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Discussion Starter · #1 ·
So a little rundown of my situation and bike.
98' CBR 600 F3

I bought a tach that came off of a 91 GSX750 Katana, wired it to my OEM tach signal wire, power and ground from the harness where the old ones were once connected via a ribbon plug. Problem is; the idle is reading between 2500-3000 which I know to not be correct thus giving me WAY higher RPMS once up to speed (25mph in second most certainly should not be reading 9500-11000 rpm). Could there be something wrong with the tach or did I fuck up and buy a tach from a larger bike that is not able to get proper readings from my smaller displacement bike?

All connections were made with electrical solder and shrink wrapped.
 

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Tach's are calibrated differently depending on the type of signal the bike outputs, as well as the ignition style. "Most" bikes use a common square wave signal, and if you tach is reading consistently (as in not jumping all over the place), that's not the issue. I'm sure your connections are fine.

It sounds like it's reading about double actual rpm- which means the tach likely came off a bike with a non-waste fire ignition, meaning it only fires the plug at the top of compression. I suspect your bike is waste fire, meaning they fire the plug at top of compression and top of exhaust(the "wasted" fire). So the tach sees twice as many ignition pulses and thus reads double what it's calibrated for.

For your case, you'd either need to get an aftermarket tach with an adjustable pulse-per-revolution, or rig up a new tach signal wire to only feed it the pulses from 2 cylinders, with a diode facing toward the tach off each coil's hot wire, which should correct the problem.
 

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Discussion Starter · #4 ·
Tach's are calibrated differently depending on the type of signal the bike outputs, as well as the ignition style. "Most" bikes use a common square wave signal, and if you tach is reading consistently (as in not jumping all over the place), that's not the issue. I'm sure your connections are fine.

It sounds like it's reading about double actual rpm- which means the tach likely came off a bike with a non-waste fire ignition, meaning it only fires the plug at the top of compression. I suspect your bike is waste fire, meaning they fire the plug at top of compression and top of exhaust(the "wasted" fire). So the tach sees twice as many ignition pulses and thus reads double what it's calibrated for.

For your case, you'd either need to get an aftermarket tach with an adjustable pulse-per-revolution, or rig up a new tach signal wire to only feed it the pulses from 2 cylinders, with a diode facing toward the tach off each coil's hot wire, which should correct the problem.
I apologize for my lack of knowledge. But could you explain what exactly your recommendation would be?


This is the ignition coil setup for the f3;

 

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Discussion Starter · #6 ·
It is a wasted fire system. Wire it to only one of the coils and that should solve your problem

Thanks for your response, I am grasping the concept of why its doubling with the different types of ignition firing. And all things seem to lead to the rewiring to one of the coils, but as my last post states that Im not really sure "where" or "how" to wire it to one of the coils? I have a diagram that I posted above, Im assuming one of the two "wires" where the electric connectors are coming from on the coil is what youre referring to?
 

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Hey Remove, I believe what you need to do is this: you have two wires that plug into each coil. Take one wire from one set of coil wires, and the opposite wire from the other pair of coil wires, and plug them into the same coil. I could be totally fucking wrong here but it makes sense to me. Polarity doesn't matter with coils. There is not dedicated positive or negative for each pair of wires from the harness that plugs onto the coils. Jmartin will set you straight I'm sure. I'm just spitballing here.
 

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That's what you need to do. Take both tach words and write them to the same coil. Right now is reading your rpms like it's a v8 since the plus are firing twice per stroke. Wiring it to one coil cuts that in half.
 

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Discussion Starter · #10 ·
That's what you need to do. Take both tach words and write them to the same coil. Right now is reading your rpms like it's a v8 since the plus are firing twice per stroke. Wiring it to one coil cuts that in half.
Awesome, I'll give this a shot when I get off work tonight! Thanks to all for the info.
 

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I apologize for my lack of knowledge. But could you explain what exactly your recommendation would be?
You have two wires going to each coil, a ground and floating hot that the ECU closes to fire the coil. The tach signal the ECU outputs is the on-off-on-off power it's firing to BOTH coils. You only want the signal from ONE coil. Get a new wire, tap it into the + side of only one coil, and run it to the tach's signal input. The existing tach power and ground are fine. Leave the stock signal wire disconnected, and sealed so it can't accidentally contact ground.
 

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Discussion Starter · #12 ·
You have two wires going to each coil, a ground and floating hot that the ECU closes to fire the coil. The tach signal the ECU outputs is the on-off-on-off power it's firing to BOTH coils. You only want the signal from ONE coil. Get a new wire, tap it into the + side of only one coil, and run it to the tach's signal input. The existing tach power and ground are fine. Leave the stock signal wire disconnected, and sealed so it can't accidentally contact ground.

You're awesome, how would you recommend I go about this? I've used Scotch locks before doing some tag light stuff. Or should I cut the wire and splice the two, then solder them to the existing connector that's already attached to the coil? I'm an extreme noob to electrics, not exactly sure what is "safe" or the "proper" way just yet.. It's all a learning process to me right now. Any and all helo/suggestions are extremely appreciated in advance.
 

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Get a few supplies and teach yourself to solder. It's easy actually. And you'll wonder why you ever spent so much money on all those fiddly little connectors.

Harbor Freight sells a couple nice butane mini torches. I love mine. Clean it by melting flux on it then drop the solder on. You'll know when the solder is "done" when you see it. It'll go liquid and cling to the wire. Heat shrink and your done.

Way cheaper, faster and easier than crimp on connectors or anything else IMO. But get a torch. Irons suck and take too long. Torches don't have cords to muck with either.
 

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Discussion Starter · #14 ·
Get a few supplies and teach yourself to solder. It's easy actually. And you'll wonder why you ever spent so much money on all those fiddly little connectors.

Harbor Freight sells a couple nice butane mini torches. I love mine. Clean it by melting flux on it then drop the solder on. You'll know when the solder is "done" when you see it. It'll go liquid and cling to the wire. Heat shrink and your done.

Way cheaper, faster and easier than crimp on connectors or anything else IMO. But get a torch. Irons suck and take too long. Torches don't have cords to muck with either.
Thanks, I have been doing quite a bit of soldering but only new wires as an "extension" or repairing connections. Basically, I don't really know what "tapping" into a wire means. I suppose I could just Google it and wing it, I just thought I'd get my knowledge for this application in specific rather than general knowledge of it as a whole for any application.
 

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Sometimes you need three hands while soldering . One thing you can do is go ahead and apply solder to each end of the wire you want to attach. That way solder is already present and you don't need a free hand to hold your roll of solder. Then just hold the two ends together and heat them again. The solder Will re-flow and join the two wires.


Or, get one of these. HF has them for about $6-8 iirc
 

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Discussion Starter · #19 ·
Oops. Forgot the pic.
aaaah, the "helping hand". contemplated getting one of these..
had the girlfriend hold the iron steady while I moved the wires and solder to it, didnt end well.
I have a 1/2" long burn on my thumb..:speechless:


Im hoping to tackle this tonight or tomorrow. Ive got wayyyy too much on my plate right now. I keep finding myself muttering "...now where did I put that 2 foot long torque wrench?? Its two god-damn feet long, how do I not see it??"
And even "why is this bike not starting? OH! Maybe it has something to do the battery on the charger right next to my feet..."
 
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