No tach on turbo diesel

Hey guys I have a 1985 524td and I don't seem to have a tach. I changed out
the clusters and still no tach. Is there a special sensor or something I
can check? Everything else works like it should. thanks for the help, ben

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"Ben Dixon" > wrote in message
...
> Hey guys I have a 1985 524td and I don't seem to have a tach. I changed
> out
> the clusters and still no tach. Is there a special sensor or something I
> can check? Everything else works like it should. thanks for the help,
> ben
>
> --
>

A diesel obviously has no ignition pulse, therefore you need a different
method to read revs. In the early 80's tachs on diesel cars were uncommon.
You need a tach that works from the alternator.




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Ben Dixon wrote:

> Hey guys I have a 1985 524td and I don't seem to have a tach. I
> changed out the clusters and still no tach. Is there a special
> sensor or something I can check? Everything else works like it
> should. thanks for the help, ben

The original cluster probably has redline marks for each gear on the
speedometer. But if you're getting anywhere near redline with a diesel, you're
doing something wrong.

Matt O.

Ben Dixon wrote:
>
> Hey guys I have a 1985 524td and I don't seem to have a tach. I changed out
> the clusters and still no tach. Is there a special sensor or something I
> can check? Everything else works like it should. thanks for the help, ben

I am a little unclear about what you mean about "not having a tach.".
Did the original cluster not have a tachometer, or did it have one that
wasn't working?

Regardless, as Chris pointed out, the tachometer pulse on diesel cars
that vintage usually came from the alternator. The alternator would have
an extra terminal (output from the stator winding taken before the
rectifier diodes). On VW's the terminal is labeled "W" (welle???),
perhaps other German manufacturers use the same nomenclature.

If your car had a tachometer that wasn't working, a good guess would be
that some time in the past the original alternator was replaced with one
that doesn't have the W-termainal. If you are comfortable with a
soldering iron, you can usually add W terminal yourself. You need to
pick the alternator apart just enough that you can get access to the
rectifier diodes. There will probably be 6 beefy ones and perhaps a
handful of smaller ones (for powering the field winding). Each of the
beefy ones is connected to ground or the main +12V output at one end and
one of the stator winding taps on the other end. You need to solder on a
wire to the junction between a diode and the stator winding. Any of
these junctions will do. This wire will carry the pulse needed for the
tachometer.

Calibration is another issue. If your tachometer is from a gasoline car,
odds are it will be way wrong for a diesel car. On a 4-cylinder, 4
stroke gas engine that vintage, the coil primary (where the tach. signal
came from) had 2 pulses pr. revolution. 6 cylinder engines had 3 pulses
pr. revolution, and 8 cylinder engines had 4 pulses / revolution. Nice
and predictable.

On diesels with the tach. signal tapped from the alternator, you need to
know the ratio of crank pulley diameter to alternator pulley diameter.
The W terminal gives one pulse / revolution of the alternator and you
need to use the ratio to calculate how many pules you get for each
engine revolution.