I have a charging system problem please advise

I am here today to seek help for charging system problems on my 1991 ST1100 34k miles. I do not know if it is related but the first thing I noticed was the high beam headlight went out. Then after a few days of riding the battery went dead. So I boosted it off my car battery and of course had the engine shut off. I just hooked the battery up for 15 min and then disconnected the jumper cables and it started. I set the choke to run at 2000 rpm to help with charging and within a minute smoke began spewing from the regulator/rectifier. I assumed it was fried and bought another one. The new one arrived days later so during that time I had the battery trickle charged and tested, by autozone, maybe a bad idea, and they said it was bad, so I took it to my motorcycle mechanic and he tested it and said it was good, but that it dropped into the yellow range for a few seconds but was still showing good. The battery is questionable I guess. I then put it on bike and started the bike with the new regulator/rectifier, which has heat fins on it, the other did not, and within a minute at 2000 rpm it was getting too hot to touch. I shut the bike off and called the mechanic and he said to check the voltage with the bike running at 3000 rpm, and it was putting out 15.5 volts. Is the alternator just bad or is there something that can be fixed that will stop it from over charging? Will replacing the battery maybe be the solution or is there something else. I have a multi meter and can check various voltages but the repair manual is a bit hard to understand and I could really use some direction in solving this problem. Thanks for taking time to read this and please help if you can. Please be very specific on where things are I am not good at reading my repair manual and locating things especially wiring connections.
I really appreciate this website and all the helpful people.
Thanks!

You will need to take off the left side plastic cover to reveal the battery and the connectors related to the charging system. First off inspect the connector to the voltage regulator, the unit you just replaced. it's a 5 pin white connector. see if there is any sign of the plastic melted or black spots on the brass contacts. If there are any signs then you will have to replace the connector and strip back the wire till you get nice shiny wire. You may not be able to replace the connector but you can use individual spade connections to the regulator.
To the left of the regulator und slightly higher look for three yellow wires that are connected with a red connector. Inspect that connection check again for melted plastic and corroded terminals. What many people have done, myself included, is to cut out that connector and splice, solder, heat shrink the corresponding yellow wires together. Before you put it back together you can do a continuity test across any of the two of the three yellow wires to check the condition of the windings. the" how to" will be in your shop manual. To the left of that connector is a black connector with a white wire and a black wire going to it, do the same checking replace if necessary. Just to the left and down from the voltage regulator is the main relay. there's a red connector there. Inspect that connector, make sure the connections are clean and tight. Use dielectric grease on all your connectors when you put them back together. Look up the red wire bypass and that will give you instructions on how to do this mod.
I think 15.5 volts is a little high and your system may be overcharging. But the 28 amp alternator will easily put out 15.1 volts at 3000rpm.
The charging system can fail because of loose or corroded connections this is what you have to do . is due diligence in this area.
do you have an oil leak under your bike, just under the alternator? this alternator is cooled by engine oil if it's leaking oil it may get overheated and burn one of the windings.
Install a volt meter, have it connected directly to the battery. you can put a switch inline so it isn't on all the time. You do this to monitor the charging system ideal it will register between 14-15.1 volts under normal conditions. If you aren't running any accessories and it's reading under 13 volts at regular rpms then you alternator maybe challenged. Not to worry as there is a way to upgrade your bike to accept a 40amp alternator. There are instructions on this site on how to do that, and your bike is still a good bike and will last many years with this modification.
all the best

or.....always try the simplest fix first - GET A NEW BATTERY!

TPadden said:

or.....always try the simplest fix first - GET A NEW BATTERY!

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the battery can't control the charging voltage applied to it, so this wouldn't be the source of the problem.

I believe a battery does contribute to the charging system, if it's faulty, the charging system will direct more energy trying to charge the battery thus taxing the alternator. When I had my 1990 I put in a fresh battery and the voltage stayed a constant 14.9. The older battery was ranging from 14.3 to 15.1.
that was my observation.

kiltman said:

I believe a battery does contribute to the charging system, if it's faulty, the charging system will direct more energy trying to charge the battery thus taxing the alternator. When I had my 1990 I put in a fresh battery and the voltage stayed a constant 14.9. The older battery was ranging from 14.3 to 15.1.
that was my observation.

