Voltage on Black / Purple Tachometer Wire

[Gordo2]

Folks,

Tossing this out to the wider community of folks who have worked on 911 tachometers.

When I connect a multimeter to the black/purple wire that comes from my tachometer - I get a voltage reading of ~ 10v (key on, and the BLK/PPL wire only connected back to the vehicle tachometer / bare open ended wire - not connected to a RPM signal source).

I assumed that the BLK/PPL wire would receive a voltage signal from a source (CDI, dizzy, signal generator etc.) - as such I'm surprised to find the wire has voltage which appears to be coming from the vehicle's tachometer.

Anyone know if this is normal, or is something mixed up with my tach wiring?

Thanks, Gordo

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For background:

I'm troubleshooting a problem I'm having getting RPM to work correctly when connected to my vehicle tach and the LM-2 wideband meter. RPM Input to Innovate LM-2 from XDi Igntion

[timmy2]

What terminal is the black/purple wire connected to on the tach?
Should be TD with the white/red from the rpm limiter. Is the rpm limiter switch imposing a voltage on it?
Pull the limiter wire and see if it goes away.

[Jonny H]

Gordo,

The voltage on the TD wire at the tacho is normal.

The TD wire does not 'drive a voltage to the tacho', rather the CDI box is a current sink for the tacho. With the wire disconnected, you'll read more or less the supply voltage (12v) on the TD pin on the tacho.

With the wire connected and the engine running you'll see a square wave of approx 6.8 volts peak. Why 6.8 volts? Because that is the internal voltage of the circuit in the CDI. Basically, the CDI box pulls the tacho TD line to ground to create the signal.

[Gordo2]

Johnny H - great explanation, thanks.

Timmy2 - not sure where the BLK/PPL wire actually terminates, I I tapped it at wire harness in the engine bay.

Gordo

[304065]

You shouldn't be measuring the voltage with a voltmeter, you must use an oscilloscope.

HELP! tach died and I need to know what can replace it!

[boyt911sc]

Originally Posted by Gordo2

Johnny H - great explanation, thanks.

Timmy2 - not sure where the BLK/PPL wire actually terminates, I I tapped it at wire harness in the engine bay.

Gordo

Gordo,

The black/violet wire is connected between tachometer terminal TD and CDI terminal TD. Please refer to circuit #95 for the USA SC factory wiring diagram.

Tony

[Gordo2]

Originally Posted by 304065

You shouldn't be measuring the voltage with a voltmeter, you must use an oscilloscope.

HELP! tach died and I need to know what can replace it!

Agree however, my black/purple wire isn't connected to a rpm source therefore an o-scope would show a flat line voltage...

I simply found a static voltage in a location I didn't expect.

Gordo