Alternator W terminal (on a 300tdi)

[julianf]

I never seem to have much luck with alternators.

Ive got this one -

I assumed the W terminal was the one right at the bottom in the 2nd photo, right?

But, when i connect up my aftermarket tacho, and set it to 4 cylinder, i get an erratic 8,000rpm on tick over, and the gauge maxes out at 9,000

That lower terminal is the W though, isnt it?

 

[Bornfree]

If your tacho has a cylinder selector it's for a petrol engine

 

[Red'n'Black]

I had a couple of mid 90s diesel cars that took the rev counter feed off the alternator. But they were calibrated for it, alt spins 3 - 4x faster than the crank and has lots of poles, so the frequency will be a lot higher than what you'd get off the points on a petrol engine.

If you're set on using this tacho the frequency will need dividing down, don't know of anything off the shelf but an Arduino or similar could be made to do it. But I'd check the output of that alternator terminal with a scope first to make sure it's doing what you think it is.

 

[julianf]

If your tacho has a cylinder selector it's for a petrol engine

that may well be my issue then! thank you