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1967 - 1972 F-100 & Larger F-Series Trucks Discuss the Bumpsides Ford Truck

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Old 02-25-2010, 02:01 PM
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3 Post to 4 Post Alternator Question

In attempting to help my brother change his alternator, we’re running into a problem. The truck is 1970 F250 with a 360. We have the new alternator in but when running, it is only reading at 12.48 volts.
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A little history, he bought the truck last week. It runs well but something was draining the battery. We bench tested the battery and then determined that the existing alternator had a bad diode causing the battery to drain while the truck was turned off.
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He bought a new alternator and it is slightly different from the original. The original had 3 posts on the bottom of the unit: 2 positive, and one ground. The new one has the 3 posts which we hooked up but also has one towards the top labeled “STA”. We have not hooked this to anything as there are no existing wires to hook to it.
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When running with the new alternator, the volt meter reads 12.48 with no accessories running. When we turn the lights on, the voltage drops to about 12.10. I did check the voltage coming off the “STA” post and it is 0.97 volts.
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My questions are:
Should the alternator be putting out about 13.7 volts when the truck is running?
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For the alternator to put out the appropriate 13.7 volts to the system, do we need to hook up the “STA” post to the starter solenoid, positive battery terminal or what? If you add the 12.48+.97 volts that’s 13.45.
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Are we simply just missing a wire that is vital to complete the charging system?
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Any info would be great. This seems like a very useful forum and we’ve already got lots of information off of it.
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Old 04-28-2014, 11:36 PM
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The regulator is usually matched to the alternator, but if you just google ford wire diagram, regulator will get power, and then the sta and fld come from the reg to alt.

Personally, I'd have gotten a 100a chevy 1 wire, drilled the 7/16 hole to 1/2 and bolted it in place. Then just run a big wire to battery and call it a day. I've given up on the 55-65a ford regulated setup. The wiring is so old and cracked it's sometimes hard to track down the issue.
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Old 04-28-2014, 11:40 PM
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Looks like you got it figured out in your other thread.

For the future, one thread of the same topic please. Keeps everything together.
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Old 04-28-2014, 11:40 PM
 
 
 
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