Is my alternator ok? Results of multimeter test (1 Viewer)

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Sep 18, 2019
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Mississippi Gulf Coast
I'm noticing that when the truck is at idle and the turn signal or hazards are on, the voltage meter dips below 12 volts at the same time I hear the relay for the turn signal clicking. So I was wondering if the alternator is going back. I did the simple multimeter test where you take the voltage at the battery terminals with the car off, then start the car and check the voltage again. With the car off, it was 12.71 volts. Once the car was started, the voltage went up to 13.88 volts. I know that I'm supposed to be looking for 14 volts. 13.88 volts sounds close enough.

If the alternator is ok, then the voltage dipping at the gauge could be attributed to a dirty connections along the circuit?
 

OSS

Joined
Jun 30, 2017
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Oblivion
The jumping needle of the battery indicator in the cab when the turn signals blink is "normal".
It's caused by the inadequate wiring harness, not the alternator.
13.88V peak voltage from the alternator on a fully charged good battery is lowish. The battery will never really get fully charged.
A new alternator with new regulator should put out about 14.3V. At the very minimum (for a new alternator) 14.1V.

If my alternator only put out 13.88V peak voltage on a charged cool battery, I personally would replace it and the regulator. But that's me.
 
Joined
Jan 4, 2011
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Vermont
Most alternators do not produce full output at idle. Normal operating can see voltage varying from 13 to 14.4 volts. A healthy lead-acid battery at rest is considered 12.6 volts. More than 14.4 volts will boil the battery and bad things will happen. As @OSS points out your 30+ year old wire harness is not doing you any favors. I recently measure the idle voltage at my battery and then at my headlights and I was losing 1.5 volts through the harness.

The needle bouncing is more likely a result of fluctuations in the alternator output at idle as the battery voltage goes up an down. Keep in mind that your truck runs off the battery. The alternator is just there to put the electrons back in the battery. You can do new battery terminals, marine grade battery cable, new connectors, clean your grounds etc and bump the electrical stability quite a bit. It should also help out the alternator as you won't lose as much through high resistance wires.
 

2mbb

SILVER Star
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Sep 1, 2003
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WC, CA
I see the fluctuating needle too, and always have. The voltage meter measures after the ignition switch, so there might be some additional loss there, as well as wiring, connections, etc.
 
Joined
Jan 4, 2011
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You might buy an inexpensive digital voltmeter and wire it up to a cigarette lighter plug. then plug it into the cigarette outlet and you can watch your voltage more closely. If your voltage is bouncing around 13.0-14.4 volts I wouldn't worry about it too much especially if it stabilizes when the rpm's increase. But if the voltage is dropping lower or it never stabilizes then you might have another issue...could be alternator, voltage regulator, wire degradation, corrosion, age etc.
 
Joined
Nov 16, 2004
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Simpsonville,SC
I got the @Forerunner kit and all needle bounce stopped, even with a new battery and alt I had it.
Super nice kit and great vendor.

 

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