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doc hopper
Samba Member
Joined: December 08, 2006
Posts: 588
Location: California
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 Posted: Mon Mar 21, 2016 6:38 am Post subject: Alternator AND Battery Failure? |
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I pulled into my driveway yesterday and at idle my alternator light came on. I revved the engine and it went out. No big deal, right?
But I plugged one of those cig lighter volt meters in and saw that even with the engine revving, the voltage never got past 12.7 volts. I'll add that in the past, I have used this same tester and got a reading of steady 14.7 volts at speed. But unlike other alternators I have tested in this manner, the voltage doesn't jump around .. .. it holds steady at 12.7.
How low does the voltage need to be for the alternator light to come on?
Later on, I plugged in the tester with the engine off got a reading of 12.0 for the battery. 8 hours of charging with a Battery Tender got it up to 12.7, but overnight, it dropped back to 12.0 volts. (75 month battery now at 60 months)
It's looking like the alternator AND the battery are in trouble.
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Eric&Barb
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Joined: September 19, 2004
Posts: 25959
Location: Olympia Wash Rinse & Repeat
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| Posted: Mon Mar 21, 2016 7:17 am Post subject: Re: Alternator AND Battery Failure? |
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Sounds like the battery is starting to go out, but would try letting it sit over a few days to see if the drain of voltage continues as much. With one of the battery cables off that would eliminate possible wiring short draining the battery.
As for the voltage not going up when revved, the battery could have been as charged up as the regulator is set for. That way the battery is not damaged by being over charged.
Would turn on the headlights and see if the voltage then soon drops below 12 volt or if the alt kicks in and keeps voltage up above that, or shut of the engine, turn on headlights for five minutes. Then start the engine to see if the voltage goes up above the 12.7 mark when the electrical system is charging what you just drained out of the battery with headlights and the starting of the engine.
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busdaddy
Samba Member
Joined: February 12, 2004
Posts: 52799
Location: Surrey B.C. Canada, but thinking of Ukraine
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| Posted: Mon Mar 21, 2016 7:24 am Post subject: Re: Alternator AND Battery Failure? |
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| doc hopper wrote: |
| How low does the voltage need to be for the alternator light to come on? |
The light isn't an indication of low voltage specifically, it tells you when there's a difference between alternator output voltage at the unit and battery voltage at the fuse box. The bulb is not grounded as you'd expect in a standard circuit, one side of the filament is connected to the alternator and the other to the bus's electrical system, any difference results in the light coming on, most times it's because the alternator isn't working, but it can also be lit by a lower voltage up front while the alternator is working fine, a glowing charge light when the headlights are on because of weak wiring is an example.
Since you were getting ~12 volts at the cig lighter it sounds like your alternator has taken a dump, is it a real Bosch unit?, or mysterious Chinese?, externally regulated (3 wires + fat red)?, or internal (one wire + fat red)?
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