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From: Qulin
Hello everyone!
I have a 1998 4.0, I bought a new/used battery from Walmart and it did something to my new alternator. It won't keep a charge, so I went back and got a New unused battery and my alternator is still not charging. The battery light went off but my gage is riding low. When I take negative cable off (while running) it dies immediately. Can anyone tell me what I should do, did my alternator go bad again? Also when I bought the first battery my alarm wouldn't shut off, with the new one it didnt make a sound, what does that mean. Did they sell me a bad battery, and in return tore my new alternator up?
Welcome to the forum
You will need a Volt meter to test battery and alternator, its easy to do but must be done or you will end up wasting time and money
Volt meters are $10-$15 and will save you hundreds of dollars down the road
Set Volt meter to DC Volts
A good battery will read 12.3volts to 12.8volts, key off
12.8v is a newer battery
12.3v is older battery and time to shop for battery sales
Read the battery voltage and REMEMBER IT
On the alternator there are 4 wires
B+ is the larger wire on the stud/nut terminal on the back of the alternator
Put the Black Volt meter probe on the alternators metal case, this is the Ground, battery negative
Put the Red Volt meter probe on the B+ terminal
Should see "Battery Volts", i.e. what you just saw when you tested battery, and remembered it
If not then 175amp Mega Fuse is blown, its on the outside of the engine fuse box
Unplug the 3 wire connector on the alternator, it has a yellow wire, white wire and green wire
Test yellow wire(black probe still on alternator's metal case)
Should see Battery Volts
If not then 30amp fuse inside engine fuse box is blown, ALT SYS fuse, or MEGA fuse is blown
Test green wire, should be 0 volts
Turn on the key
Re-test green wire, should be 12volts, may be a bit lower than Battery Volts, thats OK
If not then Cab Fuse box #15 fuse is blown, its 7.5amps, it also powers the battery light, if Battery Light comes on with key on/engine off then this fuse is OK
And thats it
If all the wires test OK then your wiring is good
Plug 3 wire connector back in
Start engine
Test Battery Volts again
13.5v to 14.8volts is what you should see if alternator is working
Under 13.5v means alternator is bad, period, not a guess if wires are OK
A bad battery can ruin an alternator
A bad alternator can ruin a battery
So the two are tied together if one fails
The Battery's only purpose is to start the engine
Once engine is running the alternator supplies ALL vehicle voltage/amperage at 13.5volts or higher, batteries are always 13v or less, so once engine is started voltage runs TO the battery to keep it charged
Just a heads up, you should not disconnect either battery cable while engine is running, this was an old test used with mechanical voltage regulators, that use relays
Yes, it will tell you if alternator is working, i.e. engine doesn't stall so battery is not being used, engine stalls so alternators is not working or fuses are blown
But it will also cause a Voltage Spike that can ruin electronic voltage regulators inside the alternator
Best to use a Volt Meter
You will need a Volt meter to test battery and alternator, its easy to do but must be done or you will end up wasting time and money
Volt meters are $10-$15 and will save you hundreds of dollars down the road
Set Volt meter to DC Volts
A good battery will read 12.3volts to 12.8volts, key off
12.8v is a newer battery
12.3v is older battery and time to shop for battery sales
Read the battery voltage and REMEMBER IT
On the alternator there are 4 wires
B+ is the larger wire on the stud/nut terminal on the back of the alternator
Put the Black Volt meter probe on the alternators metal case, this is the Ground, battery negative
Put the Red Volt meter probe on the B+ terminal
Should see "Battery Volts", i.e. what you just saw when you tested battery, and remembered it
If not then 175amp Mega Fuse is blown, its on the outside of the engine fuse box
Unplug the 3 wire connector on the alternator, it has a yellow wire, white wire and green wire
Test yellow wire(black probe still on alternator's metal case)
Should see Battery Volts
If not then 30amp fuse inside engine fuse box is blown, ALT SYS fuse, or MEGA fuse is blown
Test green wire, should be 0 volts
Turn on the key
Re-test green wire, should be 12volts, may be a bit lower than Battery Volts, thats OK
If not then Cab Fuse box #15 fuse is blown, its 7.5amps, it also powers the battery light, if Battery Light comes on with key on/engine off then this fuse is OK
And thats it
If all the wires test OK then your wiring is good
Plug 3 wire connector back in
Start engine
Test Battery Volts again
13.5v to 14.8volts is what you should see if alternator is working
Under 13.5v means alternator is bad, period, not a guess if wires are OK
A bad battery can ruin an alternator
A bad alternator can ruin a battery
So the two are tied together if one fails
The Battery's only purpose is to start the engine
Once engine is running the alternator supplies ALL vehicle voltage/amperage at 13.5volts or higher, batteries are always 13v or less, so once engine is started voltage runs TO the battery to keep it charged
Just a heads up, you should not disconnect either battery cable while engine is running, this was an old test used with mechanical voltage regulators, that use relays
Yes, it will tell you if alternator is working, i.e. engine doesn't stall so battery is not being used, engine stalls so alternators is not working or fuses are blown
But it will also cause a Voltage Spike that can ruin electronic voltage regulators inside the alternator
Best to use a Volt Meter
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