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I searched this site with no luck.

I'm wondering if my alternator is malfunctioning or if it's something else.
What's the best way to diagnose a battery charging issue? What voltages should be present (in which places) in a normal healthy truck?

I have a digital voltmeter...

Check a few posts down under molex and diaelectric grease. Just had a serious problem and solved it. Check your wires. Voltage regulator, and battery voltages. When mine is charging correctly I get 27.5 approx at the batteries. Just setting it is about 24.7, sometimes after running for awhile I get 25.4

edit: I photoed the process I went through to locate the problem.
apparently my files are too big to post. PM me your email and I can send them to you, if you wish.

What voltages should be present (in which places) in a normal healthy truck?

Its simple: expect 27-28 volts (give or take a tenth or two) at the battery terminals when the truck is running. Any less and the batteries are not being charged properly; any more and they are being overcharged. You may need to get the truck just off idle to see this voltage.

What's the best way to diagnose a battery charging issue?

1st: Check continuity of all wires from the outside of the Molex connector (nearest place to gain access to those wires outside the Molex) through the air tin to the alternator terminals. If you are lucky, one or more of the wires are open or you find corroded Molex connectors on one or both sides of the connector. Fix these first, if you find them. You should expect to get less than one ohm from the alternator terminal to the other end of the wire on the other side of the Molex.
2nd: If the wires are good and still no charging, replace the voltage regulator (approx. $30-40). It is mounted in front of the coil, inside of the front of the engine compartment (on the other side of the axe mount panel). Doing these two things should fix 90% of your charging issue (assuming the rest of the wiring is not all corroded and shot, which is not common).

After these checks, it might be the alternator ($$$$$), ground strap, battery wires, battery disconnect switch and/or relay, or some bizarre thing that only happened in your truck.

The problem could also be bad battery, they don't last forever.

Jimm391730 wrote:

What voltages should be present (in which places) in a normal healthy truck?

Its simple: expect 27-28 volts (give or take a tenth or two) at the battery terminals when the truck is running. Any less and the batteries are not being charged properly; any more and they are being overcharged. You may need to get the truck just off idle to see this voltage.

What's the best way to diagnose a battery charging issue?

1st: Check continuity of all wires from the outside of the Molex connector (nearest place to gain access to those wires outside the Molex) through the air tin to the alternator terminals. If you are lucky, one or more of the wires are open or you find corroded Molex connectors on one or both sides of the connector. Fix these first, if you find them. You should expect to get less than one ohm from the alternator terminal to the other end of the wire on the other side of the Molex.
2nd: If the wires are good and still no charging, replace the voltage regulator (approx. $30-40). It is mounted in front of the coil, inside of the front of the engine compartment (on the other side of the axe mount panel). Doing these two things should fix 90% of your charging issue (assuming the rest of the wiring is not all corroded and shot, which is not common).

After these checks, it might be the alternator ($$$$$), ground strap, battery wires, battery disconnect switch and/or relay, or some bizarre thing that only happened in your truck.

^^^ This is exactly what I was looking for. ^^^
Thanks again Jim!

The problem could also be bad battery, they don't last forever.

Very true. But the truck's charging voltage should still be correct - you can't determine the state of the batteries when the truck is running. BUT, you can check the battery voltage when the truck is being cranked to start: voltages over 20 show very good batteries, less than 20 and one or both batteries are in need of a good independent recharge or are dying.

When friends have had me help when their car doesn't start (usually out where tools like a voltmeter are not easily available) I'd turn the dome light on, then crank the engine. Usually the dome light went out which meant that the voltage dropped to very low levels. This was either a bad battery or bad battery cables. If the dome light stayed on, or mostly on, as the engine cranked then I would know that the battery had plenty of juice and the lack of starting problem was elsewhere. The dome light made a crude voltmeter by its brightness.

How to determine the voltages while the truck is running. Very simple. They also come in green and blue.

http://www.mpja.com/Mini-Panel-Meter-Sn ... 0215%20ME/

Problem solved.

It was a corroded connection in the square white plastic connector of the wiring harness where it comes through the tin engine cover from the alternator

Thanks again you guys..

Good place to use dielectric grease. Helps combat that corrosion problem.

Good call Morris.

Twin Pinzies wrote: the square white plastic connector.

That would be the aforementioned "Molex" connector.