checking alternator?

[imadoofus]

How do i check the alternator is working properly? I have a field coil alternator. Do i just check the voltage on the 3 white wires with respect to ground when the bike i s revving? how much should it be?


the dimwits at the garage conned me and i swapped a rectifier as they said my charging problem was cos of a dud rectifier. But the battery still doesnt charge.

[alains]

is it a NSR125 ? if yes i may help you
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the worst day fishing is better than the best day working

[imadoofus]

no a 82 yam.. but the techniques are similar.
tell me how?

[alains]

you need a voltmeter or a multicontroler (digital is the best) . follow the wires coming from the alternator and going to the rectifier . then follow the wire (i suppose only one) coming out from the rectifier and going to the battery . the voltage from the alternator is AC (probably around 20 volts , but use 50 volts caliber) . the voltage out of the rectifier is DC 14.5 volts . i will try that :
- unplug the wire going out fom the rectifier to the battery
- start engine and check the voltage on the rectifier and if you find a good AC voltage scrap the battery wire onto its location . if you make sparks that's look good and now with the wire still unplug check the DC voltage beetween the rectifier and the ground . if you find something around 14.5 everything is fine , so it will be a battery worn , a fuse gone or a bad main ground .
keep me inform
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the worst day fishing is better than the best day working

[imadoofus]

cant unplug battery. it is an electric starter

questions.
my rectifier works. I will check the loom from the rectifier to the red wire to the battery.
questoion is, will ground cause problems? what do you mean by check ground?


also what voltage should come on the ac wires from the alternator?
and do i check it against ground? i mean positive of multimetrt to the white wire and black to ground ( battery neg /engine block)?

[stinkwheel]

Be very carefull about fiddling with a field coil alternator! I wouldn't be poking about with the unrectified output voltage, if you somehow disturbed the voltage regulator it is capable of putting out up to 70 volts with enough amps to spoil your day.

Basic functional test is to check the voltage across the battery terminals with the engine running. If it rises to between 14 and 15v as the revs build, there isn't much wrong with your alternator.

If you have a high load ammeter (measuring in amps, NOT a multimeter that measures in milliamps) you can check the charging current too. Measure in series with the red wire going to the battery. Not sure about your bike exactly but it should go to somewhere between 8 and 12 amps at a peak. If it is doing this, your battery is charging (remember this is directional, you want amps going to the battery rather than from it, you need to get the wires the right way round).

Otherwise I would stick with static testing (engine off) of the alternator coils by measuring the resistance between the wires coming from it, this procedure should be described in your workshop manual.

There should be a set of carbon brushes somewhere on a field coil alternator too, worth checking these are within wear limits if they are easily accessable.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[imadoofus]

yeah I did read about the static testing. GUess I will try that first before i decide to nuke myself.

and also the loom to the battery.

thanks

[alains]

i will try to explain better . you must check the AC voltage with nothing on charge . to do that , you can unplug the wire beetween the rectifier/regulator and the battery . most of alternators are three-phase system , some have only 2 . take mesurment onto the rectifier/regulator beetween 2 wires (3 times if it's a three-phase) . on mine the voltage is about 30 Volts . note the voltage on each mesurment . il you have a substential difference in your 3 mesurments , your alternator is worn . in case of a single-phase no comparison possible (check on your workshop manual the genuine voltage) . now go on DC and check the voltage beetween where you unplug the battery wire and the ground . you must find 14.5 volts , otherwise the rectifier has gone . if everything OK , it's probably a battery , fuse or ground wiring problem .
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the worst day fishing is better than the best day working

[alains]

i have made a little diagram to show how i think about your mesurments
____________________
the worst day fishing is better than the best day working