Mr.T

Last measurements:
Alt+ to Bat- = 14.6
Alt case to Bat+ = 13.3

Previous measurements:
Bat+ to Bat- = 12.3
Alt case to Alt+ = 14.7

Test these:
Alt case to Bat-
This should be low, say under 0.1V. Expecting 1.3 to 2.4V based on your previous testing.

Alt case to engine block
If reading is >0.1 the alt case isn't making a good connection to the engine.

Bat- large lug (#6 wire) to bat-
If reading is >0.1 the terminal isn't making a good connection to the battery post.

Bat- or Bat+ is the actual battery post, not the terminal attached to it.

It looks like the ground path for the alternator are the alternator attachment bolts, and the #6 black wire from the engine block back to the negative terminal on the battery. Bolted connections, crimps, and terminals would be where I'd look first.

Rhino

All taken while battery light was on, meter read 13 or less, and engine was running

Alt case to Bat-. 0.03

Alt case to engine block. 0.00

Bat- large lug (#6 wire) to bat- 0.00

And then just for fun
PCM ground (both ends) to alt post 0.00
Batt - post to passenger side ground point 0.00
Block to hood ground Bolt 0.06

I think the hood ground strap and the is one in the same as the battery to block ground strap if I remember seeing it correctly. Does that seem right

Mr.T

The alternator ground path voltage drop looks OK, essentially that was the Alt case to Bat- voltage.

Where was the meter when it read 13 or less? Any other voltages measured?

Rhino

Sorry, should have said "torque read 13 or less"

Using your numbers as guidelines the only things that were > 0.01 were from block to negative ... both battery post and assorted other grounds.

Mr.T

Actually I was giving it 0.1 voltage drop maximum, and all were less than that.

Try the following when the battery light is on:

Alt+ to bat-
Bat+ to bat-
Alt+ to bat+
Alt case to bat-
Torque Pro App

This is a bit redundant, but hopefully this will narrow the possibilities down.

EDIT: Having normal charging voltage at the alternator and low voltage at the battery at the same time is the key symptom to confirm or eliminate with the above tests. If time allows, repeat the above measurements. Note if there are any trouble codes, the manual says "The PCM will store a DTC in memory for certain failures it detects". To see any stored DTC(s) on the dash odometer display: Insert your key into the ignition, turn it to the ‘ON’ position but do not start the engine. Then, push the key into the ignition cylinder and while holding it in, turn it to the ‘OFF’ position and then back to the ‘ON’ position (3) times straight making sure to end with it in the ‘ON’ position. It is important to keep pushing in on the key to prevent it from catching at the different positions along the way.

Reference note:
As you know, the PCM has the voltage regulator function built in. There's a 2-wire plug on the alternator -- The PCM senses the alternator output voltage using the red w/ gray stripe wire, and adjusts the alternator output voltage by varying the field current in the brown w/ dark green stripe wire. The PCM has to have a stable ground reference to measure these voltages and function correctly, but I'm not sure which ground that is at the moment. The TIPM has a battery voltage input that's sent to the PCM.

jchappies

Just run a set of jumper cables from the Alt case and Bat- and check all your voltages again, if they are all the same now you have a ground problem

Mr.T

Rhino -- This TSB may apply to your 2007 JK. Reads like they improved diagnostics for the Battery Light.

SUBJECT:
FLASH: Powertrain Control Module (PCM) MIL/Battery Light Illumination

Rhino

2 sets of measurements. the one after the comma is with a jumper cable run from alt case to battery neg
Alt+ to bat- 15.02, 14.90
Bat+ to bat- 13.48, 13.65
Alt+ to bat+ 1.4, 1.3
Alt case to bat- 0.04 , 0.04
Torque Pro App 13.4, 13.4

Pulled these codes with the key dance (shouldn't be shocking)
P2503 - Charging System Output Low
P2504 - Charging System Output High

Also, that TSB doesn't apply to me, build date was later than that.

Mr.T

You're very close to fixing this! That indicates a high resistance connection between Alt+ to Bat+. EDIT: Just started my Jeep and checked Alt+ to Bat+ on mine, it was 0.05 V (your measurement above was ~1.4 V).

The jumper from Alt case to Bat- didn't change anything because the voltage drop was already near zero between those two places.

Note that adding the 1.3 of [Alt+ to Bat+] to 13.65 of [Bat+ to Bat-] == 14.95 which is close enough to the 14.90 at [Alt+ to Bat-]. That's a sanity check that the problem is the voltage drop from the Alt+ to Bat+.

There's a fusible link in that wire, that's a possibility. And it could be a loose connection, bad crimp, corrosion, etc.

Arc flash safety note: For auto electrical work, remove the battery ground terminal (usually negative) first, and connect it last after work is complete.

What I'd do is disconnect this #6 red wire at Alt+ and Bat+ and measure the resistance (in ohms). Wiggle the connectors/terminals while looking to see if the resistance jumps up. Look for any looseness or corrosion. It could be one of the ring terminals at the battery or alternator. If your meter doesn't measure low ohms, suggest just cleaning and re-bolting the connections at either end.

If the Alternator was supplying ~10 amps with a voltage drop of 1.5 volts, the resistance is ~0.15 ohms. A good #6 wire + fusible link + crimped terminals is going to read around 0.01 ohms maximum. The difference between 0.01 ohms [good] and 0.15 ohms [bad] is pretty small, hopefully your meter can read that low.

Now if for some reason the amp draw was 120 amps instead, a voltage drop of around 1.2 could be expected (assuming 0.01 ohms) -- But that's a weird 1 in a million intermittent battery fault (that is normal when winching).

Previously I mentioned the sewing needle trick to pierce insulation and take readings mid-wire -- If it's the fusible link, that might be necessary to confirm it.

I suspect that the hi and lo voltage fault codes mean that it's a bad connection that intermittently heals itself quickly, it may even be going open circuit occasionally -- Essentially the PCM voltage regulator function can't keep up with quick resistance changes in the alt output wiring.

EDIT: If the Alt+ connection was ever accidentally grounded momentarily with the all the wiring connected to the battery, the fusible link in that wire run may have been damaged (adding resistance), but not melted completely open circuit.

Rhino

I'm optimistic that its resolved. Embarassingly, I didn't realize I'd attached the fusible link for the alternator to a different lug on the terminals I have and never checked it until I went to check the ohms. The lug was barely finger tight. I did check the resistance for good measure and it was fine. Tightened everything up and now have had 3 trips (short ones) w/o the light coming on. I'm not "certain" yet but the odds are good.