Sanity Check. My 3g Wiring...

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by sen2two, Aug 6, 2017.

  1. Here is how I have it installed. It's a brand new alternator and battery. Dosnt seem to be getting sufficient voltage. Dash reads very low compared to when the previous 90 amp 3g was in there.

    But that was installed by the previous owner and did not have the 4 gauge wire from the post to started solenoid or inline 150amp fuse. I added that and left everything else as it was.

    The only thing I see different when I Google instructions, is the 3rd wire from the stock plug is not going to the green wire on the stock harness. I'm not sure if this is an issue or not...

    Refer to pics... (hard to get a good shot. I have a Novi 1000 set up and the alternator is down low. Pics from underneath the car)

    Attached Files:


    Alternator wiring.



    If you have a 3G alternator, the white/ yellow wire is critical to proper operation. It is the voltage sense and regulator power lead that picks up the difference in voltage at the alternator output stud and the connection point at the starter solenoid. If you cheat and run it directly to the alternator output, it sees the voltage at the alternator output stud. It does not see the voltage at the starter solenoid connection point where it feeds power to everything else. You may have a voltage drop in the wiring between the alternator output stud and the connection to the starter solenoid. Thus you may have low voltage or less than the standard regulated voltage at the starter solenoid connection point. This makes for low voltage throughout the rest of the car: everything operates at less than full efficiency.

    Starter solenoid wiring 86-91 model cars.

    Connect the fused 4 gauge wire to the alternator and the battery side of the starter solenoid.

    Starter solenoid wiring 92-93 model cars.
  3. The ONLY wire you should have going to the power post on the back of the alternator is the 4 gauge power wire with a 150 amp fuse. Period
    THe other D sharped voltage regulator connector gets plugged into the D shaped connector on the 3g alternator
    Tap into the Stator wire from the original 2g plug and connect it to the Stator connector on the 3g alternator
    Tape off and tuck away the rest of the wires. Do not cut them

    I've don multiple Fox Body cars this way over 20 years without a problem.
    karthief and imp like this.
  4. Sadly, this car wasn't treated well before me. So some of the wiring is hacked up. Which makes it hard to follow some of the nicer diagrams like posted above.

    So, if I have this right. I should have:

    •4 gauge wire from the 1/4-20 post to the starter solonoid.
    •Yellow w/ white wire from stock Fox harness (91) to the single plug. (Field winding power input as stated above)
    •D shaped is left alone. No wires to or from anywhere?
  5. Key rect
    4g power wire from the power post on the alternator to the hot side of the starter solenoid with a 150 amp inline fuse- most people run it in front of or along the top of the radiator with wire loom
    D shaped regulator plug just plug into the 3G alternator as is
    tap into the stator wire on the 2G harness and to the single plug on the 3G-coerI woujld strip back a small portion of the wire and solder the new wire on and wrap in tape. You can use a scoche connector but those tend to come apart after time
    That and install an additional 4 gauge ground wire from the block to the chassis
    Tape off and tuck away the 2g power plug- do hook up the stock 2G power wires to the 3G alternator.

    Very easy install
    after install, with the car running use a DVOM and measure voltage at idle and at 2000 rpm at the back o the alternator and at the battery.You should see the voltage increase from 12.7-13 at idle to over 14 at 2000 rpm.

    One last thing- you may want to swap back on the 2g alternator pulley as it is the correct size
  6. Hmmm.... did it with no change.

    I'm wondering if the yellow/white wire is coming from the right place...

    Since it was already cut, I wasn't able to splice as shown in the picture. Not that is should matter. Same goes for the green wire. It was also already cut as well. Where does the yellow/white wire lead to from the alternator (Field winding power output)?

    And I just want to clarify again to be 110% sure. There are no wires coming from or to the D-shaped plug area? I basically have 2 wires connected to the alternator. One 4ga wire to the post. And the yellow/white wire from the harness to the Field winding power output.
  7. White with a black stripe is supposed to go from the three wire connector in the alternator to the small spade terminal in the side. The green wire goes to the dash battery light. The yellow with with its is supposed to go to battery + at the battery or starter solenoid constant battery + post.

    On my car I removed the original black w/ orange wires completely. Installed my own 4 gauge wire and fuse. The white with black strip only goes into the harness and loops back doesn't go anywhere else. It can be run directly from the alternator 3 wire plug to the spade terminal in the side.
  8. OK, just as in my searching, there is a lot of conflicting information.

    1. The diamgrams posted above does not show any of the D-shaped plug wires going to the alternator what so ever. It says (there are no changes), but what does that mean? I do not have the stock harness from the 3g alternator. Just the Plug which does not have the stock wiring in it.

    2. The black/white wire (middle from the D-shaped plug) goes to the spade connector (field winding power input) location on the alternator.

    2a. From the stock harness, the yellow wire with a white strip (That is with the 10ga black wires with orange stripe) goes to the spade connector (field winding power input) location on the alternator.

