#9: Re: Help with resolving charging
system issue. Author: Delmeister, Posted: Sat Jun 12, 2010
6:50 am
If you have continuity between
any of the stator leads and ground, you will have reduced output, and you
will stress some of the diodes in the RR. The exact effect will depend on
the nature of the short and how the stator is wound (Y or Delta). Any
schematics I have seen that represent the alternator internals show a Y
connection. In a Y connection, one end of each of the three coils is
connected to each other internally, and what you see coming out of the
stator are the three other ends. When you measure the resistance between
any two terminals, you measure two of the coils in series, so if you
measured 1.2 ohms, each coil would have a resistance of 0.6 ohms.
Consider a situation where the one of the wires coming out of the
alternator is pinched and perfectly shorted to ground. If you measured the
resistance between that wire and ground it would be zero. The resistance
between the other two leads and ground would be 1.2 ohms. For the case
where the Y junction of the three coils is perfectly grounded, the
resistance of each lead to ground would be 0.6 ohms. If the short is
inside one of the coils, you will have some intermediate situation. Also
all coils would pass a very high current at some point in the cycle. For
the case of the shorted lead, full output would only be possible 1/3 of
the time, and two of the six diodes in the RR would be passing a very high
shorting current.
Bottom line is stator must not be shorted to
ground. If it is, it may or may not take out the RR depending on how long
the situation persists. Make sure the continuity you measure is actually
due to an internal short rather than from a lead touching ground. If the
RR fails in such a way that the internal diodes short out (rather than
open up), then just replacing the stator could result in damage to it (the
new stator) as well.
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