W terminal output? PWM? - IDI Engine - VWDiesel.net The IDI, TDI, and mTDI source.

• W terminal output? PWM? by Alcaid on 01 Oct, 2010 00:10
• 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?
• #1 by bajacalal on 01 Oct, 2010 09:50
• 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.
• #2 by Vincent Waldon on 01 Oct, 2010 09:59
• 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
  
  
• #3 by Alcaid on 01 Oct, 2010 10:01
• That would be excellent! So duty cycle is constant then (50%?) and the frequency is the variable?
• #4 by Vincent Waldon on 01 Oct, 2010 10:12
• Yup... it's really just one leg of the alternator's winding before rectification, so a square wave.
• #5 by regcheeseman on 01 Oct, 2010 13:57
• 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
• #6 by Alcaid on 01 Oct, 2010 14:00
• Actually, i have a plan to build a VNT controller to control a GTB2056VK turbo on a 1.6TD, so rpm input is needed.