I have a 1990 5.0 and I recently have had the battery and alternator tested at auto zone and both tested good. However, I charged my battery with a charger overnight. Reinstalled in car, it started and ran fine for a dew minutes, took it for a ten minute drive- no problems.
Over the last few day I have started it once yesterday to pull it out to wash and today I was working on the ac system and started and shutdown several times
Finally I ran it to charge the ac system over about 15 minutes and then I went to drive it and my headlights were dim and the car kind of stalled. I turned off the engine and now it won’t even turn over. All lights inside and out are on and battery voltage reads 11.5 volts
Could my alternator still be bad even though the auto zone test showed good?
so this morning the battery read 12.6 volts and with the car running it read 12.5 volts. Can someone help me find the a wiring diagram for the wires coming out of the alternator? Also I have a stock set up so I don't know if thats 2G or 3G etc. I was going to buy a new alternator and then see what the output is then. Otherwise I have a bad ground or something.
Alternator troubleshooting for 86-93 5.0 Mustangs:
Never, never disconnect an alternator from the battery with the engine running. The resulting voltage spike can damage the car's electronics including the alternator.
Revised 16-Oct-2016 to add 94-95 alternator wiring diagram
Red color text applies to cars with a 3G alternator.
Do all of these tests in sequence. Do not skip around. The results of each test depend on the results of the previous tests for correct interpretation.
Simple first step: Remove the alternator and take it to your local auto parts store. They can bench test it for free.
Use a safety pin to pierce and probe the insulated connectors from the rear when doing tests with the connector plugged into its' mating connector.
Engine off, ignition off, battery fully charged.
1.) Look for 12 volts at the alternator output. No 12 volts and the dark green fuse link between the orange/black wires and the battery side of the starter solenoid has open circuited. 3G alternator: Look for 12 volts at the stud on the back of the alternator where the 4 gauge power feed wire is bolted.
No voltage and the fuse for the 4 gauge power feed wire is open or there are some loose connections.
2.) Look for 12 volts on the yellow/white wire that is the power feed to the regulator. No 12 volts, and the fuse link for the yellow/white wire has open circuited.
Engine off, ignition on, battery fully charged:
The instrument cluster must be in place and working properly for the alternator to charge. The indicator light is an essential part of the charging circuit.
1.) Alternator warning light should glow. No glow, bulb has burned out or there is a break 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 ohm 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.
2.) Reconnect the D plug to the alternator
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see 2.4-2.6 volts. No voltage and the previous tests passed, you have a failed voltage regulator. This is an actual measurement taken from a car with a working electrical system. If you see full or almost full12 volts, the regulator has failed.
Engine on, Ignition on, battery fully charged:
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see battery voltage minus .25 to 1.0 volt. If the battery measured across the battery is 15.25 volts, you should see 14.50 volts
You will need to do some voltage drop testing of several of the wires.
Start looking for these things:
1.) Bad diode(s) in the alternator - one or more diodes have open circuited and are causing the voltage to drop off as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.
2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Do the voltage drop test as shown in the Fluke tech note link. Measure the voltage drop between the alternator frame and the battery negative post. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.
3.) Bad regulator that does not increase field current as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.
4.) Bad sense wire - open circuit in sense wiring or high resistance. The yellow/white wire is the voltage sense and power for the field. There is a fuse link embedded in the wiring where it connects to the black/orange wiring that can open up and cause problems. Disconnect the battery negative cable from the battery: this will keep you from making sparks when you do the next step. Then disconnect the yellow/white wire at the alternator and the green fuse link at the starter solenoid/starter relay. Measure the resistance between the alternator end of the yellow/white wire and the green fuse link: you should see less than 1 ohm. Reconnect all the wires when you have completed this step.
5.) Bad power feed wiring from the alternator. Use caution in the next step, since you will need to do it with everything powered up and the engine running. You are going to do the Fluke voltage drop tests on the power feed wiring, fuse links and associated parts. Connect one DMM lead to the battery side of the starter solenoid/starter relay. Carefully probe the backside of the black/orange wire connector where it plugs into the alternator. With the engine off, you should see very little voltage. Start the engine and increase the load on the electrical system. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.
Alternator wiring diagram for 87-93 Mustangs.
The following are diagrams courtesy of Tmoss & Stang&2birds
Alternator wiring diagram for 94-95 Mustangs.
Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections
0.0V bolt together connections
Alternator wiring circuit
Notice the green wire connects to a switched power source. The circuit contains a 500 ohm resistor in series between the switched power and the alternator. Connecting it to switched power keeps the regulator from drawing current when the engine is not running. The resistor limits the current flowing through the wire so that a fuse isn't needed if the wire shorts to ground.
Also notice the sense wire connects to the starter solenoid and it is fused. It connects to the starter solenoid so that it can "sense" the voltage drop across the output wiring from the alternator.
Fuse link for 86-93 Mustangs
14 gauge fuse link for stock alternator.
Bussman BP/FL14 Fusible link
Dorman - Conduct-Tite 14 Gauge Fusible Link Wire Part No. 85620
Advance auto parts #85620
Pep Boys - SKU #8637594
So I bought anew alternator and same problem not charging. You might be right Mustang5L5, cause I don’t have a battery light on. I recently converted the illumination bulbs to LED, but the battery light wasn’t one of them. I did check the the connectivity for the alternator positives and grounds and all checked out
I will check out the cluster light tomorrow
Thanks to all for your help and I will keep you posted
Mustang5L5. It does seem happen AFTER i changed the bulbs. I also accidently shorted out the part of the cluster circuit because of that crappy circuit film on the back of the cluster. The battery light does NOT come on when I turn the key on. I have to also check the fuses as well. Could be that simple.
Solder some 22 gauge copper wire to the places where the traces have burned up or use some of the material from the Rear Window defrost repair kits to repair the damaged copper traces.
Use gel type super glue to stick the copper foil back to the plastic sheet. The super glue will withstand the heat of soldering as long as you don't apply heat for longer than it takes to melt the solder with a clean tip on the soldering pen.