Question about 2004 Ford Mustang

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Battery light will not go off

Replaced battery, checked alternator (shows okay) has been like this for 6 months now. runs okay, battery light just stays on. battery charge indicator went to complete zero while driving this evening, interior lights went out, radio went out, ABS light came on... help, what's wrong with my car?? Had to jump it off to get in going again. Ford wants $200 just to hook it up to the machine to tell me whats wrong... I don't have that kinda money right now. HELP!!

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I am posting a generic reply to the "charge fail" lamp (that is the correct term - when the lamp is lit = the battery is NOT being charged) problems rather than a specific cure for the ills in your car.

I suspect a faulty alternator OR an inadvertent short to battery - potential of the wire leading from the charge fail lamp to the battery charging alternator. Please try the following tests (BE CAREFUL while working - you are working near the engine + v belts + fans etc. and a fully charged battery = can be very dangerous)

Locate the three terminals of the battery charging alternator. The largest terminal would be connected to the battery positive cable (may be red in colour). The second terminal could be labeled as "W" = this is the terminal generating a frequency proportional to the engine speed for the tachometer. The third terminal is the WL or D terminal. The Charge Fail lamp is connected from battery positive (via the ignition switch) to this WL/D terminal.

Turn the ignition switch on but don't start the engine. Observe the "Charge Fail" lamp glowing in the dashboard. Now remove the wire going to the WL/D terminal. The "Charge Fail" lamp should now stop glowing. If the lamp continues to glow, there is surely a shortcircuit between the wire from the lamp to the alternator.

As this lamp current is not going to the WL/D terminal due to the short circuit, the battery charging alternator is not producing very little or no output. Check if the battery terminal voltage rises when the engine is running. If the alternator is working correctly, the battery would be at 13.2 Volts with the engine running.

The confirmatory test is to rig a spare lamp (12V, 3 or 5 watt lamp in a holder) between battery + to the WL/D terminal - if that works properly (lit when the engine is stopped and off when the engine ir running), you can connect a new wire from the lamp to the WL/D terminal - do isolate the earlier wire completely from from lamp as well as alternator end.
Hope this helps. Write back if I have confused you further.....

Apurva from Pune, India.

Posted on Mar 12, 2009

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