    So, with just that, there is conflicting information which has me pretty confused... Not only do I get conflicting information here. but in searches as well throughout Google. Seems as their is more than one right way to do this. but one thing is for sure... I have it wrong and do not know what is correct, lol...

    Edit: 99% of images on Google show the middle wire from the D shaped plug goes to the spade connector on the alternator. And the left most (closest to the spade) from the D shaped plug, goes to the post where with the 4 gauge wire.
    #8 sen2two, Aug 7, 2017
    Last edited: Aug 7, 2017
  9. This seems to be the most consistent way of images I have found.

    Attached Files:

    90sickfox likes this.
  10. That's an easy way to do it. The difference comes in the yellow with white strip wire going to the + post on the alternator. I ran my yellow with white strip to the starter relay + with all the fuse links. It seems to give a more accurate reading that way but either way should work.
  11. What I can tell you is that for every electrical modification done, 90% of the work is done by people who haven't a clue what they are doing.

    I will also tell you if I specified to do something a certain way, that I also spell out why it should be done that way. The yellow wire is a case in point; I explained exactly why it gets connected the same way as Ford designed it. The yellow wire on the D shaped connector goes to the battery side of the starter solenoid. If your D connector doesn't have 3 wires, buy a replacement pigtail assembly from the auto parts store. Part No. PT754 cost is $10.39


    #11 jrichker, Aug 7, 2017
    Last edited: Aug 7, 2017
  12. Okay, no one has the new plug on hand locally. I plan to order one, but this car needs to be back on the road now. It's my daily driver. So for the time being, spade connectors will do the job...

    The stock alternator harness was completely hacked up prior to me. And majority of the wires leading to the starter solenoid have been touched as well. Which keads me to skipping over all the stock wiring to wire this alternator in directly.

    1. The B Post (1/4-20 stud) leads directly to the battery side of the starter solenoid with a inline 150 amp fuse.

    2. The left most (Terminal A) from the D- shaped plug, is also going directly to the starter solenoid on the battery side.

    3. The Center from the D shaped plug (Terminal S) is going to the field winding power input.

    4. Does terminal "I" from the D shaped plug need to go anywhere? Or is it good how I have it now?

    Attached Files:

  13. Oh, I also added a ground using a short 4ga wire from the back of the alternator to the chassis.
    #13 sen2two, Aug 12, 2017
    Last edited: Aug 12, 2017
  14. That other wire should go to a green wire that runs to the dash battery light.
  15. Unfortunately, that green wire has most likely been cut and taped up. Or removed completely.

    So if it is ONLY for the battery warning light, then it's not actually needed, correct?

    Or does it have any other purposes as well?
  16. According to the schematic it may be able to be deleted.
  17. Did anyone bother to do more that just briefly glance at the alternator diagram?

    The green wire with the red stripe tells the alternator regulator to turn on the field power; without it hooked up the alternator won't charge.

    For testing purposes a single wire from red/green wire on the ignition coil to a 5 amp fuse and then to the green/red wire on the D shaped alternator plug will provide a signal to tell the regulator to supply power to the alternator field circuit.

    You're going to need this...

    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:
    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 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.


    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

    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; Everyone should bookmark this site.

    94-95 Mustang wiring diagrams

    Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

    Ignition switch wiring

    Fuel, alternator, A/C and ignition wiring

    O2 sensor wiring harness

    Vacuum diagram 89-93 Mustangs

    HVAC vacuum diagram

    TFI module differences & pin out

    Fuse box layout

    Mustang 5.0 Lights and Radio schematic, by TMoss:

    87-92 power window wiring PowerWindowWiring.gif

    93 power window wiring

    T5 Cutaway showing T5 internal parts

    Visual comparison of the Ford Fuel Injectors, picture by TMoss:

    Convertible top motor wiring

    Engine mounted fuel injector harness

    Location of the TPS, IAB, and the 10-pin connectors on a 5.0, picture by TMoss:

    Starter circuit

    Alternator diagram for 94-95 Mustangs.
  18. Alternators are actually easy, for Chr!st's sake! The old "I" regulator terminal connection went to the idiot light. With ammeter (factory), "I" was not connected at all. The big, heavy output wire of the Alternator goes to the Battery B+ connection through a 12 gauge Fusible Link, usually done at the fender-mounted Starter Solenoid. The Alternator "S" terminal is connected through a another Fusible, 18 gauge, to the same place. That's all there is to it.
  19. Thought I missed something last night @jrichker.

    The green wire does need to be hooked up...duh.
  20. I very much appreciate all the help. It gets difficult when stock wiring is badly messed with, on top of being overwhelmed when looking at wiring schematics you don't understand completely.

    So after going through it all, using Google to better understand the diagrams. Everything checks out OK. Exfept of course for the "I" terminal (Green/Red) for the alternator since I have nothing to it as of yet.

    I have the dash apart now. Trying to figure out how to connect this wire to the "Dash Indicator Lamp".