F-150 alternator

When the battery light on my F-150 came on (about 100,00), the dealer put on a new alternator. This has now been done 3 times in the last year, and the battery light still comes on intermittently. They have no ideas about what to do. Anyone else?

Your best bet is to take your truck to a good independent mechanic. I’ve never understood why people take their vehicles to a dealership and get the same old runaround. A dealership sells vehicles…very few of them know how to actually repair a vehicle.

Are they actually checking the alternator each time and finding that it is faulty?

I drive a '97 Escort - it obviously could be wired completely differently. But the signal for the battery light on my car is not from a direct measure of the alternator or battery power. It’s an independent signal wire. Once in a while I’ve had that wire come loose and the battery light comes on. I can put a voltmeter on my battery and/or alternator with the car running and the light on and the alternator is behaving perfectly well. I clean up and tighten the connection on the signal wire and the light goes back off.

So anyway…make sure that possibility has been eliminated. Bit if 3 alternators have been installed and - in fact - gone bad, I’d probably replace all of the main power lines for alternator and battery. If they are old and crusty and creating too much resistance they will beat up the alternator. The other question would be whether or not you have any power-hungry aftermarket add-ons (dump bed, plow, winch…?). For power hungry add-ons, sometimes an alternator has to be upgraded to higher than stock output.

The ignition switch and battery warning light circuit supplies power to the exciter inside the alternator. Normally there is 12 volts on that lead while the alternator is working correctly. When there is a problem the alternator will make a ground connection to that lead and turn on the light. Since the light is in series with the circuit it is important that the light works so the current can pass on to the alternator exciter field windings which build up the magnetic field to produce the charging current.

There is another lead that ties to the back of the alternator which is the battery sense lead. It also should be close to battery voltage when the alternator is working. If that lead has a problem on it like a bad connection the alternator regulator would think the battery voltage is low and so it would cause the alternator to produce a higher output than it normally would have to. Over time that could shorten the alternator life span. This may not be your problem but it is possible. The connections to the alternator need to be checked while the fault is taking place in order to verify something is wrong with the wiring to it. The main connection between the alternator output and battery should be checked for a problem also.