Author Topic: 28 Amp Alternator Check-up ( ST1100 ) *  (Read 9146 times)

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28 Amp Alternator Check-up ( ST1100 ) *
« on: April 05, 2009, 02:14:23 PM »
Original article can be viewed here:

Submitted by Mike Martin.


28 Amp Alternator Check-up

Tick tock tick tock.... This is the term used when someone feels it's just a matter of time before their 28-ampere alternator fails. There is normally some warning of an impending failure, as the alternator output starts to drop. According to the Honda Service Manual, normal voltage is 12.6 to 15.0 volts, with the current between 0 and 1.0 amperes, at 5000 rpm. As things go wrong, the voltage goes lower. This is the reason that you should install a voltmeter on your ST1100, particularly if you have added electrical loads.

The mode of failure is an opening of one or more of the stator windings. The exact cause of failure is a source of conjecture. The majority of failures are accompanied by signs of overheated connections at one of two locations: the multiple connector at the regulator/rectifier, or the red three-pole connector behind the left-hand side cover. A weak or corroded connection at one of these two locations will show itself by melting or burning the plastic of the connector. If the failing connection gets bad enough, one-third of the alternator output is lost. The voltage regulator will command the remaining two-thirds of the alternator to go all-out in an attempt to maintain the specified voltage. Eventually, one of the remaining two active windings fails and the alternator output drops to zero.

Some owners eliminate the red three-pole connector, soldering the wires directly. However, this cannot be done at the regulator/rectifier. Some owners regularly inspect the connections, cleaning them and coating them with dielectric grease. Other owners, particularly those who travel far from home (myself included), go ahead and upgrade to the 40-ampere alternator before the old one quits. I have never heard of a 40-amp one failing. The 28-ampere ones fail in as little as 60,000 miles, though some go for well over 100,000 miles with no failure.

Paul Kolbo's 28-amp failed in 2003 just after he left WeSTOC in Moscow, ID, heading for his home in Minnesota. He returned to Moscow and did a series of tests on his bike, which convinced him that he needed to replace the unit. To get home, he worked around the problem by buying a deep-cycle trolling motor battery and one of those plastic battery boxes used for boating. Here's a picture. He strapped the battery to the bike, wired it to the system, unplugged his headlights, and headed for home accompanied by John O. Paul didn't ride at night, of course, and charged the battery each night in his motel room. Here are his words of wisdom:

    I am one of those whose ticks turned to tocks. Be watching for the following:

    1. Voltmeter reading 12.6 or so at cruising RPM (3k or more). This means you're no longer charging but your battery is still functional and is not getting drastically drained. Suspect the alternator windings or 3P connector. Pull off the headlight connectors now (reach past the fork tubes) and head back home.

    2. Volmeter starts reading below 12. Now you're discharging and it could be the regulator. Ride it somewhere to start pulling plastic and possibly to leave it sit for a tow.

    3. People riding behind you smell something funny. Don't dismiss it as something you ate or motorhome brakes. Get down and start sniffing around. Your 3P connector might be corroded and overheating.

    There are probably other things to look for. These are the fundamental things.

Jeff Bertrand
"Before getting too concerned, try hooking the meter directly to your battery, at least temporarily, and see what that reads. The readings you get vary all over the map depending where you tapped into the system. If you still get only 13.2 straight off of the battery, the ticking should be easily audible." :^(

Now you can do all the resistance tests as specified in the Honda Service Manual to verify whether you have a toasted alternator.

2003 M. E. Martin, all rights reserved.
« Last Edit: December 09, 2012, 05:11:29 PM by Tom Melnik »