We have a 94 ford explorer. The ammeter says it's charging when we drive on the highway. But when we drive in stop and go traffic the needle goes down. Eventually the battery goes dead if we keep driving in the city.
I've noticed that the ammeter goes down each time the rpm goes below around 650 and then goes up again when the rpm is above 700. Is there a way to set the idle speed - or should it not make a difference?
Going from memory, 650 isn't very low for idle. I'm not sure how to set the idle speed, either. Maybe Jim will help. Have you had the alternator checked?
You don?t say by how much the ammeter reading goes down. But this sounds normal to me. Every alternator made gives less output at lower speeds. Every alternator made has a steep curve in the engine idle speed range. So an alternator rated at 120A for example, only gives
120A at high speed, around 6000 alternator RPM (maybe 2000 engine RPM). This same alternator might give a max of 50A at 1600 Alt RPM and 70A at 1800RPM. The engine idle speed probably falls between these two values. The lower alternator output at lower speeds is completely normal, and not a quality problem. It is part of the physics of any alternator.
So if you say 1) the alternator was checked and is OK, and 2) highway speeds are no problem, but city driving is a problem, then I suspect the most likely case is that you have added some aftermarket electrical components which are drawing too much current at low speed, and draining the battery. (Average engine speed in City driving is much lower, more idling)
Did you add anything, like a subwoofer, new electric radiator fan, refrigerator, etc ?
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