Sudden Loss Of Charging / Amp Light On

yup sounds like your alternator just crapped out. take it your local parts store and have it tested.

check your grounds as well. Do a quick test with a DVM. put the + on the power wire off the back of the alt and the ground to any chassis ground. Is the alternator charging and does it increase as you raise the rpm?

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 15 April 2012 to add simple check for regulator failure in Engine off ignition on, battery fully charged section, item 2.

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:
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 dash. 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

Familiarize yourself with the following application note from Fluke: See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .


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.




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.

Replacement parts:
14 gauge fuse link for stock alternator.

Bussman BP/FL14 Fusible link
AutoZone

Dorman - Conduct-Tite 14 Gauge Fusible Link Wire Part No. 85620
Advance auto parts #85620
Pep Boys - SKU #8637594

If it is not running, charge the battery and have the store load test it too when you take in the alternator.

Before it totally died, having the parts store do a test of the whole charging system could have saved time.

Back in the day I used to test my alternator by starting the car and turn on the lights. Then while it was running I would disconnect the positive lead from the battery.
If the engine died when the battery was disconnected, that meant the alternator couldn't handle the load by itself so it was bad. LOL
I have done it to my fox Mustangs before but I wouldn't do that on my vehicles these days because it could cause damage.
That test sure worked good back in the days of simpler cars.

OP, if your feeling adventurous, give it a try. LOL

Onefine88 said:

Back in the day I used to test my alternator by starting the car and turn on the lights. Then while it was running I would disconnect the positive lead from the battery.
If the engine died when the battery was disconnected, that meant the alternator couldn't handle the load by itself so it was bad. LOL
I have done it to my fox Mustangs before but I wouldn't do that on my vehicles these days because it could cause damage.
That test sure worked good back in the days of simpler cars.

OP, if your feeling adventurous, give it a try. LOL

Click to expand...

I don't have words strong enough to say

NEVER NEVER NEVER DISCONNECT THE BATTERY FROM A CAR WITH THE ENGINE RUNNING.

The resulting voltage spike can damage the car's electronics including the alternator.

jrichker said:

I don't have words strong enough to say

NEVER NEVER NEVER DISCONNECT THE BATTERY FROM A CAR WITH THE ENGINE RUNNING.

The resulting voltage spike can damage the car's electronics including the alternator.

Click to expand...

LOL, I knew that was coming. I said back in the day.

It used to work on the older cars in the 70's and 80's. LOL

Onefine88 said:

LOL, I knew that was coming. I said back in the day.

It used to work on the older cars in the 70's and 80's. LOL

Click to expand...

Electricity was the same in 1970 as it is today. There was a spike then just like there is a spike now. The only difference is that the only electronic device in the car besides the alternator was the radio/tape deck. There was probably enough filtering then in the radio power circuitry to minimize any damage.

Guess I got lucky huh? I and my buddys did it all the time when we were teens. Never messed anything up. There were times we would start two cars with the same battery. Once one car was started, remove the battery and install it in the other car. Gee, maybe thats why my 8 track radio stopped working in my Mustang II? Lol