How are you testing the alt?
Check the wires for discoloration and check your grounds.
Alternator troubleshooting for 86-93 5.0 Mustangs:Still having issues with the new alternator install. The alternator is not charging the battery and when we turned on the car no battery light came on. What does this mean?
Engine off, ignition on, battery fully charged
1.) The alternator warning light should glow. No glow, bulb has burned out or there is a break or bad connection in the wiring between the regulator plug and the instrument cluster. The warning light supplies an exciter voltage that tells the regulator to turn on. There is a 500 Ω resistor in parallel with the warning light so that if the bulb burns out, the regulator still gets the exciter voltage.
Disconnect the D connector with the 3 wires (yellow/white, white/black and green/red) from the voltage regulator.
Measure the voltage on the Lt green/red wire. It should be 12 volts. No 12 volts and the wire is broken, or the 500 ohm resistor and dash indicator lamp are bad. If the 12 volts is missing, replace the warning lamp. If after replacing the warning lamp, the test fails again, the wiring between the warning lamp and the alternator is faulty. The warning lamp circuit is part of the instrument panel and contains some connectors that may cause problems.
You may have a bad wire or bad connection between the D shaded plug green/red wire on the alternator and the green/red wire on the instrument cluster. Use your ohmmeter to check the connectivity; you should see less than 1.5 Ω.i have no alt lamp when ignition on and no 12 volts to the lt grn/red wire. replaced alt cluster lamp, still doesnt light up. what wires do i look for to trace it back to the alternator?
If you own a Mustang, a good analog Multimeter or DMM (Digital Multi Meter )is a must have item.would swapping out the cluster gauge with a known working one be an option?
l’ll have to see about grabbing an ohm meter to check connectivity.