W terminal output? PWM?

[Alcaid]

Hi,

does anyone know what the output is on the W terminal on the alternator? Is it PWM with varying duty cycle? At what voltage?

[bajacalal]

It's an alternating current source from one of the windings before the alternator diodes.

This allows you to determine the engine speed on a diesel engine based on the frequency of the current (with a tachometer designed for this). Even the cars that didn't have tachometers had some dashboard functions that required an input for engine speed.

[Vincent Waldon]

It's an 8 V RMS square wave... frequency dependent on RPM.  There's a thread with oscilloscope pictures... I'll see if I can find it.

EDIT: here it is: http://www.vwdiesel.net/forum/index.php?topic=13499.0

[Alcaid]

That would be excellent! So duty cycle is constant then (50%?) and the frequency is the variable?

[Vincent Waldon]

Yup... it's really just one leg of the alternator's winding before rectification, so a square wave.

[regcheeseman]

I don't know what you want it for but I'm getting a more accurate engine speed signal from he pressure switch mounted in the pump head.

It's a normally closed switch and triggers at twice crank speed so perfect to drive a gas tacho with no conditioning - just a 12v supply.

For my application I'm taking the 30 - 230Hz signal to a LM2917 F-V converter to give a 0-5 signal for an ADC to sample

[Alcaid]

Actually, i have a plan to build a VNT controller to control a GTB2056VK turbo on a 1.6TD, so rpm input is needed.