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Just got my 2000 F250 back on the road after replacing all the burned out instrument cluster bulbs. Now I have an "Alternator failure idiot light" on. See picture below. Checking the alternator with a NAPA alternator tester it tested OK. With truck off, a multimeter at the battery reads 12.3 VDC, at idle with headlights on the multimeter reading at battery is 14.1 VDC. The gauge reading is rock solid, it never moves. With the key in the ON position and the engine off, or running down the freeway at 70 MPH it stays at the same position shown in the photo. And of course now the Alt Light is shining with its new bulb in place. Anyone experience any similar issues?
There is a link in that thread to another thread which is worth a read. Your alternator is charging, but not sending the signal to the light to say that it is charging. Check you connections, alternator may be bad as well.
The socket for the alternator charge bulb is different from the rest and has a resistor built in to the socket. You might have switched the bulb socket and placed it in the wrong place. That would be the first thing I would check.
Thanks 73F700 for the insightful link to the Ford-Trucks.com site. There is some good reading there pertaining to my exact situation!
And thanks to Evan P. While I was careful and only pulled one bulb at a time (festina lente) while changing them, anything is possible. Perhaps the previous owners mixed things up.. Anyway, it is a pretty easy check to pull the gauge cluster and hunt for the 470 ohm resistor which should be in parallel with the Charge Indicator bulb. Would have never thought of that on my own.
Just to add, this is a gas 5.4 engine.
I had the same thing happen on my 99 f250. One of the brushes had worn a groove in the stater shaft and would turn the light on. Everything tested perfect. A new alternator fixed my issue with the light.
Solved. Pulled out a digital multi-meter with the truck engine running. (DMM probes go easily into socket.) Found a solid -0.01 VDC where 6 VDC should have normally been found with alternator in good condition. This condition led to ground leg of Indicator Bulb being pulled down from a normal condition 12 VDC to 1.6 VDC, lighting Bulb. The truck Volt Meter is apparently only indirectly controlled by voltage, there is a solid state module which actually controls the needle position. Will run it for another 2-3 weeks till I scare up some cash for a new alt.
Update Jan21st - Pulled old alternator. Found the 'S" single connector pin had been bent over by previous owner, as shown in 2nd image, apparently when he was dinking around with something. This "S" connector was NOT making an electrical connection. Am 90% sure if I would have been able to spot that bent pin all would have been OK. But as it is, I already purchased a new 130 amp alternator from eBay for 80 bucks, had it sitting right there; installed the new alternator and charge light has gone out and alternator is charging properly once again.
You have charging voltage, it is just the idiot light circuit that is bad. Quite likely has been this way for some time. Will be fine for a few weeks unless the charging circuit decides to go bad as well.
