GS 550 Electronic tacho operation

[Big N daft]

Looking at a couple of queries in this forum has remided me of a minor problem that I have with my '86 GS550ES. basically I frazzed the CDI a while back and decided to fit a boyer bransden elctronic iginition. Anyhoo the bransden ignition isn't designed specifically for the GS550 but for a bike like the GSX250 which is a twin, so the pickups on the rotor are designed slightly differntley. Basically the upshot is that the ignition fires twice as often as it does using the normal Suzuki CDI. Ultimately this means that the unit fires each plug at the top and bottom of each piston stroke as opposed to the normal design firing of at the top of each stroke, double double redundant sparking if you will. As the tacho is an electronic version the reading given is now at twice the RPM that the engine is actually running at. Not a major problem but as I am now in the process of making a new wiring harness I may as well sort the problem.

Looking at the wiring diagrams there doesn't appear to be any info on the circuitry involved with the tacho, not even a hint on where the pulse is taken from. I am assuming that the pulse is taken from one of the rotor pickups.

Examining the tacho mechanics and circuitry itself has left me thinking that it is basically measuring current flow through the meter and not an actual pulse count. This is based on the fact that there appears to be no "complex electronic" counting components on the PCB just a few resistors, capacitors a diode and what appears to be a resistor pack (I could be wrong on that one though).

Can anyone confirm how these tachos operate and what suggestions they would have to recalibrate the tacho to its new range.


THANX

[Billy Ricks]

Tach signal comes from the signal wire to the coils.

[Big N daft]

Thanks Billy, I'll trace the input back from the tacho through the harness and see where its taps the signal.

[Billy Ricks]

You have one color that should be common to both coils. You can use either of the other two that are not common for signal to the tach.

[Big N daft]

Thanks Billy, the strange thing is though, that the input to the tacho is on a brown wire (which is why I assumed the feed came off the rotor pickups) and I can't find where this tees off the coil feed. I can accept what you're saying electrically but I can't find the physical connection.........


Anyway I'll have a play around with it as long as someone can confirm the tacho's mode of operation and I can get an accurate reading I'll be happy enough.

[Anonymous]

There are numerous connections in the harness that are soldered. You won't see them unless you remove the tape from the harness.

Steve

[Big N daft]

I am currently making my own harness up and I have split the old harness back to no avail. Anyway its not an issue as I can sort it with my new harness, what is still the bugbear is the doubling of the tacho reading and how to sort it.

[Big N daft]

Well I finished the harness and managed to trace the pick up on the old harness back to its source, someone had pieced the wiring out and changed the colour of the cable which is why I'd had problems chasing it back. Turns out it ran off from the ignitor end of the coil feeds. Anyway in the absence of any advice otherwise I am just going to measure the resistance across the tacho coil and stick a ballast reisitor in parallel and see what the result is, I'll let you know how I get on if it works I'll see about getting a comparable coil and doing a proper job but Im reluctant to do that just yet (groan).

[SqDancerLynn1]

You may have to modify the tach itself to make it work

[Big N daft]

You may have to modify the tach itself to make it work

Any chance you could elaborate a bit, looking at the thing it appears to be a simple moving coil meter. if that is the case then basically it seems it is measuring the current flowing through it from the spark pulses to the coils. Now as the sparking pulses have been doubled (effectively doubling the current through the meter), if I place a similar resistance/reluctance in series (not parallel as I said before) with the meter then it should then draw half the current through the meter and hopefully give a More accurate reading.......or am i over simplifying it all

[kz]

There are basically two way of making the mod to the tacho:

1.
Divide the frequency of the tacho input signal with 2, possible to do with dual Flip-Flop circuit or simular.

2.
Change the tacho analog output to the metercoil itself, with a resistor in serie, I don't know how it would influate the linieraty of the tacho, but you have to test.

You must also consider, that the tacho in this case, must be able to handle the double frequency on the input as it was designed for, but it maybe can handle that.?

Hope it is of any help....

[Big N daft]

There are basically two way of making the mod to the tacho:

1.
Divide the frequency of the tacho input signal with 2, possible to do with dual Flip-Flop circuit or simular.

2.
Change the tacho analog output to the metercoil itself, with a resistor in serie, I don't know how it would influate the linieraty of the tacho, but you have to test.

You must also consider, that the tacho in this case, must be able to handle the double frequency on the input as it was designed for, but it maybe can handle that.?

Hope it is of any help....

Thanks Karl,

1. I looked at doing this but I figured as the meter isn't a digital device I was getting a bit too complex.

2. was the way I thought to go. The tacho already works now at double speed it's just that it shows red line when the engine is only running at around 7000. I'll have a play with the resisitor, I have a digimeter that has a frequency counter so I can valdiate the effect of the resisitor on the tacho. I'll let you know how it goes

[kz]

I would also go for the resistor way (Nr 2)

The first method is the more fancy one, from a technical point of view, but also more complicated.

If possible, you can meassure the output analog voltage to the metercoil.

If possible, you can messure the current as well i.e. by meassuring the voltage over an existing resistor and calculate backwards.

Then you have these values it should be possible to calculate the value of the resistor needed.

I think the metercoil is a type, don't know the english word, but translated it would be turn-coil-instrument, and they normally use very low current, so I think you will be succeselful.

If you have access to an frequency-generator and frequency counter or oscilloscope, it is also easy to calibrate it on the bench, of course you can always measure the frequency by running the bike and calibrate that way....

Hope it works out fine and I would be interessed to know how if/how it works.