Electrical Tail Light Issues...the Plot Thickens

Discussion in 'Fox 5.0 Mustang Tech' started by TBacq, Apr 1, 2014.


  1. TBacq

    TBacq New Member

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    Hello all,

    Earlier I had posted about two of my tail lights not working, the stop lights intbetween the tail/turn/brake lamps and the reverse lamps. However, upon further testing, they do receive voltage but it is consistent at around a tenth of a volt. The other lamps are dim aside from the small corner marker. All connectors are present, non corroded, and the fuses are all correct. Does anyone have any idea why here would be such a voltage drop? All other lights function, headlights are good and bright, alternator charges, battery is a little weak and you can see the headlights get brighter with revs if you look for it, but other than that all is well. I'm totally at a loss and my car is unsafe to drive. Thanks in advance, any help is greatly appreciated.

    Thanks,
    Sam
  2. 7991LXnSHO

    7991LXnSHO Active Member

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    It is time to chase the voltage down. What is going on at the brake switch? Someone have a wiring diagram?
  3. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    I sent him one for an 89 Mustang on April 1.
    @TBacq check your email to see if the wiring diagram arrived.

    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. .

    [​IMG]
    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.


    [​IMG]


    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
  4. TuneUpsNSuch

    TuneUpsNSuch Member

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    Almost sounds like a bulb short. Have you tried taking each bulb out and then checking the other?
  5. TBacq

    TBacq New Member

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    I'm definitely doing the bulb short test first. No clue why I didn't think of that sooner. Though I am interested to ask this: on an 89, shouldn't all the bulbs in the tail lights be flat style connectors? For whatever reason I have flat style one the corner markers, the stop/turn/tail lamps, and the reverse lamps, but the stop light in the middle is a round style connector. Not only that, it takes an 1156 bulb which is single filament. Should it be flat from the factory, and should it be a dual filament? I almost have the feeling the car was wrecked at one time and the rear lighting harness was Frankensteined back together...incorrect wiring would explain the stupidly high resistance. I mean the lamps that do light don't even get as bright as they should. The high mount and the license plate lights are fine...tail lights are on different wiring...trunk release is also non functioning though I feel the problem there is that the release itself is broken...haven't looked into it yet. I'm sick to my stomach over this wiring.
  6. 7991LXnSHO

    7991LXnSHO Active Member

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    After you follow the checklist, maybe it's time to call MPS auto salvage for a rear harness in better shape?

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