Alternator output testing

Technical Small Block V Twins (not new V7s)
21

Just read the great notes Don shared.

Check the charge warning bulb. Does it light? If not check and replace if faulty.

This provides Field current to start the charging process.

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22

Ok - Iā€™ll check the MP voltage tomorrow. I have a mate who has a much better idea about auto electrics than me who has offered to help. Weā€™re in Cardiff, so about as far from Colwyn Bay as you can get and still be in Wales!! Iā€™ll keep poking around and hopefully between me and my pal we can work out whatā€™s going on. We did wonder if the brushes might be the culprit as the commutator is quite blackened (see above photo) but maybe this is too obvious

23

The charge warning light illuminates - and now doesnā€™t go out. When I started the run out last week it was going out at 2500rpm but stayed lit on the way home

24

Update: the charge light now dims at 3000rpm but never goes right out.
Tested from all 3 yellows to Y with engine running and get 6V from 2 yellows and 8V from the other. My guess now would be a problem with the statorā€¦ā€¦? I tested with the yellows plugged in so used the soldered joints if thatā€™s relevant

25

It sounds as if the components are working, but something is reducing the output. Go through all the connections in the circuit & give them a good clean.
New brushes would be the next thing to try if cleaning doesnā€™t cure the problem. ,

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26

Whats the voltage to the field now? Iā€™m still wondering about the regulator.

27

Am I right in thinking itā€™s possible to connect 12V to the field terminal to bypass the regulator, which proves whether the alternator is working?

28

Have you put a voltmeter (set to DC Volts up to 25) across the battery and start the bike up, that is the true indication of the voltage being produced. The volts should go up to around 14 when you rev the engine.
Use an old style meter with a needle rather than one with a digital display.

29

Well, hereā€™s a thing. I did a quick once-over of the wiring: it looked pretty good overall but I cleaned the connectors and generally spruced it up a bit. Started her up and checked the voltage (again) across the battery. 12.3V up to 3000rpm with the gen light glowing brightly, and increasing the revs up to 4200, the voltage goes up to 14.7, so itā€™s obviously charging OK now, but should it need 4000 rpm before anything happens? Iā€™ve got new brushes on the way so I wonder if they will help?
Many thanks for all the excellent advice on here and apologies for my complete incompetence!!! Give me a box of spanners any day :wink:

30

Thatā€™s good news and yes that might be the cause. Not enough current in the field.

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31

Watch out, revs at which charging kicks in are related to the rated power of the charging/idiot light bulb. The bigger the bulb(e.g. 2 or more W vs 1.2W) the earlier the charging starts. It could be easy fix for you, just check.

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32

Iā€™m resurrecting this thread (apologies in advance). I am now getting a mere 12.3V at 4000 rpm at which point the charge light goes out. Iā€™ve cleaned the brushes, the rotor and as many contscs that I can access in teh charging system. Iā€™m beginning to wonder if there is an issue with one of the components (reg/rec or diode - whatever that is!). What is the best place to start to fix this?

33

A bit more info: I use an Optimate to charge, and when connected, I only get 11.9V across the terminals. Could this indicate a battery problem? (itā€™s a Yuasa U1R bought in April of this year)

34

OK: I think the Optimate is knackeredā€¦
Also, I get virtually no output across the 3 terminals soI guess the alternator is kaputā€¦

35

Just a thought, you mention it has a new rotor fitted. I wonder if it needs energising? Disconnect the wire to the front slip ring and touch a wire from the battery +ve lead onto the front slip ring. Have a read of this threadā€¦

36

To test the stator, get a multimeter and test each of the three stator connections
Disconnect the three yellow wires, put one probe on the Y terminal and check the resistance to each of the three stator terminals in turn. You should get around 0.4 Ohms
Then check from the stator body to each of the three terminals, there should be zero or a very low resistance.

37

Thanks Don. Flashing the stator sounds quite scary but I will try that if needed. However, I get zero resistance from Y to all 3 terminals, and also zero across those 3 and the alternator bodyā€¦ faulty stator?

38

0.4 Ohms is not a lot of resistance, check your meter accuracy before condeming the stator. Touch the 2 meter probes together and zero the meter then test the stator. I must admit it doesnā€™t sound good.

39

Hold on, but you say youā€™ve GOT some charging - the light goes out. I had once or twice a diode problem, and just soldering new one rectified(pun!) the problem:) You know, you may get just two 'waves" out of three hence such poor charging. Not sure how to check diode board, but standard manual covers that for sure.

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40

Iā€™m not getting any output from the alternator by the look of it (a couple of volts across 2 out of 3 of the yellow wires) - but I will borrow a multimeter and re-check