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Discussion Starter · #1 ·
My 2000 ST4. The tacho shows about OK at idle, but rev the motor to about 4000 and the tacho shows about 10,000. Rev it higher and the tacho goes right off the clock to about 3 o'clock. It's not a damping issue as the needle "holds" position until revs are decreased.

The tacho is fed from the ECU??? Either the ECU output is wrong, or some automotive instrument clusters have an internal voltage regulator (9V) that if they fail and supply alternator voltage (13.4) then instruments will "over read". Come to think of it, the fuel guage is showing about 2/3 when there is little fuel in the tank.

Is there a regulator in the instrument cluster??

Has anyone had any experience with this issue?

Regards, Greg
 

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usually they read low at idle, then come good. if you put a resistor between the power and signal wires that can help these, but i don't recall the rating. usually fixes one that reads low, i've never seen one read high.

or get it on a diagnostic, there's a tacho test that should put it to 3,750 from memory.

check the voltage to it, and maybe the frequency of the signal in. i think the ecu puts out 4 pulses per rotation, maybe 4 per cycle?
 

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My 2000 ST4. The tacho shows about OK at idle, but rev the motor to about 4000 and the tacho shows about 10,000. Rev it higher and the tacho goes right off the clock to about 3 o'clock. It's not a damping issue as the needle "holds" position until revs are decreased.

The tacho is fed from the ECU??? Either the ECU output is wrong, or some automotive instrument clusters have an internal voltage regulator (9V) that if they fail and supply alternator voltage (13.4) then instruments will "over read". Come to think of it, the fuel guage is showing about 2/3 when there is little fuel in the tank.

Is there a regulator in the instrument cluster??

Has anyone had any experience with this issue?

Regards, Greg
The 16M ECU in your ST4 has a 5V regulated source that's used for the TPS and the barometric sender. Probably the gas gauge, too. I would make sure the battery is getting the correct charging voltage, the health of the battery, tight connections on the battery, check grounding lugs, maybe unplug/re-plug the tacho (making sure the connections are clean.

Check the schematic. If the signal passes thru "the big" connector, make sure that isn't green inside. They are notorious for gathering moisture, and corroding. Big connector is under the side fairing, below the fuse box and relays -- just above the radiator.
 

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Discussion Starter · #6 ·
When I had the Tacho issue I was working on the bike in the shed and was using a small truck battery and jumper leads. I needed to ride it up to the house to wash it, so fitted it up with a Motobatt, and the problem went away. WTF!

I still haven't done any tests, but must have been either a bad connection, or the truck battery dragging the charge voltage regulation down. It's got me curious. I might try and simulate the issue again sometime....

Thanks to all

Regards, Greg
 

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My tacho has been acting up lately so I decided to have a look. I'd previously tightened the 3 screw nuts on the back and they we're still tight but I noticed the 3 pin plug was loose. Not the plug going in but the part attached to the back of the gauge. I carefully removed the bezel by going around the back with a box cutter and then prying the ring off.
Once inside the problem was obvious, the solder was cracked at the base of the connector. I resoldered the connector and used some hot glue for additional support then glued the bezel back on.
 
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