Power Issue Battery Light

[randypmartin]

My 1990 Bronco II was having power issues. Replaced both the battery and alternator since I hadn't done so since buying it over 3 years ago. Ran great for about 6 months and now all of a sudden the battery light is on and it's clearly struggling at certain RPMs. This truck is an automatic but has a manual 4wd shifter on the floor. The power is so low while driving at slower speeds that the floor shifter rattles like crazy. Took it to Autozone to have the power checked and the diagnostic test came back as follows:

SYSTEM TEST FAIL

Battery Test
GOOD BATTERY
State of Charge : 100%
Cranking Health : PASS
Reserve Capacity: OK

Starting System Test
GOOD STARTING SYSTEM
Min Cranking Volts : PASS
Cranking Current : PASS
Cranking RPM : PASS

Charging System Test
BAD CHARGINS SYSTEM
Voltage Regulation: PASS
Diodes: PASS
Charging Current: FAIL

The manager told me that both the alternator and battery seem 100% healthy but that there is some issue with the charging system that should be addressed by an automotive electrician... Sounds expensive... I also did a full tune up last year, including replacing all spark plugs, wires, distributor cap, and rotor. Figured I'd post here before I go spend my money at a pricy local shop. Any thoughts would be super appreciated thanks!

 

[Rowdy Fitzgerald]

My first thought would be the exciter wire. Make sure the connector is clean and snug. Test it for grounding and make sure it’s getting full voltage with the key on.

Then check the charging wire for grounding, broken wires internally and corrosion. If none this is the issue, then it’s a bas alternator.

 

[lil_Blue_Ford]

So, I’m with Rowdy on this, but I’ll add to it…

Not sure how much of a problem corrosion is where you’re at, but here I had all sorts of strange problems with my choptop at one point. Couldn’t find the problem at first, but it was electrical and everything seemed to test ok. Well, the one day I was pulling the battery to charge and test it again and realized that when I flexed the negative cable, I heard all sorts of crunching. Had it not been otherwise quiet at the time I might not have noticed. Scratched my head and then stripped a bit of insulation off the wire to find it was corroded to junk inside the insulation. Otherwise it looked fine. So that’s become one of my common things to check and repair if I even suspect it could be an issue.

I make up my own cables these days out of 4 gauge welding cable. For a long while I was using a pair of fencing pliers as a set of improvised crimpers but now I have a proper crimp tool. I put dielectric grease on the stripped end of the wire before putting it in the lug and crimping it. No-alox for household wiring (specifically for the aluminum wiring) works arguably better, but is rather messy. I also have some of the glue lined heat shrink tubing to put over the crimp lug and wire to make a “factory” type. Corrosion is a big problem around here along with rust.

That all said, the alternator could still be defective. Best way to test them is to take them into like an Autozone off the vehicle and have it tested on the machine they have in the store, it’s more precise than the tester they hook up to your vehicle and it eliminates the wiring. If the alternator passes good that way, you know it’s a wiring issue. If it fails, then there still could be a wiring issue that you would have to check for, but you know you found at least one problem.

One little side note, my F-150 I have a snow plow for which is power hungry, so I upgraded the alternator from the 95 amp it had to the bigger alternator option and added an additional 4 gauge welding wire jumper from the output over to the battery or starter solenoid, forget which. Tried using that in place of the factory wire but it kicked the battery light, there must be some sensor wire attached to the factory wire, so I hooked both wires up to the output. Worked great, I can use every amp that alternator can put out that way and the plow pump no longer really dims the headlights. It can jump start most vehicles even if they don’t have a battery.

 

[RonD]

Test it yourself or replace the alternator

Get a Volt meter, set it for 20voltsDC or just DC, test battery terminals, should see 12.3v to 12.8v, Volt meter is working

KEY OFF
On the alternator will be the B+ terminal, stud and nut terminal, test voltage on that terminal, should see "battery voltage", what you saw when you tested battery
If so move on, if not then fusible link is blown

Unplug the 3 wire connector on alternator
Test yellow wire, should see "battery voltage" if not then fuse or fusible link is blown

Test green wire, should see 0volts
Turn ON the key
Retest green wire, should see close to battery volts, can be slightly lower
If not then green wire is chafed and grounding out somewhere

And thats it, thats all there is to testing, pretty much ANY vehicles, charging system, 3 wires to test

If the 3 wires test OK then replace alternator, for sure, 100%