Intermittent Charging on a Yamaha XS750


| 12/20/2017 12:00:00 AM


Motorcycle Classics tech expert Keith Fellenstein

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Q: I have had an intermittent charging problem with my 1978 Yamaha XS750 Triple since the day I bought it. I bought it two years ago with 18,000 miles on it, and now it has 20,000. The previous owner installed a new regulator, so the first thing I did was install a new battery and repair the wire connections at the stator (it had been dropped and it pinched one of the wires). I use an amp meter on the battery side. When charging, it shows 5 amps at 2,000rpm with the lights on, which I think is a bit low, but according to the Yamaha manual I have is what it should be. But I find when I ride it the charging becomes intermittent and eventually drains the battery. As an experiment, I took it on a 75-mile round trip, with a friend in front of me, for safety reasons, and one following. It may be coincidental, but it seemed when I would hit a bump in the road it would sometimes alter the charging state. I have checked and rechecked all the connections and repaired a few of them to no avail. Would the headlight perhaps cause this, or do you have any suggestions?
Dennis/via email

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A: I think you found the obvious fault in the pinched wire, but perhaps there is some hidden damage in the stator windings that shorts out when jarred. You could try a used alternator from eBay, or go whole hog and get a rebuilt one from Rick’s Motorsport Electrics. The latter is more expensive, but you know you’ll get a good unit. If you’ve checked all the visible connections, that leaves the ones you can’t see on the inside of the case.



Moto-Resto-LLC
1/24/2018 10:16:46 AM

Amperage check is a step past where he should have started and doesn't really tell anything tangible for his problem. He can later, but first things first. Recheck all the wires and connections first. Especially, the B+ wires that lead from the regulator to the battery. They are likely Red or Red with White tracer. Check grounds too. Also, check the wire (maybe black in color) that is the voltage monitor of the regulator. That is only energized with key-on. If that plug is good and contacts fine, move on. Put the meter on DC volts and check the charging (with a known, good battery) while engine running and varying RPM's as he did. 13.4-13.8 range good. Wiggle the wires while doing this, stator wires in particular since one was pinched. If he doesn't have a decent voltage while running state, or it drops in and out, disconnect the stator from the Reg/Rectifier. Change meter to continuity and check between the three stator wires for continuity, not to ground (that's next). Again, wiggle the wires while doing. Should be continuity and below 1 ohm, .5-.6 would be great. If that ranges in and out while moving the wires, trace which one is the problem and fix. However, if that proves-out, meter each stator wire to ground. Should be no continuity to ground on each stator wire to chassis ground. If that passes, change the meter to AC. Keep the stator unplugged. Carefully meter each stator wire (between the stator wires themselves, while engine running, rotating the leads to capture each wire back and forth -- not to ground!) for AC volts. 19 to perhaps 40 to 60 volts each at higher RPM's good. If all tests pass and wiggling the wires doesn't change the results, do a Rectifier test per the service manual, checking the forward and reverse bias of the diodes. If that passes, the regulation part of the R/R is bad. As per a tech at Rick's Motorsports Electric told me recently, the only way to test the regulator side of an R/R is to change it with a known good unit and check for charging. But the above tests will help isolate any wire problem, stator short (or open) and the rectification side. Hope it helps.


Moto-Resto-LLC
1/24/2018 10:16:45 AM

Amperage check is a step past where hes should have started. Recheck all the wires and connections first. Especially, the B+ wires that lead from the regulator to the battery. They are likely Red or Red with White tracer. Check grounds too. Also, check the wire (maybe black in color) that is the voltage monitor of the regulator. That is only energized with key-on. If that plug is good and contacts fine, move on. Put the meter on DC volts and check the charging (with a known, good battery) while engine running and varying RPM's as he did. 13.4-13.8 range good. Wiggle the wires while doing this, stator wires in particular since one was pinched. If he doesn't have a decent voltage while running state, or it drops in and out, disconnect the stator from the Reg/Rectifier. Change meter to continuity and check between the three stator wires for continuity, not to ground (that's next). Again, wiggle the wires while doing. Should be continuity and below 1 ohm, .5-.6 would be great. If that ranges in and out while moving the wires, trace which one is the problem and fix. However, if that proves-out, meter each stator wire to ground. Should be no continuity to ground on each stator wire to chassis ground. If that passes, change the meter to AC. Keep the stator unplugged. Carefully meter each stator wire (between the stator wires themselves, while engine running, rotating the leads to capture each wire back and forth -- not to ground!) for AC volts. 19 to perhaps 40 to 60 volts each at higher RPM's good. If all tests pass and wiggling the wires doesn't change the results, do a Rectifier test per the service manual, checking the forward and reverse bias of the diodes. If that passes, the regulation part of the R/R is bad. As per a tech at Rick's Motorsports Electric told me recently, the only way to test the regulator side of an R/R is to change it with a known good unit and check for charging. But the above tests will help isolate any wire problem, stator short (or open) and the rectification side. Hope it helps.


Clint
12/28/2017 10:06:51 AM

These are not a permanent magnet alternator so the connections from the fuse block to the alternator need to be in good condition as well. If he is still running the original fuse block it may be the issue. They have failed with no visible damage to the tabs. An intermittent break in the alternator would be very rare.