2002 F-150 battery light on during load

2002 F-150, 4.2 V6, 5 sp manual, AC.

Under load - RPMs at 1500 or higher - the battery light comes on and engine stumbles slightly. Truck starts normally each time. So far...1. New battery 2. Checked alternator/battery 3 times - all good. 3. Serpentine belt looks ok. No failing bearing sounds from any pulley.

What should I chase next?
Thanks, Doug

I would pull the alternator out again and have someplace else test it as a second opinion, sounds like one or more of the diodes have failed, either that or there is an extra electrical load in the system somewhere that shouldn't be there that is stealing all the juice that would normally be used to run the engine with.

Thanks, Torch

Lemme clarify my test...I took truck to 3 different Autozones and had them test with cart diagnostic....all 3 passed with flying colors. Would pulling alternator and testing on bench somewhere reveal more? Thanks, Doug

For any suspected battery or alternator problem, I always check three voltage readings. I don't know how much of a help they will be in your particular case but you have to start somewhere, eh. The three voltages are:

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Voltage after sitting parked overnight, but before starting engine - should be around 12.6V (indicates that there is no excessive current drain with the ignition in the Off position, and that the battery is capable of holding at least some level of charge).

Voltage with engine idling - should be towards the high end of thirteen-point-something volts - maybe as high as 14V (indicates that the alternator is producing output current, and that that current is reaching the battery. Also indicates that the rectifier is functioning, converting AC to DC).

Voltage when revving engine slightly - should climb to around 14.2V and then hold steady at that voltage regardless of further increase in engine speed. (Indicates that the voltage regulator is functioning, and clamping the upper voltage threshold at the correct level).

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You've said the truck is starting normally, so therefore the battery must be at least reasonably healthy if it's capable of cranking the engine, so I expect you will indeed find 12.6V after it's been sitting for a few hours or overnight - I wouldn't expect a problem there.

For the second reading, If the battery light is off whilst idling then there's a good chance that you will have a normal voltage at that time (close to 14V).

What would be interesting would be to see what happens with the third reading, as the revs rise, at which time the voltage regulator is supposed to kick in and limit the upper voltage threshold at around 14.2V and then hold it steady at that voltage - I would expect to see an anomaly in that reading if not in the others.

I can't think of a better starting point, so I'd start by making those checks.

Thanks, Selectron...good data. Had a friend mechanic look at it. He says the alt is behaving properly but must have set a ?chip? to send abnormal data so batt/alt idiot light comes on...and chip can't be reset. Alt may be failing slowly...so I'll watch voltages and behavior and just be ready to replace soon. 5 years on a Ford OEM...seems like a short lifespan. I just replaced the OEM alt (whopping 55 amps) on my 1983 Mercedes 300SD.....23 year lifespan.

Yesterday, truck turned over so slowly...i said heck with thinking about this one anymore...bought a rebuilt alt at autozone and installed. This one must have been crapping out intermittently and I never caught it. Next time I experience this...ain't gonna put so much thought into it - just change it out.