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alternator wiring question

Posted: Sun Aug 18, 2013 3:09 am
by michael3006
It's been along time since I had alternator problems, and never with my SD23.
Am I right to say that with only 3 wires from the loom to the alternator.
1 will be 12v from battery to alternator.
1 will be alternator output to voltage regulator.
1 will be earth.
If that is right any suggestions on telling which was which on an alternator full of sump oil where the vacuum pump seal failed
Thanks for any ideas


Original Alternator identification plate after I scraped off the oil.


Posted: Sun Aug 18, 2013 8:06 am
by plenzen
I believe that Al has covered this in a post someplace you may try the "search" feature.
If i recall correctly he has drawings and output voltages/amps etc

Posted: Sun Aug 18, 2013 6:02 pm
by asavage
(Click on image for larger)

The page in the Factory Service Manual that that's from is here.

The thread where we talk about the LR150 alternator in the 720 pickup that has the SD2x engine is here

To summarize: on the LR150 as used with the SD2x in 720 pickups, the B and S wires are both unswitched 12v+, and the L terminal is the idiot light circuit, which is absolutely necessary to kick-start the alternator.

None of those three wires is earthed. I know nothing of your LT140, but Hitachi tends to use the same design over and over across many of their alternators.

The 5mm stud on the back of the alternator is the one that charges the battery, not those three wires.

The primary cause of failure of this series of Hitachi alternators is physically failed diodes. It's extremely common. The oil leakage from the vacuum pump into the alternator is often due to a groove worn in the rotor's shaft. The fix is a dual-lip seal that seals on the shaft in two places. The part number for that seal is in the thread referenced above.


Posted: Mon Aug 19, 2013 1:18 am
by michael3006
Thankyou both very much.
With that info I can get somewhere, literally.