General Ford Ranger Discussion General discussion of the Ford Ranger that does not fit in any other sub-forum.

ranger battery light

Old 02-10-2019
Thread Starter
Join Date: Nov 2018
Location: Spring hill florida
Posts: 2
ranger battery light

I have a 2000 ranger 2.5 battery light keeps coming on the battery, starter, alternator checks good.
I cleaned the battery posts and terminals .The truck starts fine and the truck amp gage reads fine.
I am going crazy. Any ideas ? need help!!!!!

thank you.
Old 02-10-2019
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iTrader: (1)
Join Date: Jan 2014
Location: Vancouver, BC
Posts: 13,060
Welcome to the forum

Battery light circuit is pretty simple to understand

Cab Fuse box, key on 12volts---------------Battery light Bulb----------------------(light green wire)-------------------------Alternator(voltage regulator)

When you turn on the key the battery light bulb gets 12volts from a fuse, and if engine is off, alternator is at 0volts so other connection on bulb is a "ground", so bulb lights up

When you start the engine the alternator takes over and supplies ALL voltage in the vehicle, it sends out 14volts, battery is only 12volts so it absorbs some of this voltage to keep it charged up

So Battery light bulb now has 14volts from fuse panel and 14volts from alternator wire, so bulb is OFF, a DC voltage light bulb needs two Different voltages to light it up, which is important

If battery light comes on it means there MUST BE two DIFFERENT voltages at the bulb, simple as that

The Voltage regulator inside the alternator is what the battery light bulb is connected to, its the Light Green wire on the 2 or 3 wire connector on alternator

This light green wire also provides the "startup voltage" for alternator, so there is a circuit inside the voltage regulator that can fail and cause battery light to come on even though battery shows 14volts with engine running

No way to test voltage regulator for this, the fact the alternator is generating 14volts even at idle(13.5volts minimum) and the battery light is coming on would indicate to me that the voltage regulator needs to be replaced.

You can test green wire, unplug 3 or 2 wire connector on alternator, use volt meter to any ground and then to green wire
Turn on the key, should see battery voltage on green wire, 12.3v to 12.8volts
Move wiring harness around, see if maybe the green wire has a short to ground somewhere, that would also cause battery light to come on because "ground" is same as 0volts

If you have the original factory alternator I would be tempted to change the whole thing, its 18 years old, so bearings are getting close to the end

Last edited by RonD; 02-11-2019 at 09:59 AM.
Old 02-11-2019
Join Date: Aug 2018
Location: Winston, Ga
Posts: 114
Ditto on the wiring issue.
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