Alternator troubleshooting for 86-93 5.0 Mustangs:I am having issues with my alternator not working. I have a 3g alternator with a 4 gauge wire running to the starter solenoid and the two black and orange wires capped off. The battery is a brand new optima and the alternator is brand new, but when I read voltage at the battery it is always reading under 12 volts even when the rpm is up. I run after market guages and read somewhere that the factory guage battery light had to be connected for the alternator to work. Is this true?
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 28-Nov-2018 to add warning that the instrument cluster must be in place and working for the alternator to charge
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.
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. .
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
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
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring; http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.
94-95 Mustang wiring diagrams
http://www.veryuseful.com/mustang/tech/engine/#95-95Diagrams
Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif
TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif
Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
Mustang 5.0 Lights and Radio schematic, by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxLights-Radio_diag.gif
87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif
93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif
T5 Cutaway showing T5 internal parts
http://www.veryuseful.com/mustang/tech/engine/images/5_Speed_Cutaway_Illustrated.jpg
Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg
Convertible top motor wiring http://www.veryuseful.com/mustang/tech/engine/images/mustang88VertTopMotorCkt.gif
Engine mounted fuel injector harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangEngineHarness.gif
Location of the TPS, IAB, and the 10-pin connectors on a 5.0, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/TPS_IAB_Pic.jpg
Starter circuit
https://www.stangnet.com/mustang-forums/attachments\21328
Alternator diagram for 94-95 Mustangs.
http://www.veryuseful.com/mustang/tech/engine/images/Mustang-94-95-Alt.gif
It's simple: connect one end of the 500 ohm resistor to switched power like the ignition and the other end to the green/red wire that connects to the alternator. Use the diagrams I posted to help out.Thanks, I plugged the factory guages back in and that fixed the issue. Now I just have to figure out how to wire up that resistor so I can use my aftermarket guages.
Sorry, I was meaning 8 gauge wire (not 8g alt haha).What's the amperage rating of your alternator? Did the shops test this? Is it possible that your regulator can't keep up when the power requirements are increased?
An unrelated story... years ago I had a 2006 Ford Freestar with a 6G alt (PCM controlled the voltage regulation). Every now and then the PCM would do something weird and the system would charge at 18V. My temp fix (or semi permanent solution) was to unplug the voltage regulator (that the PCM controlled), restart the car and blip the throttle to 2000 or 2500 RPM (I can't remember which). This put the alternator into a default charging mode, it would run steady at 13.5V irrelevant of load on the system. Because it was more consistent this is what we ultimately did because I had no intention of hunting down PCM issues in a van with 450,000 km. Eventually we retired it to the scrappers.
Pics of your alternator do not look like any 8G images I have seen looking online. The closest in appearance 1 wire units I see are either 60 or 100amp. Either might have issue with full electrically load you described. Check into the output (if the shops did not already give you this info). That might be your issue.
Me neither. But I’ve never seen it any other way either.I have NEVER, seen the tensioner setup that way.
That’s kind of what I was thinking. At least I know where the power wire is plugged in...but it doesn’t seem super normal either - it plugged into some relay boxHere is what I would do. Since you have no idea what alternator this is and from the pics it looks like a 2g with incomplete wiring, I would get a 1 wire alternator with a wiring kit or instructions and then you know what you have, make sure it has enough amperage to run ac, lights and the amp for the stereo.
Yep. I just wanted to make sure that with what I’ve going on right now, a 3g 1-wire would work. Like I I think mentioned earlier, where some people just tape off some of the connection wires when they go 1-wire, I don’t even have those wires anywhere. Just the one 8 gauge.Like thief said.....or buy a 3g alternator and wire it up.
That’s kind of what I was thinking. At least I know where the power wire is plugged in...but it doesn’t seem super normal either - it plugged into some relay boxHere is what I would do. Since you have no idea what alternator this is and from the pics it looks like a 2g with incomplete wiring, I would get a 1 wire alternator with a wiring kit or instructions and then you know what you have, make sure it has enough amperage to run ac, lights and the amp for the stereo.
I’ll have to test that later this week...they made me go back to work after a glorious 5-week “work in my mustang” vacation...I mean quarantine.A quick question... when you have the truck running at idle, A/C on, lights on, what ever else you would want ... what is the measured voltage at the battery? What is voltage engine idling and none of accessories are running?
No additional grounding should be needed as grounding through the engine is normal.
Why does everybody keep calling it a truck? Sometimes when buying a project that is mostly done you have to correct things that the previous owner didn't know was wrong or at the build time better was not available.
Tally up all the amps you need including the amp then call a rod shop like speedway and talk to them about alternator and wiring.
Do you have a fire extinguisher in that truck?
Just wonder'n
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