Alternator won't charge consistently on 1988 GMC suburban
Voltmeter shows alternator putting out a charge when vehicle is idling. The voltmeter will continue to show and increase in voltage. once the voltmeter shows a significant charge, I can begin driving and turn on all electrical devices, heater fan, lights, radio, back window, etc. However, as soon as I brake or let my rpms drop off, the voltmeter shows that I am draining the battery. If I stop, let it idle, it will build back up and I can go again with everyting on.
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Check the alternator and the charging system. You may have a weak or failing alternator. If you have a voltmeter, get the battery reading with the engine off. A charged, good battery should show at least 12.6 volts. If you have to boost it, it must be low on charge. Recharge it or have it tested, a dead cell in the battery would not let it charge up.
Now take a reading on the battery with the engine idling, or idling as low as possible without dying. A good alternator and charging system will make the battery now read at least 13.5 volts. If your reading is lower than this, you probably have a weak alternator.
-Most Alternator not charge, because the brushes are won out.
But here is the basic to determine what is wrong before replace any part.
-Turn on the car let it run to normal idle then use voltmeter to check
power at battery terminal. If it's below 13 volt. The alternator need to be replace. But if it show 13.5 to 14 volt. The alternator is good. The battery is bad. It doesn't hole any charge. Need new battery.
Keep in mine. each battery is last ONLY 5 Years.
First, you state "the battery is only getting 11.58". This implies the alternator IS charging albeit too low. I would connect the voltmeter to the main terminal on the alternator and take a reading before starting the motor, then again with the motor running at around 1500 RPM (fast idle). This will indicate whether the alternator is working or not.
Getting back to the ign light, does it remain at the same brilliance before starting the engine and after?
You don't say what year your truck is but since the 70's the alternator and voltage regulator are housed together. Use a voltmeter to check the output from the alternator to a good battery when running it should be putting out 13-14 volts if not there is your problem.
First off, never disconnect battery when vehicle is running. It could stress out the alternator and blow the diodes. Now you need to put a volt meter on the battery while the vehicle is running to see if the alternator is putting out 14 volts. If its putting out 12 or less, you have likely blown the alternator. Remove alternator and have tested free at Orielys or napa. I dont trust autozone.
start engine unhook positive cable if it dies its bad alternator or fuesable link between batt and alternator also a volt meter will tell u if its charging a good alternator will charge 13.50 to 14 volts
that sounds about right . the voltmeter just tells you how many volts the battery is holding,if you turn your lights on high beam turn your wipers on and your heater all at the same time,as soon as the current being used is greater than the alternator can charge , you will see the needle on the voltmeter drop back that is normal and logical. if you are running spotlights etc you may need a higher wattage alternator a lot of vehicles come out with 55watt alts.and the owners change the alts. to 100 watt alts... NOW as long as yours is not dropping below 12volts it is not draining your battery so there will be no ill effects it just means the alternator is working close to its capacity-------------------- there you go ,I would not be worrying too much unless it flatens your battery Idony think you need worry "youdoit"
Investigate the battery and the alternator.
The battery's only job is to start the car.
The alternator's job is to power the electrical supply for the car.
The battery can go bad at any time, and the alternator is probably is putting out only enough electricity to only keep the car running. And not enough to power charge the battery and power the headlights.