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Where were your voltage measurements taken, across the battery terminals, or somewhere else? You may have been seeing different voltages based on the weak battery not being able to drive the load effectively over time, that wouldn't surprise me.

A weak battery can require more alternator output to maintain the desired operating voltage, I agree with that. But unless the battery has some serious problems that isn't going to cause the charging voltage to go above its normal ~14.4v range. If the battery is in that bad shape, then a simple test would likely reveal the battery problem. It makes sense to test the battery, rather than just replace it without testing its condition.

Generally if you have a weak battery, the alternator voltage won't necessarily be any different than with a strong battery, it will just have to dump a lot more current into the battery to maintain that desired voltage. If you allow that condition to continue for a long time, then the alternator may eventually burn itself out from the excessive current draw.

dwalby said:

Where were your voltage measurements taken, across the battery terminals, or somewhere else? You may have been seeing different voltages based on the weak battery not being able to drive the load effectively over time, that wouldn't surprise me.

A weak battery can require more alternator output to maintain the desired operating voltage, I agree with that. But unless the battery has some serious problems that isn't going to cause the charging voltage to go above its normal ~14.4v range. If the battery is in that bad shape, then a simple test would likely reveal the battery problem. It makes sense to test the battery, rather than just replace it without testing its condition.

Generally if you have a weak battery, the alternator voltage won't necessarily be any different than with a strong battery, it will just have to dump a lot more current into the battery to maintain that desired voltage. If you allow that condition to continue for a long time, then the alternator may eventually burn itself out from the excessive current draw.

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Just to be clear, the voltage range between the 28 amp alternator and the 40 amp alternator is different. My 97 sits at 14.3 whilst my 1990 was in the 15.1 range and that's what the shop manuals sate.
I concur with what you say.
I had the voltmeter hooked up across the battery. My observation was just that,once I changed to a new battery my readings were slightly different from the previous.
Aside from that, I managed three seasons and 50,000km before my alternator gave out. That 1990 is in other hands right now and is having the alternator upgrade.

Having personally experienced charging issues on my 1990 I advocate checking all the charging connections to save grief later. Given the moderate mileage on the OP's bike there's still a good possibility that he will garner many trouble free miles with the 28amp unit.

kiltman said:

Just to be clear, the voltage range between the 28 amp alternator and the 40 amp alternator is different. My 97 sits at 14.3 whilst my 1990 was in the 15.1 range and that's what the shop manuals sate.

Having personally experienced charging issues on my 1990 I advocate checking all the charging connections to save grief later.

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agreed, I mentioned checking all the connections in the other thread he had started on this same topic. I had a similar problem with a Yamaha years ago where the voltage at the battery was around 15.0v with no aftermarket loads on the bike, so it shouldn't have been that high. I eventually found corrosion on a connector that was causing an additional voltage drop of about 0.7v before the point where the alternator monitors the loaded system voltage. When I removed that corrosion the charging voltage went back to its normal 14.2-14.4v.

I think the spec on the "normal" range does go up to 15.0v or so, but unless you're really loading the system to its full capacity I wouldn't expect to see that high a voltage. On your 28A alternator, did you have a lot of aftermarket loads, or did it just run that high with the stock electrical loads? In my experience with cars and bikes 14.2-14.4v is the typically "normal" range. I'd be uncomfortable with a system than ran continually at ~15v, perhaps that's why the 28A system tends to fail, its overloaded and running at full power all the time?

dwalby said:

the battery can't control the charging voltage applied to it, so this wouldn't be the source of the problem.

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Your WRONG - a shorted battery cell will control the voltage (sucking whatever the regulator/ alternator will put out). Again it MAY not be the problem but it is the first culprit I would replace; especially after being told it tested bad.

TPadden said:

Your WRONG - a shorted battery cell will control the voltage (sucking whatever the regulator/ alternator will put out). Again it MAY not be the problem but it is the first culprit I would replace; especially after being told it tested bad.

