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Old 01-02-2016, 17:50   #16
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Re: Yanmar 1gm10 alternator wiring question

Are you sure it wasn't .12 volts AC not 12 volts AC? I think that might be possible, a little ripple riding on the DC.

At any rate skip any AC testing for now. Only rarely would it be needed on 12volt boat systems.
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Old 02-02-2016, 12:30   #17
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Re: Yanmar 1gm10 alternator wiring question

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Are you sure it wasn't .12 volts AC not 12 volts AC? I think that might be possible, a little ripple riding on the DC.

At any rate skip any AC testing for now. Only rarely would it be needed on 12volt boat systems.
That would make some sense although that amount of ripple is would suggest (at least IMO) some of the diodes are becoming leaky.
Perhaps if it was 1.2 volts AC, some of them have failed and could be part of the problem.

Given the OP's unfamiliarity of electronics, it might best to have the alternator checked out at his local auto electric shop.
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Old 02-02-2016, 15:42   #18
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Re: Yanmar 1gm10 alternator wiring question

Four Winds: This is very possible. I was on the "200" AC setting on the multimeter...I think you probably got it. I will have to test DC and I will let you all know how it turns out. Thanks everyone for your ideas!!!
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Old 02-02-2016, 17:27   #19
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Re: Yanmar 1gm10 alternator wiring question

The alternator outputs DC voltage. Do not measure on the AC setting.
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Old 03-02-2016, 02:04   #20
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Re: Yanmar 1gm10 alternator wiring question

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The alternator outputs DC voltage. Do not measure on the AC setting.
Using the AC setting is a good diagnostic tool to determine the serviceability of the alternator diode pack. If the diode pack is serviceable, the alternator will output 0V AC or very low AC volts say 10millivolts AC. Leaky diodes might have readings in the hundreds of millivolts (AC). Leaking diodes is a common source of RFI especially of SSB reception.
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Old 20-02-2016, 14:29   #21
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Re: Yanmar 1gm10 alternator wiring question

This is the OP. I ran the motor again yesterday (after putting on a new mixing elbow, fuel pump, and changing out the secondary fuel filter and rebuilding the seawater strainer )

This time I followed the directions you all gave me and used the 20 DC setting on my voltmeter.

The alternator, I think, is totally gone.

I put the voltmeter on the red wire sticking up from the "Batt" terminal on the back of the alternator and the black wire sticking up from a nut that goes nowhere on the back of the alt. Both those wires are clearly marked in the pictures attached. Each time I did this, a bunch of seemingly random numbers appeared on the voltmeter up to 12.7. Within about 3 seconds, though, the reading settled on a big fat zero.

Is this clear evidence that the alternator is fried?

Thanks everyone!!

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Old 20-02-2016, 14:50   #22
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Re: Yanmar 1gm10 alternator wiring question

No. You should have 12 volts on the alternator posts with the engine OFF.

I don't just make volts and send it away. It needs a good clean connection to the battery to do that.

Get 12 volt power to the alt + post first, before starting up.
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Old 20-02-2016, 14:59   #23
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Re: Yanmar 1gm10 alternator wiring question

No. Doesn't clearly indicate anything to me.

Was the engine running?

Random numbers means the test connections weren't stable.

Zero volts on a battery is very unusual in my experience.

Some suggestions. Remove those two unneeded wires and forget about them.

Measure and record the voltage of the battery at the battery terminals.

Start the engine and measure the voltage again at the battery. Wait a few minutes and measure again.

Note if the voltage I higher each time. Or maybe less.

Simple test almost anyone can do. Almost.
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Old 20-02-2016, 15:05   #24
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Re: Yanmar 1gm10 alternator wiring question

As twin boat said. That B terminal should read the same voltage as the battery terminals. No need going further until this is verified.

You saw 12.7 breifly. I'm guessing that is the battery's voltage and you recently used a charger to charge the battery. But there is a bad or loose connection somewhere. Hence the flakey readings.
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Old 20-02-2016, 16:41   #25
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Re: Yanmar 1gm10 alternator wiring question

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As twin boat said. That B terminal should read the same voltage as the battery terminals. No need going further until this is verified.

You saw 12.7 breifly. I'm guessing that is the battery's voltage and you recently used a charger to charge the battery. But there is a bad or loose connection somewhere. Hence the flakey readings.
What 4winds and Twin Boat said plus what is emphasised above.

Probably the vibration of the engine running causes it to drop to zero, it is shaking up the dodgy connection; especially considering it is single cylinder, even the best 1GM10 vibrates a bit
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Old 20-02-2016, 19:29   #26
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Re: Yanmar 1gm10 alternator wiring question

I forgot to include critical information. The batteries are measuring across their terminals the same whether the engine is running or not, and the "charge" light on the control cluster is on no matter what...so I am positive the system is not functioning but not positive that the problem is the alternator.

I was pressing the connections onto the voltmeter with my fingertips...it was awkward but I think I had a good connection...
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Old 20-02-2016, 19:50   #27
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Re: Yanmar 1gm10 alternator wiring question

Take the boot of the red wire.

Remove the wire that goes nowhere.

The heavy wire that is left gets put back on and tightened. The other end should go to the big positive post on the starter.

Turn on the battery, engine off and check for battery voltage at the red wire and the case of the alternator. Ignore or remove the black wire.

If you don't have 12 volts, the red wire is bad or loose on the other end. It is possable it is not connected.

Until you get 12 volts to the alternator red wire from the battery, the alternator will not work, nor will any voltage readings anywhere matter.

The charge light is on because the alternator can't charge if the big red wire is not connected to the battery somewhere.
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Old 20-02-2016, 19:53   #28
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Re: Yanmar 1gm10 alternator wiring question

Ok, that is better info. I assume you know the battery to be good also.

Two things.

Remove, inspect, and clean every connection related to the battery and charging. At the battery, battery switch, starter, and alternator. Look carefully for corrosion, loose crimps, etc. Wire issues are almost always at the terminal ends and not mid run. Does your harness look original and well wrapped? Or have any signs of previous repairs?

Take the alternator to a shop and have it bench tested.

Does the warning light glow brightly or does it dim and brighten with engine revs?
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Old 20-02-2016, 20:02   #29
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Re: Yanmar 1gm10 alternator wiring question

What twin boat said. Again if you don't have battery voltage at the big red B wire on the alternator, engine off, it can't charge.

You said 12.7 briefly on the extra wire. That wire could be dodgy. Take it off. Check at the B post, then you're checking the wire that actually charges the battery.
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Old 20-02-2016, 20:10   #30
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Re: Yanmar 1gm10 alternator wiring question

Have you ever cut the key switch off without first killing the engine with the fuel stop lever?
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