90 Gt Electrical Problem-starting

Discussion in 'Fox 5.0 Mustang Tech' started by BKM48198, Sep 10, 2013.


  1. BKM48198

    BKM48198 Member

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    I went out this morning and my car wouldn't start, when I turned the key the lights all went out and after a min or so came back on, the under hood light had some power but was very dim and then came back on bright so I tried the key again and the same things happened. After work I replaced the starter and solenoid thinking that was probably the problem but after replacing them I still had the same problem so to check the starter I used jumper cables from the battery to the starter wire on the solenoid and it spun the motor so I tried it again with the key on and it started up, so I backed it in the garage and let it run for a few mins. and turned it off and tried the key and everything worked, started right up, did that 4 or 5 more times and had no problems. I don't want to go somewhere and have it not start on me so I'm a bit worried about driving it, any ideas what might have caused this or what to check. The old starter was bad, bench tested it and all it did was hum. I do have an alarm on the car and I replaced the connectors for it at the battery before replacing the starter.
  2. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    loose cables somewhere in there


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  3. John Dirks Jr

    John Dirks Jr Active Member

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    Try cleaning the cable / terminal connections at the battery. Disconnect them, clean with a wire brush and reconnect. I smear Vaseline on the battery posts before clamping the terminals on. It helps keep the corrosion away.
  4. mikestang63

    mikestang63 Mustang Master

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    Start at the battery with a DVM and work your way back along the cables on both the starter and battery side with the key off and on. . Check for voltage and continuity drops. Check to make sure all of the grounds are attached, clean and snug. There are several important grounds. Sounds to me like you have a loose or missing ground.
  5. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    No Crank checklist for 5.0 Mustangs

    Revised 05-Oct-2010 to update Fluke references.

    No crank, slow crank and stuck starter solenoid problems have the same root causes – low battery voltage and poor connections. For that reason, they are grouped together.
    Use the same initial group of tests to find the root cause of both no crank and stuck solenoid problems.

    Since some of the tests will bypass the safety interlocks, make sure that the car is in neutral and the parking brake is set. Becoming a pancake isn’t part of the repair process…


    1.) Will the car start if it is jumped? Then clean battery terminals and check battery for low charge and dead cells. A good battery will measure 12-13 volts at full charge with the ignition switch in the Run position but without the engine running.
    A voltmeter placed across the battery terminals should show a minimum of 9.5-10 volts when the ignition switch is turned to the Start position and the starter engages or tries to engage. Less than this will result in a clicking solenoid, or slow cranking (if it cranks at all) or a starter solenoid that sticks and welds the contacts together.

    Most auto parts stores will check your battery for free. It does not have to be installed in the car to have it checked; you can carry it with you to the auto parts store.

    The battery posts and inside of the battery post terminals should be scraped clean with a knife or battery post cleaner tool. This little trick will fix a surprising number of no start problems.

    The clamp on with 2 bolts battery terminal ends are a known problem causer. Any place you see green on a copper wire is corrosion. Corrosion gets in the clamped joint and works its way up the wire under the insulation. Corroded connections do not conduct electricity well. Avoid them like the plague...

    If the starter solenoid welds the contacts, then the starter will attempt to run anytime there is power in the battery. The cables and solenoid will get very hot, and may even start smoking. The temporary fix for a welded starter solenoid is to disconnect the battery and smack the back of the solenoid housing a sharp blow with a hammer. This may cause the contacts to unstick and work normally for a while.

    A voltmeter is handy if you are familiar with how to use it to find bad connections. Measure the voltage drop across a connection while trying to start the car: more than .5 volts across a connection indicates a problem. The voltage drop tests need to be done while cranking the engine. It's the current flowing through a connection or wire that causes the voltage drop.

    See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .

    [​IMG]

    2.) Check the battery to engine block ground down near the oil filter, and the ground behind the engine to the firewall. All grounds should be clean and shiny. Use some sandpaper to clean them up.

    3.) Jump the big terminals on the starter solenoid next to the battery with a screwdriver - watch out for the sparks! If the engine cranks, the starter and power wiring is good. The starter relay is also known as a starter solenoid.

    The rest of the tech note only concerns no crank problems. If your problem was a stuck solenoid, go back to step 1.

