New Alt, Battery Draining When Running

Discussion in 'Fox 5.0 Mustang Tech' started by 90Stocklx5.0, Mar 23, 2014.

  1. Hi,
    My battery is draining with the car running . I replaced the alternator , checked the fusible link. Any ideas of what it could be?
  2. Do you have a digital multimeter?

    With the car running at idle, the alternator should be outputting around 14 volts. If you have pullies, that number might be a little lower..maybe 12.5-13V at idle.

    Throttle up to around 1500RPM and you should have 14+ volts.

    If you don't have these numbers, your alternator is not outputting enough voltage to charge the battery.
  3. I put the meter on it, and I have correct voltage and it still drains the battery. I'm thinking a possible short to ground somewhere. I took the battery out , and put it in a diff car and it holds a charge, so I can rule out bad battery
  4. Does your DMM measure AMPs in DC volts? Most do, but you need one that will test up to 10A.

    If so, remove one lead from the battery. Touch one lead of the DMM to the battery, and the other to the cable to removed. Get a reading of how many amps the car draws in this state. Should be under 1Amp.

    If it's higher than that, pull a fuse out and recheck. Do that until you can isolate the circuit with the high load and then go from there
  5. I'm drawing 8 amps until I pull out fuse 13 instrument illumination. I think the green wire from the alt goes to the instrument cluster for charge indicator. Could this be the prob or maybe just the circuit itself?
  6. At 8AMps, i'd say this is your issue.

    I'd need to grab some wiring diagrams and look. Don't have that currently. But I'd say you'd narrowed it down. Possible short somewhere on that circuit, or in the cluster
  7. Thanks for hanging in there with me, I appreciate all the help!
  8. [​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.

    Something is probably wired into the dash using the dimmer switch power feed. Look for aftermarket radio, stereo or alarm system to be the culprit.
  9. If the alarm is the problem what would I need to do to fix it? Other than remove the alarm completely.