that way or to the tach pin of the ECU.

The Honda CDI output to the coil is probably too hot for the tach. It's stepped way up from 12v. That's why there is a separate coil to drive the CDI vs the charge/headlight circuit. I haven't measured a Honda scooter, buy my Yamaha was somewhere around 90-180v on an oscilloscope. So that does not *look* to match up with the electrical diagram above. But then a coil may have transients, and the tach input could have some protection - could work fine on some tachs <shurg>

I think the likely safe connection point is the AC stator coil (Headlight). That should be similar electrically than the auto coil, and should give 1 pulse per rev. Isn't the tach set on 4 cyl calibrated for 2 sparks/rev? So the tach might read 1/2 true revs on a scooter. Not that bad, we could use any old 6k-8k tach, and cover 12-16k. Just have to do the arithmetic...

Or if we were more ambitious, could tack on 2 external magnets to the flywheel and add a pickup to read true rpm.

But don't risk your nice 2" tach to an experiment yet - If I dont find a tach to try by this weekend, I'll hook up my beater auto tach/dwell meter to the scooter.

mousewheels · Post by **mousewheels** » Sun Mar 08, 2009 5:49 pm

Update - connecting a tach to the AC headlight circuit as the closest electrical match to auto coil primary.

Pulled the headlight and connected tach/dwell to the AC.

Started the scooter and the tach did all the right stuff, reading is in range, and changes with rpm. Oddly, the 8cyl range seemed to be right on, around 800rpm. The meter says to double the reading for a 4 cyl engine. And since a 4 cyl fires twice per rev, my thinking was the tach woud read 1/4 true rpm

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As a check, another auto tester was used, this one has an inductive pickup.

At first glance, looked great, '740' rpm. But note the switch is on the 0-6000 range - '3700rpm'

To check the engine analyzer was working ok, connected it to my car, on cyl 1's plug, measured 900. So it works fine on an auto, but not on a scooter. Tried another scooter - Yamaha, and the tach read way high on it too.

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Good news is nothing blew up, both meters readings went up/down with rpm. But the puzzling result is that the cheap tach/dwell read high, and so did the inductive tach.

chevyguyjay · Post by **chevyguyjay** » Sun Mar 08, 2009 7:40 pm

thanks for the info mouse. i just thought it would be neat to mount this thing on my aero.