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Re: not charging at low idle
How have you determined "not keeping up"?
We need to diagnose whether the culprit is the alternator or battery
(or something else like an accessory drawing more than normal current)
With a digital voltmeter (not analog) , measure battery with car off, then compare battery at idle.
Off reading = about 12.6 - 12.8 volts DC
Idle reading must be higher than above or no charging will take place,
Idle volts typically 13 - 13.5 and should increase a bit with RPM until
regulator limits the increase.
Some possibilities are :
bad diode in alternator (can be replaced separately in many alternators)
bad regulator (built-in to modern alternators)
bad connection at battery terminals (wire brush or use battery terminal cleaning tool)
bad battery (shorted cell, often caused by allowing water level to go below tops of lead plates)
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Probably using the AC takes a lot of power. Requires the alternator to keep the battery charged, drawing more power making the engine to slow.
The same thing happens when you press on the brakes, but not as obvious.
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.
Your idle speed may be too low. It is common for the Alternator to slow down with the engine RPM. You can get a free scan of your charging system at Autozone or Oreillys. You may have a lazy Regulator which is not allowing more power through.
Some cars like Ford products have a sensor which automatically increase idle speed to keep the Voltage up. In your brand you need to ask a Dealer. When your car has more equipment demanding power than the Alternator produces, the Voltage will drop.
You may want to install a new Serpetine belt and check your Tension pulley for a tight fit. If the belt is sloppy it can effect the charging system.
These trucks have what is called a smart charge system. It normal for the alternator light to elluminate when the truck requires no charging or the idle is low. The most likely reason you have low idle is throtttle body coking which requires cleaning or a bad idle air control valve. Also clogged injectors will cause a low rough idle but if that was the case misfires would show up and can be seen while they happen using a diagnostic data scanner.
And the first thing we always look at is the air cleaner. If you have a K/N filter the oil can contaminate the MAF senor. If the sensor cannot see and feel the air then it won't be able to add fuel causing a low idle, The MAF can be sprayed with brake cleaner to clean it. Usually you will get a p0171 and or a p0174 if the maf was causing a lean condition.
charge it up .crank engine while its idling get a volt meter hook it to battery .volts should be 13.5 to 14.5 if its 12 to 10 or lower.alternator is not charging.if alternator is charging . turn engine off battery should stay 12.5 volts if it drops after engine is off to 11.0 or 10 volts battery weak replace it.