Re: Battery charges when idling but not at speed. 81...
This sounds like the altenator belt is loose. When the altenator is under a load the magnetic field causes the altenator to require more fource to rotate. At higher engine rpms the belt can be slipping on the altenator pulley. There is one other possibility I can think of and that would be that the voltage regulator is faulty and and causing an overvoltage at higher engine speeds and therfore opening up the field circuit to prevent the altenator from over charging the battery this is a safety function of the regulator. I would check the belt first.
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Tom, When running voltage should be 14 to 14.8 volts, check all connections,fuses/relays,fusible links if equipped, bad battery, alternator defective, not wired up properly, broken wires loose alternator belt, etc. Diagnosing Alternator Problems EricTheCarGuy
Not sure I follow your problem. The alternator puts out 13-14 volts at idle but very few amps. If you turn on the headlights at idle the voltage may drop to 12 until you raise engine speed. The amperage goes up with engine speed. Is the battery voltage at least 14 when the engine speed is 1500RPM or more ?
SOUND LIKE ALTERNATOR NOT CHARGING BATTERY.BEFORE BUYING A NEW ALTERNATOR CHECK ALTERNATOR WIRES AND FUSE.IF YOU HAVE TO REMOVE ALTERNATOR.TURN OFF RADIO BEFORE REMOVING NEGATIVE BATTERY CABLE.SO YOU WONT LOSE PLAYING CODE.
The instructions for installing a new alternator usually read to fully charge the battery (12.3 - 12.8 volt range). Otherwise, the alternator will be damaged when it initializes. I have seen some extreme damage result from installing a new alternator with a low-charge battery. The torque required to turn an alternator when damaged in this scenario is unbelieveable!
It sounds as if the alternator is not putting out enough charge at idle to keep the car's battery at minimum necessary voltage levels. When you drive, the RPM are higher and the alternator spins fast enough to charge the battery. At idle speed, the output from the alternator isn't enough to replenish the battery's charge. Find out whether you have a separate voltage regulator, or if it's built into the alternator (they often are). I think you're looking at a new alternator though. You could go to an auto parts chain store and have a charging system test done to see if everything is OK - at AutoZone or Advance Auto Parts, they'll do it for free - it'd be worth the time to have them do that, to see whether it's the battery or alternator, or neither.
If the alternator isn't putting out as it should be the first thing
that needs to be checked is the battery because it really has an
influence as to how the alternator charges. Since you have a new/good
battery...hopefully the cables where they attach to the battery are
corrosion free...and the alternator has been replaced several times and
tested the problem shouldn't be with those 2 components.
The next place to look is the AIR pump belt because it drives
the alternator belt. It has a tendency to loosen up and when it does
the alternator doesn't turn as fast because of the belt slipping on the
AIR pump pulley. To verify this with the engine off place your thumb on
the fan blade of the alternator and try to push/turn it. It shouldn't
move at all and if it moves easily then you have probably found your
problem. There is 1 bolt in behind the AIR pump that you will need to
access from below and loosen otherwise the pump will not move when you
turn the tensioning bolt as well as 2 in front.
There is another possibility and that is that the idle
speed is too slow to allow the alternator to charge properly with
everything on. If you raise the idle speed up by pressing your foot on
the accelerator and your voltage returns to the 14 volt range then try
checking the adjustment of the minimum air/idle screw, throttle
position sensor/TPS and the air gap for the idle speed control/ISC
motor and maybe you can get the charging voltage to a normal range that