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Mike S.
Mike S., ASE Certified Technician
Category: Ford
Satisfied Customers: 7368
Experience:  ASE Certified Master Technician
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I have a 1997 f150, 4.6, While driving the, alternator stops

Customer Question

I have a 1997 f150, 4.6 ,
While driving the, alternator stops charging, and than you can watch the volt gauge start dropping, I turn everything electrical off, with in a few miles (3-4) the volt gauge is down to 8 volt or less, have change the alternator, new battery,
Submitted: 1 year ago.
Category: Ford
Expert:  Mike S. replied 1 year ago.

Confirm that you do have voltage to the alternator thin wire/s with the engine off and key on run.

Expert:  Mike S. replied 1 year ago.

Actually, here is the wiring diagram.

Expert:  Mike S. replied 1 year ago.

The yellow/white big cable at B+ should have 12 volts with key off.

The orange/light blue, at terminal A, same as above.

The light green/red at terminal I should be hot with the key on run/engine off.

Expert:  Mike S. replied 1 year ago.

If all is ok, with a voltmeter check the charging voltage right there at the back of the alternator at the B+ terminal, while the engine is reved up to at least 1,500 rpm. It should be at least 14 volts. If it is, check the voltage again while reved up a that 175 amp fuse, and then again at the connection to the starter. Wherever it looses that voltage would be the spot where the cable is most likely bad and might be rusted/corroded and need to be replaced.

Customer: replied 1 year ago.
I have 12.56 volts key off
I have 14.28 volts running
I can drive some time all is well, next time I will be driving and it will stop charging and the volt will drop to 8 volts or less, draining the battery, will no crank, battery is to weak , if i let it sit the battery will come back up. if i jump it it will start and is charging fine.,
Expert:  Mike S. replied 1 year ago.

Ok, then you have basically 2 options. 1 is waiting until the problem happens again and performing the tests I mentioned or 2 just assuming that it is a loose connection and re-tightening each connection.

Expert:  Mike S. replied 1 year ago.

I would do the seconds first, re-tightening each connection and then if it happens again be prepared to perform the tests.