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Agreed, the charging system is made up of three main components, none of which are worth anything without the other.

1.Alternator
2. R/R
3. Battery

Batteries, grounds and connections are always the first things to be checked (correctly) with a charging problem, and most any other electrical issue. Any problem with any of the three can cause your bike, car, whatever to do weird things.

jmcarruth said:

Agreed, the charging system is made up of three main components, ....Batteries, grounds and connections are always the first things to be checked ......

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Lead Acid (wet cell) batteries fail internally when the battery has been in a low state-of-charge for weeks or months due to sulfation. There is no simple method to measure sulfation, other than the batteries performance under load and how it retains a stable voltage profile on discharge ...... A sulfated battery WILL draw as much voltage as the alternator/ regulator WILL put out.

dwalby said:

agreed, I mentioned checking all the connections in the other thread he had started on this same topic. I had a similar problem with a Yamaha years ago where the voltage at the battery was around 15.0v with no aftermarket loads on the bike, so it shouldn't have been that high. I eventually found corrosion on a connector that was causing an additional voltage drop of about 0.7v before the point where the alternator monitors the loaded system voltage. When I removed that corrosion the charging voltage went back to its normal 14.2-14.4v.

I think the spec on the "normal" range does go up to 15.0v or so, but unless you're really loading the system to its full capacity I wouldn't expect to see that high a voltage. On your 28A alternator, did you have a lot of aftermarket loads, or did it just run that high with the stock electrical loads? In my experience with cars and bikes 14.2-14.4v is the typically "normal" range. I'd be uncomfortable with a system than ran continually at ~15v, perhaps that's why the 28A system tends to fail, its overloaded and running at full power all the time?

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With no loads the 1990 would run 14.9-15.1 When Cruise was turned on it would drop a tenth and with heated gear down to 14.6 just before the alternator went it was in the 14.3 mark with just the stock load, and when the alternator packed it in it would be 11.6.
The OP probably has a bad battery, but since the cover is already off, a few minutes to check all connections may be in order. Who knows the previous owner may have dealt with the Yellow wire mod.

TPadden said:

Your WRONG - a shorted battery cell will control the voltage (sucking whatever the regulator/ alternator will put out). Again it MAY not be the problem but it is the first culprit I would replace; especially after being told it tested bad.

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If the battery had a shorted cell, wouldn't the problem be too low a system voltage? I'd have guessed that the shorted cell would prevent the charging voltage from getting that high, but I don't really have any experience with a shorted cell. Or does the alternator just keep raising the voltage, and the battery just keeps sucking the load but doesn't drop the voltage?

edit: just saw this from one of your later posts, thanks:

" A sulfated battery WILL draw as much voltage as the alternator/ regulator WILL put out. "

dwalby said:

If the battery had a shorted cell, wouldn't the problem be too low a system voltage? I'd have guessed that the shorted cell would prevent the charging voltage from getting that high, but I don't really have any experience with a shorted cell. Or does the alternator just keep raising the voltage, and the battery just keeps sucking the load but doesn't drop the voltage?

edit: just saw this from one of your later posts, thanks:

" A sulfated battery WILL draw as much voltage as the alternator/ regulator WILL put out. "

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A sulfated battery may appear to charge normally indicating 13 volts or better, and even be good for a start or two, usually the next day it will once again be indicating low voltage. It will eventually not take enough charge to start and can even suck enough voltage (as a parallel load) to not take a jump.

Thanks for your advise! I will let you know how it turns out hopefully within the next week.

I heard that replacing the battery every 2 years is about the norm and its way overdue. I will do that first. Thanks!

Do you know a good place to buy a battery? The reviews on autozone batteries are bad so I guess I need to just find an inexpensive great battery from Bike Bandits etc.

Sounds more and more like just a battery problem and that's a huge relief

Excellent points... Thanks!
I measured between the terminals of the battery.

Thanks again for the info. If my voltage reading was 15.5 at 3000 rpm, it seems odd that a reading of 15.1 on your bike did not cause any overheating of the voltage regulator/rectifier. It just doesn't make sense that a difference between 15.1 and 15.5 could cause the regulator to get so hot.