high voltage

Joined
Mar 24, 2017
Messages
12
Location
albany ny
Bike
ST1100
Welp. I recently acquired a cheap 1991 that I was told had a bad alternator. I pulled the alternator and had it checked with a megger. All good. I reinstalled the alternator and dynamic checked the 3 phase coming off. Balanced and responsive. I soldered as many connections as I could find and bought a used oem VRR. It seems to charge fine but I'm regularly seeing 15.5 volts at 4k RPMs. I also read 12.8 vdc with the ignition off and 12.1 vdc with the ignition on. Thoughts? Don't worry about it? Buy a new battery? VRR?

I've got to put some more miles on this bike before I double its cost with a 40 amp upgrade.

Thanks!
 
Joined
Mar 13, 2012
Messages
5,046
Location
soCal
Bike
'97 ST1100
STOC #
687
I think the folks with 28A alternators say 15v is typical, so you might be a little bit high at 15.5. I had a bike years ago that was charging at 15+, and it turned out to be corrosion in one of the main connectors in the wire harness. That corrosion was causing an extra voltage drop of about a volt or so between the battery and ignition switch. The charging system was reading the system voltage at the 'on' side of the ignition switch (downstream of that drop), so it was charging at 15.5 instead of 14.5 because of the extra 1 volt drop in the line. Replaced the connector with a new one to remove the corrosion, and the charging went back to 14.5v.

Haven't had to trace this out on the ST1100, so I can't point you to the exact locations to check, but poke around with a voltmeter and see if you are getting any unusual voltage drops between the battery and the 'on' side of the ignition switch. I think something like 0.5-0.7v might be normal, if its anything higher than that check the connectors in the path to the ignition for corrosion.
 
Joined
Sep 10, 2011
Messages
2,211
Location
West Michigan
Bike
'98 ST1100
STOC #
8470
Like Doug said, you may have an excessive voltage drop somewhere in either the +12 volt side or the ground side, too.

I would measure the voltage drops between certain parts of the system. You could start by measuring the drop across the ignition switch , frame and negative battery terminal, positive battery terminal and main fuse, etc.

But, you will always measure a voltage drop - whenever current is flowing through a conductor, there will be a voltage drop. So look for an excessive voltage drop - compare all the measurements and see if there is one that is much more than the others.

What's a normal voltage drop ?? Who knows , how many of us have checked the voltage drops BEFORE we had a suspected problem ? It depends on which circuit you are talking about. In general, the higher the current, the higher the normal voltage drop will be.
 
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