What is wrong if the battery light is on in a 1999 Ford Expedition and it has 11.5 volts at battery and you replaced the alternator and it dies if the battery cable is pulled while running?

Answer 1

Check out the battery cables. Make sure that they are connected good and not corroded or damaged. Also check the connections at the alternator.

I had a 97 Expedition with a battery indicator, 12.4 volts at the battery with the engine off and 11.6v with the engine running. Replaced the alternator and then had 11.9 v with the engine running. Checked the alternator fuses on the firewall and found the 175a alternator output fuse blown. Replaced the fuse and voltage then read 14.3 with the engine running. Not sure if the fuse was blown before the alternator was replaced or not as I never thought a 175a fuse could blow without fireworks.

2 cents:

Fuse..... Also check for defective replacement alternator, alternator rebuilt with incorrect regulator, damaged instrument panel cluster, or damaged or poorly connected wiring.

Rocky_B Sounds like a dead battery cell. 1) Engine OFF - A servicable battery will show 12.5 volts at the terminals, anything less and you have dead or dying cells - Replace the battery. 2) Engine RUNNING - A good alternator should put out 14volts or more. 13-14volts=weak alternator, If voltage at the terminals is the same or lower after starting, your alternator might not be putting out anything or you have bad diode pairs in the voltage regulator. With most newer models the voltage regulator is attached to the alternator, so you replace both. If the voltage regulator is not attached to the alternator, you still want to replace both as voltage regulators tend to become mated to the old alternator's output over time. replacing just the alternator could cause the voltage regulator to fail.