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I have an 02 4.0 that the battery was having charging issues. I got the battery tested they said it was reading fine and it was probably the alternator. So I bought and replaced that only to continue driving and had the battery die again. So I brought it do a different spot where it read about 6-7v. Then I replaced the battery. But now my volts on the guage cluster are reading high when driving.
You need to get a Volt meter, it will save you alot of money over the years, they cost maybe $15
Battery and charging system are easy to test with a volt meter
Set meter to DC volts
Car batteries read 12.3volt to 12.9volts(key and engine OFF)
12.9v is a new battery
12.5v is a 3 year old battery
12.3v is a 5/6 year old battery and time to shop for battery sales
12.2v or less is a drained or bad battery
Simple
Your whole charging system consists of just 3 wires
Test the battery volts and remember it, that's "battery volts" for the other tests
Key and engine OFF
On the back of the alternator is the B+ terminal, a stud/nut terminal, test its volts, should be Battery Volts exactly
Unplug the 3 wire connector on back of alternator
Test the Yellow wire, should be Battery Volts exactly
Test light green wire, should be 0volts
Turn on the key, engine OFF
Re-test light green wire, should be Battery Volts, can be 0.1 or 0.2 less
Inspect white wire, short jumper wire to spade terminal on the back of alternator, should not be frayed and should have a good connection to single terminal
If all 3 wires test OK, then truck system is fine, 100%
Plug 3 wire connector back in
Start engine
Test battery voltage, should now be above 14volts, up to 14.9volts is OK, but NEVER EVER above 15volts
Let engine idle for 5min or so
Voltage should be dropping as time passes
Should settle in under 14volts after 5min, 13.5volts to 13.9volts is expected
If volts are not dropping then alternator is bad, well voltage regulator IN the alternator is bad, not uncommon in 3rd party alternators
New parts are NOT tested, they are made, boxed and sent out, YOU are the tester, that's why they have a warranty and cost less
Motorcraft(Ford) parts are tested, and that's why they are SO EXPENSIVE, labor to test is big dollars now
For an alternator that is not that hard to swap out its worth saving the money and getting 3rd party alternator, but you will come across "bad out of the box" parts
