Motorcycle Repair: overcharging honda shadow 1983 vt 750c, negative battery cable, regulator rectifier

Question
Hi Wayne,
 I have a 1983 vt 750c honda shadow that is overcharging the system. I've had the bike for years and it use to fry the battery once a year then while i was sitting at a red light it backfired real loud but still ran and i went home and found the plugs to the regulator rectifier had melted. i corrected the melted mess and then the bike was frying batteries more often than once a year. i had then parked the bike for more than 5 years and now it's time to fix the problem. i bought a new regulator rectifier and also a new alternator and a brand new battery. the bike starts and runs well but it is still overcharging the output at a idle is 15.6 and at 5000 rpm it is 15.8  in the clymers book and everyone else this is too much it should not be over 15. this bike is the grandfather of all shadows and it is known for electrical problems. any help would be great. thank you.

Answer
Hi Karen,


The overcharging is usually related to
a few different things.

One is that the regulator on the VT750
used the mounting bolts as a ground
to channel the excess charging current to the
bike's frame.

If the mounting area of the regulator or the place it bolts
on is dirty, rusty or has paint on it
then the regulator will not be able
to transfer the excess voltage to the frame.

Two, any bad ground or loose connector can
cause a voltage increase. On many of these
bikes we had to cut the old connector
plugs off the stator and regulator and
solder them direct to keep the
connectors from melting.

Three, a weak cell in the battery or
bad battery cables or connections
can cause problems. Especially clean
the frame where the negative battery cable
bolts on. Clean any paint or corrosion
away and make sure all battery connections
are tight.

Some VT750 had a voltage sensing wire
on the regulator, it was a black wire
that helped the regulator to know
the condition of the battery and
adjust the charging rate.
If it has one it should be checked
for breaks or if it has been disconnected.

It is possible a newer regulator may not
use this wire at all though.

You may have to confirm the battery
cell condition even if new and also
it is possible to get faulty new
regulators so don't totally
eliminate that if all else fails
to help.

Get the regulator body grounded really
solid and clean the battery negative
frame ground and check if the regulators
green wire is grounded well also.

Good luck!
Wayne S.
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