Help with diagnosing bad alternator
 

Thinking I have a bad alternator on my 99 Ranger. I have "rescued" teen daughter three times now with jump starts when the truck would not start. Always when it has been raining (headlights on, wipers on, etc) The first time I suspected alternator but local shop said battery was good but low and alternator was charging. I put a meter on it and it reads 13.4 to 12.96 when running so it appears to be charging to me too. I can also remove negative cable from battery and it will stay running. BUT, as soon as I turn the headlights on with negative cable removed the engine dies immediately. I am thinking the alternator is on the way out and it cannot operate under any kind of load. I am thinking with too much load the alternator cant handle it and the battery has to take up the slack until it too gets too low to run the vehicle. Thoughts?

FIRST.........NEVER unhook a battery cable with engine running
On older vehicles without electronics this was a common way to test if alternator is outputting any voltage, it causes voltage spikes which WILL damage current electronics.
Also when jump starting disconnect the battery on the Jumping vehicle(the one that starts), if it has electronics, you can fry it's electronics when other engine starts.

An alternator has 3 "fields" in it's case, each field produces AC power when the rotor is turning(engine running).
Each field has a pair of Diodes that convert the AC volts to DC volts.

Most common issue with alternator failure is field failure, either the wiring or diode pair, this limits the voltage output at lower RPMs.
I.E. dimming lights when at idle, no that is NOT normal, lol.

If you drive mostly in the day time and are above 1,200rpm most of the time 1 field failing won't cause an issue, except for dimming lights at idle :)

Your voltage test says you have at least 1 failed field maybe 2, depends on the amp rating of the alternator.

Engine off battery voltage should be 12.3v to 12.8volts
12.8v being new battery
12.3v being 5 year old battery
At 12.2v or less you will have issues starting on cold mornings

Engine running
Just after starting battery voltage should be above 14volts, 14.4v-14.9v is normal
This is called "recharge voltage", the voltage regulator inside the alternator can detect battery voltage(amps) has been drain from starting the engine, so it "recharges" the battery from this draining.
After a few minutes the voltage will start dropping until it reaches "maintenance charge voltage", about 13.5v, this won't "cook" the battery it just maintains it at full charge and runs all the vehicle electrics.

Your voltage report shows fail field(s), at idle when you turn on the headlights the voltage may drop but will come back up to 13.5v(or higher) within a few seconds.
If voltage is at 12.9v, raise the idle, if voltage goes up then it is a failed field.

Thank you I will gladly take the dope slap for disconnecting the negative cable. So after reading your post I am assuming the alternator is likely defective. Am I correct?

Car batteries last 5 to 7 years depending on conditions.
1999 is 16 years old, so..............could be end of life on 3rd battery?

Battery's have six 2.1volt cells in series, so 12.6volts total, slightly over 2.1v new, for 12.8v

Common battery failure is a shorted cell, the plates inside a cell get a short and this drains the whole battery when it sits, with key off check battery voltage and leave meter on for a minute or two and watch if voltage is dropping, if it is then you have a shorted cell, and battery is self draining, BUT......also do the same test with battery disconnected, if voltage doesn't drop then you could have a short in the electrical system causing the battery to drain.

Thanks for the info but the battery tests good and is only 1.5 yrs old. I was asking if , based on the other information I provided if you believe the alternator is the culprit ?

Yes.........but, I would test voltage at idle and at approx. 2,000rpms, if voltage is higher and steady at 2,000rpms then yes I would say failed fields.

thanks, I just finished up replacing the alternator and that was it for sure. After new one installed I have no voltage drops, dimming of lights, etc. Everything is running and voltages are where they should be.

Thanks for posting the fix to your problem