Good or Bad Alternator???

[primo74620]

A few days ago a suspected that my alternator or voltage regulator was bad. Today, I had the truck running with a brand new battery and checked it with a Voltmeter. With the truck(74' 620) idling the meter hooked up to the battery read 14.6. As I turned on the lights the meter read 15.7. I also noticed that my charge light at the dash would blink at times. Can anyone give me some feed back on this?

[john510]

I think with those voltage readings your alternator must be charging the battery.Check the voltage with the engine off.Check the fan belt,make sure its tight enough.

[primo74620]

I think with those voltage readings your alternator must be charging the battery.Check the voltage with the engine off.Check the fan belt,make sure its tight enough.

 

So is it normal for the voltage to jump from 14.6 w/ the lights off and 15.7 when I turn the lights on? With the engine off and hooking up the voltmeter to the battery I am getting 12.6-8!

[john510]

It sounds like its charging,i couldnt tell you for sure why the voltage increases with the lights on except maybe the voltage regulator is doing its thing to keep the battery up.Someone else here will have answers or suggestions for you im sure.The blinking charge light could be a loose belt or bad connection at the alternator or voltage regulator.Check the connectors on those.That voltage at the battery with engine off is about right.

[primo74620]

It sounds like its charging,i couldnt tell you for sure why the voltage increases with the lights on except maybe the voltage regulator is doing its thing to keep the battery up.Someone else here will have answers or suggestions for you im sure.The blinking charge light could be a loose belt or bad connection at the alternator or voltage regulator.Check the connectors on those.That voltage at the battery with engine off is about right.

 

Thanks John, I appreciate the info.

[ggzilla]

It doesn't normally go as high as 15.7, but if the battery is low it might.

[primo74620]

It doesn't normally go as high as 15.7, but if the battery is low it might.

 

It is a new battery. I was told that the alternator may be bad since it is going that high when I turn the lights On. But other than that it reads fine.

[banzai510(hainz)]

Primo something is wrong,

I would think its the volt reg.

I assume you have a external volt reg on this since its a 74.

 

I have seen where people have a external volt reg and put on a Internal reg alternator. Gives a wondering around voltage.

 

you dont want over 15 volts it will slowly cook the batter(acid ooze out the top and ruin what paint you have left )

 

 

just make sure your grounds are good also incase soemthng obvious

[racerx]

Take it to Kragens, now Oreily, and have them check it..

[Z-train]

The alternator light comes on when the voltage exceeds parameters-be it low OR high.15.7 volts is pushing the upper end.I suspect the flickering light is trying to tell you that you are close to having too many volts.

[Spades]

when you start getting over 15 volts, something is wrong. Often the alternator will push more voltage when a battery is low or a component(like an internal or external regulator) is going bad. The more voltage, the less amperage required to charge, hence why a even though a car battery is 12 volts, a cars electrical system tends to be designed to run around 14 volts during operating. 13.7 volts from the alternator may work, but will eventually undercharge and fry the battery, 15.7 may work, but will eventually overheat and overcharge the battery...it may take some time, but that is most likely why the charge light is on.

 

I wouldn't condemn the alternator or regulator so fast without doing a proper load test on your new battery though...I have seen too many alternators replaced because of bad batteries. Most new digital testers will test the impedance of a battery for a simulated load, and that will tell you if the cells are bad, but an old carbon pile tester like a VAT-40 will give you a better idea of what the alternator, starter, and battery are drawing for amperage and voltage(as well as how much the alternator is actually putting out).

 

If the new battery tests out ok, and you have an external regulator and no access to a proper amperage tester, I would start with the voltage regulator...it is a cheap part that is usually readily available and often fails long before the generator/alternator will.