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Old 01-10-2012, 08:51   #1
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Alternator help

I'm hoping someone on the forum can give me some help / advice.
A little history: I have a Perkins 4.108 diesel with a motorola 55 amp alternator. About a week ago I turned on the ignition and the ignition circuit fuse blew. I took the starter out of the circuit and it still blew. The only other load on the circuit is the field wire to the alternator. Disconnected it and everything worked fine - except the alternator of course. I removed the alternator and noticed that the field wire had some of the insulation worn off and was apparently shorting against the case. Since I had it off, I took the alternator to a shop and had it tested. It tested good. I put heat shrink on the field wire and reinstalled the alternator. The engine fired right up without blowing the fuse but no output from the alternator. I double checked the alternator connections - still no output. I disconnected the field wire and checked that it had 12 V from the ignition circuit. Still no output. Any ideas???
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Old 01-10-2012, 08:58   #2
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Re: Alternator help

try revving the engine,alternators need to get above 1200 rpm before they kick in
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Old 01-10-2012, 09:03   #3
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Re: Alternator help

Quote:
Originally Posted by atoll View Post
try revving the engine,alternators need to get above 1200 rpm before they kick in
Self-excited alternators do, but this one has an excite wire as mentioned in the original post, and will not need to "kick in"
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Old 01-10-2012, 09:19   #4
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Re: Alternator help

Not sure from the description whether it is an internally or externally regulated alternator. If it is externally regulated, there is a regulator somewhere between the ignition and field wires. If internally regulated, you are not talking about a field wire--more like an excitation wire.

In any case, if you trust the shop didn't do a wall job on you, the next step would be to take a wire from the positive terminal of the battery and connect it to your 'field' terminal. If the alternator works, that points to your ignition wiring as the problem.
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Old 01-10-2012, 11:00   #5
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Re: Alternator help

[QUOTE=donradcliffe;1048639]Not sure from the description whether it is an internally or externally regulated alternator. If it is externally regulated, there is a regulator somewhere between the ignition and field wires. If internally regulated, you are not talking about a field wire--more like an excitation wire.


It is a standard automotive type alternator with the regulator attached to the back of the alternator. The field wire comes out of the regulator.
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Old 01-10-2012, 11:29   #6
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Re: Alternator help

See ➥ Basic Alternator Test - Page 1 of 2 - Goto page 2: http://cruisersforum.com/photopost/showphoto.php?photo=1694 Cruisers & Sailing Photo Gallery

And ➥ Basic Alternator Test - Page 2 - Goto page 1: http://cruisersforum.com/photopost/showphoto.php?photo=1695 Cruisers & Sailing Photo Gallery

And "Troubleshooting Marine Engine Electrical Systems"
http://assets.fluke.com/appnotes/ele...r/B0270b_u.pdf
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