Author Topic: Alternator Rotor Wiring Repair - 40 amp ( ST1100 ) *  (Read 8358 times)

Offline KoTAOW

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Alternator Rotor Wiring Repair - 40 amp ( ST1100 ) *
« on: October 02, 2009, 08:56:25 PM »
Originally posted on

Written by Bob Sowell, STOC 5297:


Alternator Rotor Wiring Repair

The Symptoms:

On my way to work I noticed that my voltage had dropped to 11 volts on my 2002 ST.

I was 30 miles from home, so I turned around and headed for the house. I got to about 5 miles from home and the bike quit running.

The bike is at home now and I am starting to trouble shoot the problem. I have searched this forum and did not see any write ups on this same problem.

The manual says I have to pull the alternator apart to determine the exact cause.

So far I have checked all of the fuses and they are good.

There were no indications that I was about to have a problem... the charging system just quit charging. As I said the bike is an 02 and it has about 130,000 miles on it. I have two volt meters on the bike and they both indicated that I had about 11 volts when I turned around and headed home. The bike kept running until the battery voltage dropped below 10 volts. I don't know exactly how long the bike ran on just the battery, but I know for sure that it ran about 20 minutes after I noticed the problem. The battery is about 4 months old.

The Fix:

This post is intended to document what was done to resolve the issue with my dead 40 Amp alternator so that future STíers that might suffer the same problem will have a reference starting point to resolve it.

Current (pun) status. I put the repaired alternator in my bike last night and it is generating 14.2 volts at the battery.

First I want reference the following link for excellent pictures of an open 40 amp alternator:

Unfortunately the pictures that I took with my cell phone pale in comparison but they are what they are.

To get to the alternator you must first pull the swing arm. This is very well documented on several sites so I wonít go into detail here. Also note that there is a youtube video that shows the installation of the alternator passed the frame (it is a tight fit).

<a href="" target="_blank" rel="noopener noreferrer" class="bbc_link bbc_flash_disabled new_win"></a>

Once I had my alternator removed from the bike, I separated the two halves. As noted in an earlier post I found a broken wire under the slip rings.

The picture below shows the broken wire from the rotor:

The picture below shows the modification I made to reroute the rotor wire. It also shows where the epoxy
is that had to be removed to access the wire to the slip ring:

This final picture shows the short jumper that I spliced into the rotor wire (the original wire was not
long enough to reach. Finely I soldered it all together and then re-measured the slip ring resistance
and it all measured ok.

What isnít shown above is that I epoxied over the splice that I made.

Will this repair hold up? Iíll let you know when my bike gets to 200,000 miles (that will be in about 70,000 miles from now).
« Last Edit: December 10, 2012, 06:04:00 PM by Tom Melnik »