Alternator question

[mobileortho]

My alternator stays on when the engine's off, it won't even turn off when you disconnect the pigtail. It seems to be charging fine, not sure if it needs to be replaced. Any ideas?

[wildfifty2]

Originally Posted by mobileortho

My alternator stays on when the engine's off, it won't even turn off when you disconnect the pigtail. It seems to be charging fine, not sure if it needs to be replaced. Any ideas?

Your alternator should only turn when the engine is running as it is driven by the belt. Am I missing something?

[mr48chev]

Stays on How? In other words what indication causes you to believe that it "stays on"?

[mobileortho]

You can hear it humming.

[dwcsr]

pull your belt off and if the pulley spins the Alt needs to be replaced

[mr48chev]

Originally Posted by dwcsr

pull your belt off and if the pulley spins the Alt needs to be replaced

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I've read some strange suggestions on the net but that one tops the list. You can motor a generator but I have never heard of it with an alternator.

From what I can find out by a search the alternator is getting feedback that is telling it to charge when it sitting static.

[dwcsr]

Originally Posted by mr48chev

?????????????????????????????????????????????????

I've read some strange suggestions on the net but that one tops the list. You can motor a generator but I have never heard of it with an alternator.

From what I can find out by a search the alternator is getting feedback that is telling it to charge when it sitting static.

A car/truck 12 v alternator is a 3 phase synchronous motor why would it not be able to run under power? I'm surprised you have never seen and alternator turn by itself. It does need to have a diode problem to do this. I'm always glad to be on the top of the list.

Google "alternator as a motor" you will find all kinds of crazy stuff they do with alternators as motors. search youtube, you'll see real crazy stuff there

[Nordic]

Do you have a diode in the harness.... possible backfeed to the alt.

[59stepside]

I'm just thinking off the top of my head....alt gets power from ignition switch on power for the exciter I believe. So I would just check with a voltmeter your connections. Depending on the style, you should have the main wire that runs directly to the battery and a smaller wire with it that should be terminated near the battery or junction block near battery which senses the voltage. This wire is usually just jumper back to the main wire at the alternator. (Which is not very effective btw). And thirdly there should be power from ignition switch keyed on. If this is getting power with ignition off that could cause your problem. With truck turned off and power applied to that wire, it will sense below its set voltage and constantly try to charge/run.

[59stepside]

So you can see the two red wires, they will have 12-14v. The third wire labeled brown but could be any color, usually white should only have voltage with key on. If I remember correctly. If I am wrong please correct me.

[mobileortho]

Got an online coupon for $50 off @ Advance Auto so I just bought a new one, problem solved.

[59stepside]

Hopefully. :-)

[wildfifty2]

Originally Posted by mobileortho

Got an online coupon for $50 off @ Advance Auto so I just bought a new one, problem solved.

$50 coupon... where do I sign up??

[mobileortho]

Originally Posted by wildfifty2

$50 coupon... where do I sign up??

It was part of a Dec. promotion. I made an online purchase and they sent me.

[Highsider]

Originally Posted by mobileortho

It was part of a Dec. promotion. I made an online purchase and they sent me.

Cool to get a credit that large.

Last fall an Advance Auto lifetime guaranteed heater motor failed on my rv.
When I exchanged it, the new one was cheaper and they handed me a $1 cash refund.