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Ron, ASE Certified Technician
Category: Ford
Satisfied Customers: 35490
Experience:  23 years with Ford specializing in drivability and electrical and AC. Ford certs and ASE Certs
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My battery light is on in my 1999 Ford F150. I put in a different

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My battery light is on in my 1999 Ford F150. I put in a different alternator, cleaned the battery cables, put in a new battery, checked all the fuses, and the light is still on. When test driving it once, I noticed that if I hit a bump, the light would go out temporarily, then come back on. I would like your ideas. Thanks! Mike
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Ron :


The battery light is controlled by the internal voltage regulator and in most cases its comes on when the alternator/regulator has failed or the connection to the regulator.Disconnect the alternator to verify the light goes out proving the Light Green/Red wire is not shorted to ground in the harness. If the light is still on, locate and repair the short to ground in the harness.
If the light does go out, at the alternator with everything connected and the engine running, check the voltage on the Orange/Light Blue wire in the 3 wire connector at the alternator looking for full battery voltage. If the voltage is low on that wire, locate and repair the voltage drop in the circuit. Start with checking for a problem at fuse #11 20A in the engine compartment fuse box. If the voltage on the OG/LB is good, check the voltage on the White/Black wire in the 3 wire connector looking for half of system voltage with the alternator charging. If the voltage is correct, inspect the connections at the 3 wire voltage regulator connector. If the terminals are all good and tight, suspect a faulty regulator in the alternator.

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Customer: replied 6 years ago.
When you say to disconnect the alternator, do you mean to disconnect it where it bolts on to the alternator? I disconnected this wire, and the light battery light is still on. Do I need to disconnect all of the wires going to the alternator in the first step?
Yes you disconnect the connector. The light should go out, if not the wire is shorted to ground.
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