Battery light on

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Daryl

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get a multimeter and set it to restistance/ohms. Put one lead on the lug where it attaches to the motor, put the other on the batter and that will tell you the resistance in the wire. It should be very very low/close to 0.
So which setting is that?! Here’s a pic of mine:image.jpg
 
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Ah, the Greek alphabet Omega letter. Thank you!!
 
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Daryl

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That might not be it. Only way to be sure is to ohm it out from one end of it to the other, or just from that connector to the (-) battery.
Cobra Jeff,
So in my picture on the left, I place the tip of the red wire of the voltmeter on that metal L-shaped portion of the bracket that the nut is holding in place and the black wire on the negative post at the battery?
 

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yes sir. Before you do that confirm the meter is working by simply touching the red/black on the meter together and it should show up as 0.
 

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"OL" is the typical reading that there is no continuity or in other words there is an open break in the cable or there's just no continuity because of immense corrosion.

The setting all the way to the bottom-right (radio wave looking thing) will give an audible beep to let you know there is any continuity at all. Real handy for tracing wires that snake through a lot of hard to reach places but you can still find the ends of them. Alligator clips extend the reach of the leads. Or you can wrap some wire around the tips of the leads and tape them up so they don't slip off.
 

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I actually had this problem before and did the same thing and now I can't remember what number I was looking for. If you said something like 50 ohms I/we could say for sure that is a problem and fix it but .1 is still a little higher than I would like it to be. Do you have a piece of scrap wire laying around about the same length to do a quick test? The gauge/thickness isn't important here, I just want you to do the same test on that wire and see what you meter reads.
 
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Daryl

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I’ve got some thin gauge bailing wire. How do I do it?
 

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I did a little research I think it could be high enough to start causing some problems here and there. Skip the test and remove the ground wire at both ends, clean the connector and the area it goes to with a wire brush till it is clean and then make sure it is tight enough that it does not wiggle.
 
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Daryl

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I did a little research I think it could be high enough to start causing some problems here and there. Skip the test and remove the ground wire at both ends, clean the connector and the area it goes to with a wire brush till it is clean and then make sure it is tight enough that it does not wiggle.
Will do! It’s a day off so I’m bouncing between doctor appointments (great gettin’ older!), but I’ve got a few hours gap and I’ll get to it then
 

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also inspect the wire. Make sure it is still flexible/bendable and there are not any spots where the insulation has failed and the wire under might also be.
 

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0/1 ohms is acceptable but then again it depends on the path it takes to reach chassis ground (ultimately the negative battery terminal). If it's going through a lot of sheet metal, then 0.1 is going to have to be acceptable. But this is coming from a guy who rigged up his own star-point ground system going directly to the battery with 1/0 gauge wire. What can I say? I'm going full bore on my car.
 
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0/1 ohms is acceptable but then again it depends on the path it takes to reach chassis ground (ultimately the negative battery terminal). If it's going through a lot of sheet metal, then 0.1 is going to have to be acceptable. But this is coming from a guy who rigged up his own star-point ground system going directly to the battery with 1/0 gauge wire. What can I say? I'm going full bore on my car.
Well, the negative battery cable runs the OE path. No aftermarket this or that; no battery in the trunk, etc

If this helps, IDK??
 
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Sidebar: does anyone know where I can get a negative battery cable replacement?

Worst case scenario, my boys @ NAPA said they could make me one if I bring in the original
 

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the only thing stopping you from making it yourself is maybe a large gauge wire crimper. It is just a piece of wire with ring terminals on one side and the battery terminal on the other. Any car audio shop could hook you up as well.
 

cobrajeff96

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I'm pretty sure you don't have a grounding issue on that segment. How far away is it from the neg battery terminal?
 

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I am not positive he does either but when I go out to my garage and grab a piece of wire and measure a 10 ft long piece of it I got .009-.01. The fact that this piece has 10x the resistance of a fresh piece that is 3x longer leaves me to wonder if the ground that feeds the motor is just bad enough to START to cause problems with out failing completely. This info along with how easy/cheap it is replace a ground and how important leaves me recommending to try swapping it out.
 

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Lets make this easy for ya.
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.

The alternator MUST have the instrument cluster in place and working properly. The alternator warning light is a part of the charging circuit. No cluster, then no working alternator.

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.

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. .


attachments\568093
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.
attachments\568094


The following are diagrams courtesy of Tmoss & Stang&2birds

Alternator wiring diagram for 94-95 Mustangs.
Mustang-94-95-Alt.gif

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
 

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