After market tach connection

[audijunky]

I bought an after market electronic tach. It has four leads. two for power and one to connect to the coil, and the other to ground. After looking at the wiring diagram in the Clymers, it looks like the only wires going to the tach are for lights and the fuel gauge. Any one know how the old tach worked? It is also for a two cylinder engine/which I guess only has one coil? SO I guess I should be able to hook it up to both coils low voltage supply side? Let me know if you guys think this will work if I cant use the wires that are already present from the old Tach.
Gracias in advance

[randy]

Steve,

Confusion:

Four wires, two for power (+ and Ground), one for coil, what's the other ground for?

DON"T connect it to both coils, only connect it to one or the other. Either will give you the right RPM reading. If you hook it to both, you'll only short the coils together and the bike won't run.

Hook it to the return side, either the Gray or the Orange. (The red/white is the feed side of the coils)

BTW, thanks for the avatar!

[oseaghdha]

Originally posted by randy
Steve,

Confusion:

Four wires, two for power (+ and Ground), one for coil, what's the other ground for?

Maybe the light.

[TopCatGr58]

Hey Steve,

You can't use the signal (white) wire from our wiring harness for your aftermarket tach, that signal is from the alt., not the coil. Randy is correct(of course, he's one of the electrical gurus) in connecting it to the (-) side of ONE COIL. Your AM tach probably has 2 power leads, one for the actual tach, the other for the light!?

IF the 2 cylinder engine it was designed for had only 1 coil then it probably won't read correctly (Half-Fast)because that engine/coil would have fired 2 times per Revolution, the second pair of firing happening on each cylinders exhaust stroke. Our coils for each pair fire only once per revolution because the paired cylinders are moving together both up or down, so the timing can be more delayed and shared, comp for 1, exh for the other, wasted spark design again, but different since the pistons are in the same position on the crank for each pair.

In a 2 cylinder engine the pistons are usually opposing each other, one up and one down, so the comp stroke of one follows the compression stroke of the other, then both go thru the exh stroke again one after the other a 1/2 crank rotation at a time, but the coil triggering mechanism comes around again and fires the coils on the Exh stroke of each cylinder, so you get 2 triggers per revolution.

So...when it shows 2k rpm, the XS will actually be turning 4K!

However, if the engine the tach came from had 2 coils, then it would read correctly! Good Luck!
T.C.

[Pat Kelly]

My 400cc Honda Hawk (2 cylinder) had both pistons moving up and down together. This had it acting similar to a single cylinder 2-stroke (right fire, left fire, right fire, left fire). As one cylinder fired and was on the power stroke, the other was on the intake stroke.
Chain-driven counter balancers kept things smooth.

[audijunky]

Thanks for the advice fellas, I believe the Tach is for a V-twin. It stops at 8k. only one I could find at a decent price. Any way If I hook it up to one coil ,I should be reading half the actual RPM. This Q is for the Electrical GUru, Could I hook it up to both using an in line diode. So that the voltage only goes one way(to the tach) and not to the other coil. Could that possibly give me the actual RPM? I guess the other ground is a return for the voltage coming from the coils. Maybe the light ground and the coil ground are not connected internally in the tach? About the wires There is a + and - wires for the light. There is one wire that connects to the coil(s) and one wire to connect to ground.
Thanks fellas

[randy]

The diode may work, use a diode on each coil so one can't backfeed the other, anode on the coil and cathode to the tach. Small 1N4001 should be sufficient.

[LoserShoes]

On our bikes both coils fire once every 360 degrees. The difference is one set fires 180 degrees apart from the other.

It depends on the firing configuration of the engine it was designed for. Some Vtwins fire once for 720 degrees with two coils and the trigger off the cam.

Also the voltage at the coil is not 12v, because of the TCI, but that should not be a problem.

Got a link to some tech data for the tach you have?

Steve

[randy]

It depends on the firing configuration of the engine it was designed for. Some Vtwins fire once for 720 degrees with two coils and the trigger off the cam.

Steve, what I think you meant was that each cylinder of ANY four stroke fires once every 720 degrees?

So, for a twin, one firing every 360

A triple, every 120

A four, every 90

[TopCatGr58]

Uhhh Randy,

I believe your thinking cap is on too tight!

The XS11 4 fires once every 180 degrees of the crank, even if you ignore the wasted spark cylinder, takes 2 complete revolutions for a complete combustion cycle=720, divided by 4 cylinders=180.
Remember there are 2 pickup coils, they get a signal every 180 degrees.

A triple should fire every 240 degrees. Just wanted to FYI!
T.C.

Pat, I had that Yamaha TX500A that came with a counterbalancer, but I didn't get too much into the mechanics of it, but now that you mention it, I think both cylinders came up at the same time as well, hence the counter balancer!

[randy]

Thanks for the wake-up call, TC. Before typing I should make sure brain is in gear!

[audijunky]

the only info I have on the tach is that it is for a two cylinder 4 stroke. Not sure if this helps at all. Sure doesnt help me. Also when looking at the wiring diagram in the clymers, It looks like the 2 coils share the red/white wires and have seperate grey/the other color returns (if you will). So where should I start. Should I hook up one coil to the lead from the tach and see what happens? THen if I get half the rpms use the dide config. on both coils. I think I will try this plan. What do you think?

[chopwatchagot]

I certainly cannot add anything to the electronics discussion going on here but I would ask the question: What is your goal here? There are used tachometers on eBay all the time, and probably for less than the aftermarket deal. Just a thought.

Dean

[LoserShoes]

Yea the red is common (hot) and the grey and brown are pulled to ground by the transistor in the TCI. Just touch the tach lead to one or the other. Either the tach will respond, or it will kill that pair of cylinders. If the you get a misfire when you do it, the tach is loading the circuit too much (unlikely but could happen) and can't be used.

Of course the other wires from the tach need to be connected too

Good luck, with the lack of tech info all you can do is try it.

Steve

[audijunky]

Thanks for the replies fellas. I think Ill take your advice and just try it. The goal of this is asthetics. I have no stock lights or instruments, everything after market. headlight, signals, brake. The reason I am trying to use this specific tach is it matches my new speedowhich is fully functional.. The stock cluster was way too bulky for my tastes.