Alternator issues

I have a 2004 st1300 108k miles and my alternator went out. I found a guy that says he rebuilds them out of Merrillville, IN. I sent it to him and about a 5 day turn around got it back. I did some test when i got the alternator in was putting out around 14.2 volts and with amp meter i clamp on the positive lead with the big fuse had about 10 amps charging. I took a ride and in about 33 miles it went out again. I called the guy and he said no problem send it back, he sent me another rebuilt one, first thing i notice was the tag had st1100 it looked the same so i put it in. Again did the test 14.2 volts and 10 amps charging. Once again took a ride and about the same miles 32 it went out again. Either i have more current draw on than it can handle or i got another bum rebuilt alternator. The bike ran good both times and the tack and speedo goes out suddenly. Anyone got any ideals on this? I dont know if its pulling too much current or i have something shorting out but the miles where about the same on both times.
Thanks

I've never measured the current flow from a working alternator, but if its supplying 10A to the battery, where say 8A is leaving the battery to drive the active loads (lights, ignition, etc.) and the other 2A is actually charging the battery, that would seem reasonable. I would assume that an excessive load that could burn out an alternator that fast would blow a fuse first, but anything's possible. Hard to say if the identical failure mode is due to an identical improper rebuild, or something wrong with your electrical system. The fact that your original alternator went out might suggest there's something overloading your electrical system that caused the initial failure, but not necessarily. If your measurement actually indicated that 10A was charging the battery, in addition to driving the other ~8A or so of load, then that is probably not normal. A 14Ah battery like most motorcycles use typically needs 1.5-2A of charging current.

I'd try to measure the current load from the various systems with the ignition on and the bike not running, and see if anything abnormal shows up. See if you can find a small resistive load that you can measure for voltage drop to calculate the current flow (like a fuse or section of large gauge wire). Then start it up and measure the load again. Sorry I can't be of more help, perhaps one of the folks who have rebuilt alternators can provide more info.

Edit:

The fact that you measure 14.2V at the battery while under load is normal. If you have a very high current draw you usually create a larger than normal voltage drop across the ignition switch, fuses, etc. and the alternator senses that drop and raises the battery voltage to compensate. Or a corroded connection provides a false high voltage drop to the alternator and it bumps up the voltage accordingly, which usually overcharges the battery in addition to taxing the alternator unnecessarily. The fact that you aren't seeing that may suggest that your system is normal and your rebuilt alternators were not done to spec. How much do you know about the guy rebuilding them, has anybody else you know used him successfully?

I'm thinking the same as you are on the draw, i did disconnect the headlights and watch the amp meter it drop to 6.8amps and also put the fuse back in and tuned on the high beams with really didn't bump up the amps to 10.8. He warrenty's his work and says he can test them but as far as his repairs i don't know if he is using less than parts to fix it. It be helpful if i could actually read the amps it is putting out while it is running. I seen somewhere a wiring diagram of st and i need to look at the wiring harness and see if something is bare and maybe grounding out which at that amperage would think it would be noticeable.

Two years ago my ST1300 alternator went out of service due to a failed rotor winding. A local shop with decades of experience that I took it to told me they could repair it but couldn't bench test it because the gear-driven design lacked a pulley and they tried but could not cobble up something to spin it up. They felt sure they could replace the rotor but wouldn't warranty the repair without bench testing it.

I then talked with the guy in Merrillville and he also told me he could rebuild it but the Denso parts were getting expensive and finished cost might not be much better than OEM replacement cost. Seeing how the thing had 116,xxx miles on it I didn't want to just replace the rotor for $215 parts plus labor and shipping and have the worn brushes, bearings, and the original regulator/rectifier left in it. If I wanted a real rebuild job the cost would approach $400 for labor and parts. A new one was $509 IIRC.

Sorry to hear your problems but I decided I wanted a new one with Honda warranty.

Update on the alternator. I did some tests on the voltage and current draw and discovered the 3 year old battery had a issue. First it would start the bike like normal but it would draw 10-12 amps constantly to charge it that made the alternator to over heat and fail. The current draw was 19 amps and it go out in about 30mins. I replaced the battery with new and with the bike running total amps was 7-11 amps. I have to say i got to take out the alternator many times during this issue and started out taking 4 hours to the last time 1 hour to completely remove it. The old saying you can do it if you put your mind to it, is very true. lol Thanks to this forum and others that have posted some tricks really helped, especially removing the fI which at first was very difficult.

stman1300 said:

the 3 year old battery had a issue. First it would start the bike like normal but it would draw 10-12 amps constantly to charge it that made the alternator to over heat and fail. The current draw was 19 amps and it go out in about 30mins.

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Glad it turned out to be a simple fix once you narrowed it down. With that kind of current going into it, did the battery get noticeably hot?? I'm even wondering if that much current could be dangerous with the potential of exploding the battery, that's 5-6x what is recommended for charging it. If you have all your access points still available, measure and record the voltage drop between the alternator output and the battery positive terminal now that its back to normal. Then if you have to troubleshoot this again if you see any higher voltage drop than what you have now, you know you have another excess load issue (or a potential corrosion issue, but that should be visible). I'm guessing with an extra 10A flowing in the circuit, you probably had at least an extra 1V drop across a fuse or fusible link between the alternator and battery. Did you happen to notice this during your troubleshooting? Turns out my above guess of possibly having an additional 10A load in the path was correct, but at the time I wrote it I didn't consider it to be the most likely cause because I've never heard of a battery doing that before, so thanks for the info.