Only diesel engines use the W lead driven tachs, AFAIK. Until somewhat
recently diesel engines had no pulsing signal that represented engine
speed other than from an internal connection point in the alternator.
Alternators meant to provide this signal have an extra lead that you can
connect to. Bosch and VW call this a W lead and Vanagon alternators have
a marked spot on the back of the case for the W lead which may or may
not be actually present. Since gas engine Vanagons don't need the W
signal only some replacement alternators have it installed in the
alternator. This so fewer alternators need to be stocked because the
same alternator works across more applications and the cost of the
rebuilder adding the lead is nearly nil. Since the alternator is belt
driven and often by a compound pulley system there is not any known
exact frequency that means an exact engine speed until the tach is
calibrated for a given pulley setup.

Mark

Rocket J Squirrel wrote:
> I've settled on the VDO "Cockpit Black" series of meters for Mellow
> Yellow's upcoming gauging project.
>
> Looking at eguages's offerings in the 2-1/6'' diameter size, 4000 rpm
> range, I find two very similar gauges.
>
> One is the 333 365, the other is the 333 369. They have very similar
> prices and seemingly the exact same specifications:
>
> For use on 4,5,6,8 cylinder application engines with standard
> coil ignitions, most square wave electronic control ignitions
> with tachometer output terminal, and alternators with "w"
> tachometer terminal.
>
> Eguauges does not know what difference there is between them. After some
> searching, I found this:
>
> 333 365 is 4,000 RPM (400–1300Hz)*
> 333 369 is 4,000 RPM (800–2400Hz)*
>
> where * = For Alternator use only
>
> Can someone help me figure out which gauge will read correctly when
> connected to the coil? Maybe they both will, but this difference of 2:1
> in the frequency spec gives me pause.
>
> --
> Rocky J Squirrel
> '84 Westfalia: Mellow Yellow ("The Electrical Banana")
> '74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
> Bend, OR
> KG6RCR
>