New Battery or Alternator (both)?

[GilpinGuy]

Jumped in the truck (2001 F150 5.4l) and it fired right up. While on the gas the battery light was coming on - let off of the gas and it went out. The needle gauge was fine. I drove it for about 6 or 7 miles then the gauge started falling also. By the time I got home, all of the electronics inside were fading or going out altogether. Good thing I was close to home.

I whipped out my battery/alternator tester. With engine off - low voltage, dead battery. With engine running, low voltage also. I bought this truck in August and the seller said the battery was replaced within the last 18 months, so I'm assuming it's alternator time.

Thoughts?

[Irving]

Agree on alternator. When running, it should be showing 13-14+ volts.

[SideShow Bob]

Agree on alternator. When running, it should be showing 13-14+ volts.

Unless the battery has a dead cell drawing down the alternator output, or loose/corroded connection from the alternator to where it connects to charge the battery.

Jump start the truck and remove the battery + cable and see if it dies. And while it is running see if a wench will stick to the back of the alternator from the magnetic field created by the alternator.

[newracer]

Could be either or both, easy enough to pull both and haven them tested at an auto parts store.

[newracer]

Unless the battery has a dead cell drawing down the alternator output, or loose/corroded connection from the alternator to where it connects to charge the battery.

Jump start the truck and remove the battery + cable and see if it dies. And while it is running see if a wench will stick to the back of the alternator from the magnetic field created by the alternator.

It would be the negative cable not the positive. However this is not good thing to do any longer as it could cause damage to many parts of the electrical system.

[GilpinGuy]

Question about alternators:
I'm not sure if my alternator is 95 amp or 110 amp. There's only a $10 difference in price. Go with the higher amp to be sure?

[fportmen45]

Question about alternators:
I'm not sure if my alternator is 95 amp or 110 amp. There's only a $10 difference in price. Go with the higher amp to be sure?

Yes.

[brutal]

Yes.

Is it a fair assumption that the voltage regulator/regulation circuit in ECU whatever, can manage the increased output?

In other words, is a typical "tow package" just a larger alternator and a trailer wiring circuit/harness?

[Wulf202]

Replacing the voltage regulator is usually the easy solution. Typically costs 1/4 the cost of a rebuilt one

[SideShow Bob]

Replacing the voltage regulator is usually the easy solution. Typically costs 1/4 the cost of a rebuilt one

If you are going to take apart the alternator to replace the rectifier/regulator, you might as well replace the brushes & clean up the contact strips. Then you will have a reman alternator.