    4.) Then pull the small push on connector (small red/blue wire) off the starter solenoid (Looks like it is stuck on a screw). Then jump between the screw and the terminal that is connected to the battery. If it cranks, the relay is good and your problem is in the rest of the circuit.

    5.) Remember to check the ignition switch, neutral safety switch on auto trans and the clutch safety switch on manual trans cars. If they are good, then you have wiring problems.

    Typical start circuit...
    Diagram courtesy of Tmoss & Stang&2birds
    [​IMG]


    6.) Pull the starter and take it to AutoZone or Pep Boys and have them test it. Starter fails test, then replace it. If you got this far, the starter is probably bad.


    Starter solenoid wiring for 86-91 Mustang
    [​IMG]


    Starter solenoid wiring 92-93 Mustang or earlier Mustang with upgraded high torque mini starter.
    [​IMG]

    Electrical checks for the switches and starter solenoid

    Remove the small red/blue wire from the starter solenoid. Use a screwdriver to bridge the connection from the battery positive connection on the starter solenoid to the small screw where the red/blue wire was connected. The starter should crank the engine. If it does not, the starter solenoid is defective or the battery lacks sufficient charge to crank the engine.

    If the starter does crank the engine, the problem is in the clutch safety circuit (5 speed) or Neutral Sense Switch (auto trans) or ignition switch.


    See the Typical start circuit diagram above for wiring information for troubleshooting.

    You will need a voltmeter or test lamp for the rest of the checks. Connect one lead of the voltmeter or test lamp to ground. The other lead will connect to the item under test.
    Look for 12 volts on the white/pink wire when the ignition switch is turned to the Start position. Check the ignition switch first.
    No 12 volts, replace the ignition switch.

    The next step will require you to push the clutch pedal to the floor (5 speed) or put the transmission in neutral (auto trans) while the ignition switch is turned to the Start position.
    Good 12 volts, check the clutch safety switch (5 speed) or Neutral Sense Switch (auto trans) for good 12 volts on both sides of the switches. No 12 volts on both sides of the switch and the switches are defective or out of adjustment. Check the wiring for bad connections while you are at it.
  6. BKM48198

    BKM48198 Member

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    Both battery terminals have been cleaned, I have added a short ground wire from the battery to the body also, cleaned it, I have not taken the ground off the motor yet and cleaned it, I'll do that tomorrow after work. Yesterday on my way home from work in 98 degree heat the end of my radiator came off, I had just bought the thing 2 months ago and it is the newer style allum with plastic ends crimped on. So after working all day then waiting 2 hours for a flatbed I got it home and just pulled the rad. out and went and got another, by then it was too humid to keep working on it. Once I got home today I put the rad. in and replaced the thermostat, it was only 94 today and humid.
  7. mikestang63

    mikestang63 Mustang Master

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    Man, sorry to hear that.. Rains it pours right... Not to pile on, but that is why I don't buy radiators with the crimped on plastic tanks. I would look at the ones with the welded all metal end tanks as they tend to last longer. Many people have their own opinions, but I've used Griffin, Be Cool, Champion aftermarket radiators with success.

    You should also add a 4 gauge ground from the A/C bracket or block to the ground post behind the battery.
  8. BKM48198

    BKM48198 Member

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    It's always one small thing or another when you drive a 24 year old car as a daily driver, I already had an extra ground from the terminal to the body and had checked the ground from the terminal to the block. I do have 2 grounds on the back of the block that go to the body also. The problem was coming and going, it wasn't constant but after going thru all the grounds, new terminals, starter, solenoid and checking voltages everywhere I could think there could be a problem and everything was checking out good I just happened to bump an inline fuse holder that goes to my car alarm and the under-hood light went dim. I replaced the fuse holder and haven't had another problem since. I didn't cut it apart but it must have had some corrosion that was getting worse day by day. I would have preferred to have gotten a radiator without the plastic ends but I needed the car for work and my wife needed her car for a doctors appt. so I got something that night. When I put this rad. in I mixed up some JB WELD and applied it all the way around the clips so hopefully it won't happen again before I can get a better rad.
    It took me a while to get back on here to let everyone know what I found to be the actual problem because on top of all this I caught the Flu and lost almost a week due to it, but my Mustang is running good again. ......For Now!

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