Topic: 78 550 electrical problem

[njedgexj]

friday night i went out riding, came home, put the bike away and then next morning went into the garage and noticed an odd smell, kinda like battery acid. i noticed that my battery was dead and the wires pluged into the rectifier, the black platstic boots on them where alittle melted? i know i left the key off the night before. any ideas of what would cause some sort of short to make the battery drain like that and melt those wires?

[TwoTired]

The charging system is connected directly to the battery without a switch and with any fusible link.  The battery drain isolation is provided by the rectifier diodes that only allow current to flow from alternator to battery.  However, if these diodes fail in a shorted condition, the windings of the alternator are subsequently connected directly across the battery terminals.  The alternator stator windings have very low resistance, causing very high current draw.  The interconnecting wires and connector terminals cannot dissipate the heat without melting the wire insulation.  I've seen such failures also melt the connectors and plating off the contacts in the connectors.  The wire harness can also have the insulation of adjacent wires damaged, as the ground currents travel the wire terminated to frame near the coil mount.

Did you charge or jumper the battery before your ride?  Did you reverse the charger connections?

This is the most likely cause, I think.  Although, there are some battery failure modes that can draw excessive current from the charging system and melt wire insulations, too.  A failed cell in the battery can keep the voltage at the regulator low and it will tell the alternator to put out full power, even through the remaining cells can accept no more charge.  However, if your battery can be recharged to it's full 14.5 volt potential, then the rectifier is the more likely culprit.  The rectifier can be checked rather easily with a multimeter.

Best of luck!

[njedgexj]

thanks for the informative response, no i did not have to charge the battery before i took it out. i was riding most of the day and then took it out for a final night ride.  what tests should i perform with my multimeter to see if the rectifier is bad?
thanks

[TwoTired]

The rectifier plug has a green and red wire.  Disconnect it and use a multimeter to measure continuity with one polarity and infinite resistance with the opposite polarity.  This would indicate a good rectifier.

Use the diode function if your meter has such a setting.  This will supply enough voltage to make good diodes conduct and measure continuity.  Of course, shorted diodes will measure continuity in both directions, which is what I would expect with your reported symptoms.

If you do have to replace a bad rectifier, I would carefully check the stator winding resistance, replace any damaged wiring, and verify the battery is good before returning the bike to service.  A tedious task, but to avoid further issues and gain confidence in reliability, worth the effort.

If by some chance your rectifier is still good,  then you'll have to dissect the wiring to find the source of the high current drain.

Good hunting...