Power Issue Battery Light

[randypmartin]

I know this is a Ranger forum but My 1990 Bronco II should have the same basic components as the same year Ranger and was hoping to get more eyes on this. I've been 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!

 

[dvdswan]

Battery light on | The Ranger Station

 

[RonD]

Welcome to the Forum

There is a reason someone works at Autozone and is not working as a mechanic, nothing personal about it, it's strictly based on knowledge level and experience

All you need is a Volt Meter and some knowledge to test a Charging system, and its VERY VERY easy to do
If you don't have a $15 Volt meter then take it to a mechanic
But the $15 will be money well spent

Set Volt meter to 20v DC or just DC Volts
key off/engine off, best to test the system AFTER it has sat for a few hours, overnight even better

Touch the red probe to Battery Positive and the black probe to Battery Negative
Should see 12.3v to 12.8v, under 12.2v is an issue
Remember this voltage

Move the Black probe to Alternator's Metal housing, and you should see EXACTLY the same as "battery volts", if not you have a bad or loose Negative battery cable

For the rest of the tests the Black probe will stay on the alternator's metal case(called a Good Ground), assuming it tested as OK

Touch the red probe to the B+ terminal, on the back of alternator, it has 2 larger wires connected, its a stub/nut terminal, may have a rubber/plastic cap on it, pull it back and test voltage
Should see EXACTLY the same as battery voltage, if not Fuse 3 in Engine fuse box is blown or corroded

Unplug the 3 wire connector on alternator
Test the yellow/white stripe wire with red probe, should see EXACTLY battery volts, if not Fuse 7 is blown or corroded

Test Light green wire, should see 0 volts
Turn key ON(engine off)
Retest light green wire, should see battery volts but can be 0.1 or 0.2 less
This green wire is the Battery Light wire, also the ON/OFF switch for alternator

If this green wire doesn't have 12v with key on alternator can not work
If this green wire were shorted to a ground then battery light would stay on and alternator would stay OFF

There is also a White short jumper wire, no test for that just make sure its not frayed and is plugged in to its single tab on back of alternator

So test 3 wires
B+
Yellow
Green
If these are all OK then YOUR vehicles WIRING for charging system is OK

If they all test as OK plug the 3 wire connector back in and start the engine
Test Battery Voltage again
Should now see 14.2 to 14.8volts, alternator is on and working
If you still see 12volts then replace alternator, it is bad, 100% for sure
Its truly NOT a complicated system, 3 wires

Car battery's are only used to START an engine, thats all, the alternator runs ALL the electrics in the vehicle at 13.5volts, at the lowest, so voltage flows TO the "12 volt" battery when engine is running, to keep it charged up for the next "start up"

New batteries("right out of the box") are 12.8 to 13.0 volts
12.5 volts is a 3 year old battery
12.3 volts is a 5/6 year old battery and time to look for battery sales

12.2v or less is a done battery, or badly drained, may or may not be able to recharge it, but it won't last another year

Car battery's are made with thin plates inside so they can quickly discharge 60-80 AMPs to the starter motor to start the engine
And then to be quickly recharged after start up, that's what they are designed to do, NOT to power a vehicle
Any time they have a long steady drain or voltage drops below 12.3v the thin plates start to short internally so you get a self draining battery, "end of life" for car batteries is when this happens, "can't hold a charge" is the common term

They make Deep Cycle batteries for RV, Boats, ect....., these are made with thick plates so are made to be drained under 5volts powering lights rechargers, ect................., and then be recharged when possible
They can start an engine but not as well

The point is that a failing alternator CAN ruin a battery, and a failing battery CAN ruin an alternator
So its not unusual to fix one and have the other fail as well soon after, if not both at the same time