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Old 15-06-2015, 12:12   #1
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Mando alternator overcharges

The new Mando 65 amp alternator on the YC Grand Banks (Lehman 120 hp) is driving me nuts. Its a 3-wire alternator, with output, exciter, and sense wires. The sense wire is jumpered to the output, which should keep the output voltage less than 14.5v, but it just keeps going up. If I rev up the engine it goes over 15.5v. Installing a ground wire on the alternator case does not help, and neither does putting a new regulator in the alternator. The alternator puts out about 25 amps at idle, and over 45 amps at cruise. The alternator shop swears that the unit tests normally on the bench, but I haven't seen it with my own eyes.

The excite wire was going to 12v switched, and there was some discussion on Mercruisers about going through an idiot light. I tried a 1.3 watt light in the excite circuit with no change, and then put a 100 ohm pot in the circuit and going from 2 to 102 ohms had no effect. I'm running out of things to try, and I think my options are another brand alternator or converting this one to external regulation. Any suggestions??
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Old 15-06-2015, 12:55   #2
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Re: Mando alternator overcharges

If an alternator shop says it tests good and swapping to another know good regulator (my assumption) doesnt fix it then maybe its not the regulator or the alternator. How are you determining output voltage? How do you know that meter is calibrated correctly?
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Old 15-06-2015, 13:12   #3
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Re: Mando alternator overcharges

If you disconnect the excite wire, does it still have output? Can you measure the current on the excite wire with a clamp-on ammeter? Maybe the sense wire is disconnected inside?

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Old 15-06-2015, 14:36   #4
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Re: Mando alternator overcharges

The output voltage is the same on three different digital voltmeters.

If you disconnect the sense wire after the alternator is putting out, it continues to put out. If you disconnect the sense wire and then start the engine, the alternator won't put out until you rev the engine up and it starts to self-excite. The sense wire connects to a stud which comes out of the voltage regulator, and that stud shows battery voltage.
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Old 15-06-2015, 19:08   #5
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Re: Mando alternator overcharges

Don't know this particular alternator/regulator, but shouldn't the sense wire go to battery voltage rather than output? Otherwise how does the regulator know actual battery voltage?
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Old 15-06-2015, 19:09   #6
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Re: Mando alternator overcharges

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Originally Posted by donradcliffe View Post
The output voltage is the same on three different digital voltmeters.

If you disconnect the sense wire after the alternator is putting out, it continues to put out. If you disconnect the sense wire and then start the engine, the alternator won't put out until you rev the engine up and it starts to self-excite. The sense wire connects to a stud which comes out of the voltage regulator, and that stud shows battery voltage.
Sure sounds like a regulator issue...are you certain the one you swapped in was good?
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Old 15-06-2015, 23:52   #7
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Re: Mando alternator overcharges

The replacement regulator was new. I thought I had found the problem when I discovered that the regulator was grounded to the alternator case under one of its mounting bolts and the case was painted in that area. However, when I scraped the paint off and tried again it did not solve the problem.

The alternator output is wired to the battery, and tying the sense wire to the alternator output post rather than the battery means that the battery should see LESS than the regulated voltage, due to the voltage drop in the output wire.

I also measured the voltage difference between the alternator case and the battery negative terminal--less than 0.02 volts at 25 amps output, so the case is well grounded.
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Old 16-06-2015, 06:31   #8
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Re: Mando alternator overcharges

Is the internal regulator adjustable? Maybe take it back to the alternator shop and watch the bench test to see if you get the same high voltage.
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Old 16-06-2015, 23:27   #9
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Re: Mando alternator overcharges

Nope. no adjustments-I think there is a brush block available which will give me external terminals for both sides of the brushes, so I can make one the field and ground the other. Of course, the brush block costs more than the internal regulator with integral brushes, but that's life. I'll post again when I've exorcised the demons.
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Old 21-06-2015, 19:24   #10
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Re: Mando alternator overcharges

The final note to this saga was that I soldered two jumpers on the regulator/brush assembly to make it a B Circuit externally regulated alternator, then bought a simple external regulator at Napa for $20. Problem solved. Moral of the story--don't buy a Mando alternator.
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