Motorcycle Tach issue. It works, unless the headlights are on..

[Daniel G]

I have a g4x Monsoon on my cbr900rr turbo.

The ignition system was originally TCI, wasted spark, with a tach signal wire out from the tci unit. It had no issues.

The link does run the tach without the headlights running, but is also in a permanent fault state (Aux1). Charging voltage is 14.3, on first start up when battery voltage is below 14, the tach jumps on and off, and then comes right.

I was happy to ignore the fault, but first time I used the headlights, tach dropped to 0. No signs of life at all. Battery voltage is still at 14 or just above depending on rpm. I have also checked for any high resistance on the tach signal wire and earth. Both ok.

Any ideas?

Thanks, Daniel

[Vaughan]

Might be that the original tacho output was from a coil negative and it was looking for a big voltage spike with a threshold of ~14V. If so you will either need to modify the factory tacho (common for MR2s and Celicas when going ot individual coils) or a tacho booster wired inline.

If you want to confirm the signal from the ECU is still operating correctly you will need to put an oscilliscope on it, the ECU pulls low and has a weak pullup to battery voltage, if the ECU isn't actually pulling up to battery voltage when the tacho is connected (scope with tacho connected, scope without, check height of square wave) then maybe a stronger pullup will fix it.

[Daniel G]

Thanks for your reply. You were correct.

I tested by connecting tach feed wire to coil negative. The tach was stable here. (But displaying half revs)

I will put a tach booster inline.