Alternator W terminal to picaxe

[Captain Haddock]

Hi guys, not been here for a while due to other things...
Is there a way to take a feed from an alternator's W terminal (tacho pulse output for diesel engines) to a picaxe and check to see if an engine is running or not?
I'm well rusty with all things picaxe and electric in general so any hardware and software tips are appreciated.
Thanks.

 

[PhilHornby]

For other readers, who, like myself, have never heard of a "W" terminal, I found this snippet on some random forum :-

But along the way I found something interesting with the Dakota Digital DSL-1 module I was using. I had to tap into the alternator windings to create a "W" terminal to drive the Dakota. The W terminal on my other vehicle gives a ~7v ripple, it is a rectified half of the ~14v AC produced by the alternator coils.
But the DSL-1 didn't like this rectified half signal, it was only when I removed the diode to give it the 14v AC signal (+/-7v) that the DSL-1 accepted the signal and worked as expected.

I'm not sure what the expected frequency would be though.

What's the intended purpose of this signal? Could you take a really simple approach and use a connection from the Oil Pressure Warning Light? (which is normally off, when the engine is running... )

 

[inglewoodpete]

Alternaively, monitor the battery voltage. It will be higher when the engine (and alternator) are turning.

 

[papaof2]

The oil pressure light (if available - some few vehicles still have gauges ;-) is the simpler thing to monitor - ON or OFF and no ADC to configure.
Either will work.
I'd expect the alternator's output to be in the human audible spectrum.

 

[tmfkam]

Personally, I'd use an optocoupler, the LED connected through suitable resistor (and possibly reverse protection diode) to the alternator, the 'transistor' connected to the PicAxe.

 

[inglewoodpete]

Personally, I'd use an optocoupler, the LED connected through suitable resistor (and possibly reverse protection diode) to the alternator, the 'transistor' connected to the PicAxe.

Always a good thing to use when connecting microcontroller circuits to vehicle electrics.

 

[Solar Mike]

I used to build and install genset control systems for large multi KVA diesel generators, to get a reliable feedback signal to indicate the motor was running (turning over), we used a magnetic coil sensor placed near the starter motor flywheel teeth.
If the engine was running at all, this would be picked up by the frequency of the coil output; thus the controller would not allow the starter to engage if the engine was running. Looking at other engine sense points like oil pressure sensors, battery alternator sensors etc were too unreliable for failsafe operation.

Cheers
Mike

 

[hippy]

An opto for the interface is probably the easiest and safest approach. Connect that to a HINT pin so one can interrupt a countdown loop and reset the countdown.

When the interrupt occurs the engine is running. If the countdown was zero the engine has been started. If the countdown reaches zero then the engine isn't running, has stopped. Something like the skeleton below -

MainLoop:
  countdown = 0
  SetIntFlags ....
  Do
    Pause 1
    If countdown > 0 Then
      countdown = countdown - 1
      If countdown = 0 Then
        Gosub Stopped
      End If
    End If
  Loop

Interrupt:
  If countdown = 0 Then
    Gosub Started
  End If
  countdown = TIMOUT_MS
  Return

 

[Captain Haddock]

Thanks for the ideas guys.

 

[Mark.R]

Why not use the charge light which is normally labelled up as D+, a throw back to dynamos. This terminal is normally wired through the warning light and to the ignition live, B+, so when the alternator is not charging the light is on as current can flow through the bulb, though the alternator and to ground, when it is there's two positives so no current flow and the lamp goes out. If you connect a relays coil between the D+ and B- then current will flow when charging and the relay will operate and tel you the engine is running, same can be done with the oil pressure switch to double test.
This is how caravan split charging works.

 

[Captain Haddock]

The idea is for engine alarms so the buzzer only sounds if the engine is running and an alarm is triggered else just the light comes on, it seems volvo use the alternator field winding lead that goes to the charge light and a zener diode so the buzzer only gets power when the alternator is charging so this is the way I'm going to head I think.
I was over thinking it with the idea of a picaxe powered alarm control setup.
Thanks everyone.

 

[hippy]

it seems volvo use the alternator field winding lead that goes to the charge light and a zener diode so the buzzer only gets power when the alternator is charging so this is the way I'm going to head I think.

The alternator will only be emitting a signal or voltage when the engine is turning so a tacho signal from that feels to me the most obvious candidate for running or not.

Other signals may work but it seems there's a degree of potential whataboutery which can be discarded when going straight for the tacho signal or alternator output.