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From: Radoje Micic (no email)
Date: Thu Oct 02 2003 - 16:00:18 EDT
>I have VDO tachs on my engines ( 8.2 Detroit's)one
>tach does not operate until the engine RPM's reach
>2000.
>
Most alternators need small electrical current ("excitation") in order
to provide power output and, consequently, tach signal. Taking small
amount of output power back ("self-exitation") is OK, but there are no
output power if there are no exitation current. (sort of chicken-and-egg
situation). Usually, initial exitation current is provided via signal
lamp for no-charging alarm; lamp is connected between battery plus (via
key switch) and excitation terminal of alternator. As long as alternator
doesn't produce its own power, current is taken from battery and fed
into the field winding (lamp glows). When alternator gets excited, it
makes its own 12 V (and, btw, tach signal), and lamp between battery 12v
and alternator's 12 V doesn't pass any current through - lamp doesn't
glow. If you have twin engines, you can easily oversee that one of your
alternators doesn't charge, because other one charges both batteries.
If we speculate, that signal lamp died, than you never get "no-charging"
visual signal. You might tolerte it, assuming it is just a signal lamp -
but it is not! It has also stsrtup exciting role, i.e. without this lamp
alternator really don't start charging! With sufficient RPM, reminent
magnetism produces just enough electricity to turn power output high
enough for self-exciting, after which you can decrease RPM and still
get alternators power output & tach signal - until regulator recognizes
battery full and shuts excitation below self-mainteining treshold.
To find, if my speculation is correct, check following:
1. Does charging signal lamp of respective engine illuminates, when you
turn key switch ON, and before you start the engine? If YES, problem is
elsewhere. If NO, it is that; replace the lamp, and start over. If it
wasn't lamp, it is wiring.
2. When you increase RPM, tach "wakes up" at 2000 RPM. At what RPM it
stops? If my speculation is correct, it depends on charge status of the
battery; as soon as it is tooped up, tach will stop; same if you run
also other engine and have batt selector switch in possition "BOTH".
To conclude, I beleive you just need to replace signal lamp, and leave
alternator alone. If not, check wiring, then alternator.
Good luck!
Radoje
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