Starting, charging, and battery question - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 6 Old 10-08-2016, 12:39 PM Thread Starter
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Starting, charging, and battery question

We're getting hit by Hurricane Matthew and i haven't really had ability to figure my son's 91 LX issue out. But here's what happened. He calls Thursday night and battery was completely discharged (checked w/voltmeter). But no warning the previous day. I checked the alternator once we got it jumped, and it was putting out 'marginal' charge voltage, just a tad above 12V. The battery was old enough and there was so much rain I went ahead and got a new one. He drove the car not even 24 hours and less than 7 miles on two trips in the city, and it is now discharged again! I was able to jump it quickly this time.

I can definitely trouble shoot alternator output and static battery voltage, but what would cause his battery, with normal load (no lights, no aftermarket amp/radio) to discharge so quickly? I did check the starter relay area - had forgotten about the stupid Ford fusible link harness mess and stuff. Could it be the harness between battery/relay and alternator? I've heard stories about Ford cars/trucks that catch on fire due to these.


1995 Impala SS 396 LT4 intake/heads with GM/Crane 847 Cam Tri-Y headers T56 4.56 gears 58mm TB RAISS intake Cat delete OBD2 PCM Bryan Herter Programming
1974 F-100 390 HP
1991 Ford Mustang LX, 5.0L, Bone Stock
1996 Buick Roadmaster. LT1. Bone Stock
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post #2 of 6 Old 10-08-2016, 12:50 PM
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with the new battery, and engine running what was the voltage of the battery?

also, when you check battery voltage, you must check when under load

therefore you crank the engine while checking voltage

a static check is meaningless


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post #3 of 6 Old 10-08-2016, 01:09 PM Thread Starter
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You are right. But what could discharge the battery so quickly, regardless?
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post #4 of 6 Old 10-08-2016, 01:18 PM
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the engine running

injector can be 7amps, there are 8
ign
fuel pump, ~10amps

with a fresh battery, expect maybe a couple of hours of driving without a charging alt....

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #5 of 6 Old 10-08-2016, 01:42 PM Thread Starter
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Fuel pump definitely not running while car is off. My son probably drove the car 15 min. total. The battery discharged with the car off!
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post #6 of 6 Old 10-08-2016, 04:07 PM
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well then, measure the current draw on the battery, when it is off

report how many amps it is.

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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