harley tach question

first off, i'd like to say what a nice forum you guys have... there is a lot of knowledge floating around here and now i'd like to pick at it.

i've got a cafe'd out 1999 harely sportster 1200 (non-s) that i recently removed the tach from (that thing was just too ugly). but now i hit the rev limiter when getting on it from a red light and i need some sort of tach or shift light. ive been doing a lot of searching on this forum and have found schematics to a digital tach that i think will work for me. my question is, does anyone know what kind of signal is supplied by the line that formally went to the tach? i'm thinking its a single pulse signal, but i'm not sure. also, is there any way to test it to find out what type of signal it is?

any info would be very helpful and greatly appriciated.

thanks
kiel

Hi Kiel

I'm willing to bet that a digital tach is not the way to go, but a shift light might be very good, or both in combination would be OK. The reason I don't like a digital tach is that they are too slow to update, or your eyes are too slow to see the updates, one or the other. An analog tach gives your brain the ability to anticipate when to shift based on the speed of the needle moving towards redline. A digital tach is much harder to interpret. The shift light is a good idea, but you would want to be able to tune it to the rev value that you are comfortable with on the 1-2 shift since that is the one that comes up the fastest.

I don't know bike electronics but it stands to reason that the feed is a pulse stream with rate proportional to engine revs. Need someone else to chime in with the facts though to confirm. To test this, you need to get your hands on an oscilloscope and figure out how to use it. With a scope, its pretty easy to see what you are dealing with by probing the old tach connections while revving the motor.

thanks for the info, i guess its time to find someone with an oscilliscope. i shouldn't have said a "digital" tach, but rather a ten LED array, utilizing the lm3914 led controller, that will flash like a shift light when a hit a predetermined rpm (prolly 300-400 rpms before the hard limiter kicks in). my plan is to have two red 10mm superbrights flanking the other 8 LEDs, which will prolly be six green 3mm LEDs and two yellow 5mm LEDs. this way, when they all flash, it will be quite noticable, even in the day time.

-kiel

The signal being sent to the stock tachometer is usually tied into one of the ignition coils but you will need to condition this signal as not to damage any equipment or your new tach circuit from voltage spikes. You should try to find a wiring diagram for your bike, and on it, you will be able to tell exactly when the tach signal wire comes from. -Jacob

jacob.zurasky said:

The signal being sent to the stock tachometer is usually tied into one of the ignition coils but you will need to condition this signal as not to damage any equipment or your new tach circuit from voltage spikes.

Click to expand...

how would i condition it, with a capacitor?

thanks again,
kiel

ragingxtc said:

how would i condition it, with a capacitor?

thanks again,
kiel

Click to expand...

For bikes without an ECU, I usually tap into the low voltage winding of the coil and use an optocoupler to isolate it from the rest of the circuit.

This is the primary ignition scope pattern.
**broken link removed**