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Ford 9N, 2N & 8N Discussion Board

Re: 8n altenator problem

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Posted by soundguy on September 27, 2012 at 18:23:25 from (

In Reply to: 8n altenator problem posted by amos mc on September 27, 2012 at 17:30:08:

DO verify that you have correct voltage output using a common VOM.

DO disreguard the advice about an ammeter being useless on a 12v conversion.

A dash volt meter may show 12ish volts for a LONG time after your alternator has died, while the low 3a draw of the points is running off the battery. By the time that draw makes the voltage start to sag under 12v and be visible on the commonly availbe 12v dash meters that usually read 8v, 12v and 18v and let you guess at everything in between.. by the time you are dipping enough to nitice ona shakey trractor.. you have a severly discharge 12v battery that liekly won't restart your tractor if you shut her down. a 12v LA battery has a nominal voltage of 12.6v.. at 11.7v it is at 10% capacity.. IE.. severly discharged. I've added a link for a pdf to explain battery voltage vs discharge state. Volt meters are virtally useless without an idiot lamp or ammeter to back them up and actually tell you something abdout which way current is going.

a center 0 ammeter, at a glance, tells you if the battery is charging or discharging based on what quad the needle is in.

charge rate vs time of operation also tells you about the battery.

it's normal too see a initial high charge after starting that tapers off soon as the genny or alt makes up current used to start a machine. if you see a continous high charge rate.. you know there is an issue. for instance.. a shorted cell... you will see excessive charge rate.... a volt meter maght just show 12-14v and you'd think it's fine.. shut down, and poof.. you have a dead battery.... ammeter tells you that.. voltmeter don't

if I had a voltmeter only.. I'd have to have a discharge indicator lamp... atr very least. OR BOTH.. but if only 1, an ammeter tells you more about the condition of the electrical system. volt meter tells you one thing only.


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