My DaD's 4.6l '98 4x4 has it's battery light on but it starts the same as usual and the gage shows 14 volts. What's the trouble?

 

How are you getting 14 volts ?

The gauge range is 8 volts on the low side, and 18 on the high side, no other numbers on the scale.

 

Measure the voltage at the battery terminals using a voltmeter. The idiot gauge in the instrument cluster is crap. My guess is that your truck is needing a new alternator.

 

OK, so that is the battery via meter is showing 14 V, not the gauge.

Might want to check the harness to the alternator for problems with it ( some times the harness has corrosion ) and the inline fuse for the alternator field.



Here is a thread from jbrew with the info in it.

https://www.f150online.com/forums/ar...nator-why.html

 

On the gage it's dropped a volt in about 4 starts(of the engine) atleast now i know what ta check. I thought it would be the alternator too but i'll check the harness und stuff before i replace it.... it looks like i have to pull a bunch of junk off ta get at it to change it!

 

OK, we tested the bat at the terminals reading 12.6v. Cleaned the posts and cables(really corroded) and started and run it for about 30sec with heater on full and lights on bright, shut it off and the battery at the terminals read 11.2v.
If the alt wasn't charging it would have drained more than that after a start with the heater and lights on and only 30sec for charging wouldn't it?

 

If the battery was good, it definitely wouldn't have drained it that much just from starting and running the heater and lights. Problem is, the 12.6 doesn't mean much without the surface charge being removed. You'd have to turn the lights on for AT LEAST 30 sec. to remove the surface charge (if you don't have a load tester to do it with) and then test it to see what the voltage actually was before you started and ran H&L. The battery definitely sounds weak (read bad) just from what you've said so far. The alt. may also be bad, but the tests need to be done in progression to know what all is bad.