Sudden Alternator Problem...or is it?

[Dirt Road]

This morning I made my usual commute to work on my ST1100. It's a '97, 169,000 miles on the clock, and it's been virtually trouble free the whole time. My 35 mile ride to work was uneventful, but right at the end it sounded like it was occasionally dropping a cylinder. When I got off and examined it it sounded fine and every pipe was hot, so no obvious dropped cylinder. Forgot about it and went about my day. About five miles into my ride home the problem came back and started getting worse.
Here are the symptoms:
-Slowly lost power over the course of 5-10 minutes, getting deteriorating faster right at the end.
-It recovered briefly if I kept the throttle wide open.
-checked the plugs: dusty black, but not horrifically so. Very dry tips. Oily threads.
-I gapped the plugs, and reinstalled. No immediate difference and bike won’t start.
-Cranked the bike for a couple minutes with only hiccuping, but eventually it fires up.
-chugs like skipping a cylinder for about 15 seconds, then runs normally again.
-At this point I noticed something significant: the voltage seemed to be stuck around 7volts, even when revved up.
-back home now (thanks to friend with truck, didn't want to risk it): it starts fine and idles, but the voltage on the dashboard voltmeter doesn’t go over 7volts.

Is this the alternator? I'm assuming this bike already has the 40amp (it has tons of farkles). How should I go about replacing it? It looks like Walmart sells the same mitsubishi alternator. Will that work on this bike, or is the google lying to me?

EDIT: I know my profile says I have a 95. That one's a donor bike now, so if I can steal its alternator I will. ...I also have a 91....I think I have a problem....help?

 

[Bush]

First things first. Are the battery connections clean and tight? How old is the battery? Do you have another voltmeter to check the voltage at the battery, when running? Obviously, 7V. is not good at all, if that is the actual voltage when running. If it is, then your alternator is not charging at all and your battery is struggling to supply enough power to generate a sufficient spark, all on its own. This doesn't last long and the engine dies.

Look under the bike, left side at rear of the engine to see what alternator you have in there. You will notice either an automotive style alternator, with fins on it, or a smooth round case with a wire loom coming out the back of it. The former is the 40 amp and the latter the 28 amp. It would be much more likely that your alternator has packed it in if it is still the 28 amper, with that many miles on it.

It could also be the voltage rectifier/regulator (VRR) that has died. Some testing will be required, but let us know some further details re my questions.

EDIT: As Paul pointed out, your bike will have the 40 amp alternator. It was late and I missed the "97 reference early in your post, having focussed on the bike listed in your profile.

 

[vinny]

Fried red main connector...?

 

[paulcb]

1996 and later have the 40A alternator.

Long shot... make sure your starter button is not sticking partially closed... this will keep the starter engaged, pulling a lot of current and dropping the voltage.

After starting, check to see if your headlight is on. If it's on, the button is not sticking. If not, pull on the button and see what happens. Good luck.

 

[John OoSTerhuis]

Charge the battery and then load test it. If it will take a charge and pass the test, then move on to connections, etc.

John

 

[Dirt Road]

First things first. Are the battery connections clean and tight? How old is the battery? Do you have another voltmeter to check the voltage at the battery, when running? Obviously, 7V. is not good at all, if that is the actual voltage when running. If it is, then your alternator is not charging at all and your battery is struggling to supply enough power to generate a sufficient spark, all on its own. This doesn't last long and the engine dies.

Look under the bike, left side at rear of the engine to see what alternator you have in there. You will notice either an automotive style alternator, with fins on it, or a smooth round case with a wire loom coming out the back of it. The former is the 40 amp and the latter the 28 amp. It would be much more likely that your alternator has packed it in if it is still the 28 amper, with that many miles on it.

It could also be the voltage rectifier/regulator (VRR) that has died. Some testing will be required, but let us know some further details re my questions.

EDIT: As Paul pointed out, your bike will have the 40 amp alternator. It was late and I missed the "97 reference early in your post, having focussed on the bike listed in your profile.

I checked with a proper voltage meter, and the battery is reading 12 volts with the engine off, 11.3 with the engine running. Just walked in the door from work, so I'll be checking the grounds next and I'll report back.

EDIT: a question: if the regulator goes on the fritz, can that inhibit the charging of the battery or would that not make a difference?

 

[Bush]

EDIT: a question: if the regulator goes on the fritz, can that inhibit the charging of the battery or would that not make a difference?

Certainly. The alternator is capable of multiple times the 12V. of the battery, so the VRR plays quite an important role in supplying the required voltage to the system.

You should be seeing 14+ V. with the engine running. 11.3V is a discharge state of the battery. A good fully charged battery should read around 12.8V with the key off.