I am having issues with my 1997 B2500 Van with 5.2 that I cannot seem to figure out what's happening. Starting first thing the van runs fine, I show the voltage is good and no problems but once the van has ran for a while (100 miles of travel or more) it stops charging the battery and eventually lose all charge. I slowely start seeing the voltage drop late in the trip (unfortunately when I'm away from home) and if I'm lucky and it's not getting dark I can make it back otherwise the headlights will kill it quickly and I end up stranded. I replaced the alternator but still seeing the exact same behavior with the new one. Any help would be greatly appreciated. It's my only vehicle so I can't afford to have this keep happening but also unemployed so cannot afford to have a professional repair.

 

Can you confirm the alternator is actually charging, meaning the voltage is higher with the engine running compared to before starting it ? I'm wondering if you're just running off the fully charged battery.

The voltage regulator is part of the PCM unfortunately. A common fix is to use an external regulator which requires a little bit of wiring. Plenty of information on the web about this.

 

Just checked with multimeter

Not running / Everything off
Battery 13.12v

Running / Nothing on
Battery 14.45

Running w/ headlights, rear fan and front fan on high
Battery 13.2-13.5

 

Ok, so the charging system is working initially and then stops. I suppose it's possible the control circuit in the PCM fails as it heats up. Before condemning the PCM check a couple of things: Measure the voltage at the dark green wire going to the alternator. It should be low to no voltage when charging is working, what does it show when charging stops ? Also check the ground connection from the alternator and make sure the battery temp sensor is still in place under the battery.

 

​​​​​Just got back from driving it for about 45 minutes with nothing on. The following is a check after returning:

Running / Nothing on
Battery 14.37

Running w/ headlights, rear fan and front fan on high
Battery 12.99

Not running / Everything off
Battery 13.01v

I will check the voltage of the requested lines shortly and follow up. Thank you for helping.

 

Need to see if the alternator is putting out voltage. If not likely the voltage regulator failed in the PCM. You can wire in an external regulator

 

Not seeing green wire but might be green under Insulation. The wire circled in red is showing 14.46-14.49v at idle.

 

The green wire should be one of the wires up on the top

 

I can see a harness that has a small green w/ white stripe, a small orange w/ black stripe, and a large black wire. All 3 are running into the plastic block that connects to the top of the alternator. Unable to tell which goes to which terminal but the idle voltage on the left (purple) is 9.6-9.9v and terminal on right (white) is 13.82-13.83v.

 

Sorry, I went by the truck service manual but yours is a van. In any case the terminal circled white is coming from the ASD relay and provides battery power to the field coil. The one circled purple is going to the PCM and the PCM switches the connection to ground rapidly basically creating pulse width modulation. Depending on the on/off ratio the measured voltage changes.
The voltages look good, check them again when charging fails. If you see the same voltage on both terminals that would indicate the PCM has stopped switching the ground connection.