If the system is not developing at least 13.75 volts with the engine running, how could the mechanic say the generator is okay? Did he perhaps bypass the voltage regulator, or, as I mentioned previously, could this be an intermittent problem? By far the most common cause of intermittent failure to charge is worn brushes inside the generator, but you would have solved that already by replacing the generator.
Does the "Battery" light on the dash turn on when you turn on the ignition switch? If it does not, check for a broken white / green wire going to the generator, or a blown 10-amp "C-10 Meter / Gauge" fuse in the fuse box behind the left kick panel. I can't tell what else that fuse feeds, but I suspect you'd have additional symptoms.
Next, check for 12 volts on the black / yellow wire at the generator. To be accurate, these voltages must be measured with everything plugged in and connected. Back-probe through the back of the connector next to each wire. You should find 12 volts on this wire when the ignition switch is on. If that is missing, check the 15-amp fuse, C-8, also behind the left kick panel.
Next, check the voltage on the white / blue wire at the generator. This circuit appears to be very similar to Chrysler's circuit which I am very familiar with. If this works the same way, you can expect to find between 4 - 11 volts. The lower the voltage, the larger the magnetic field is being created, and the more output current you should be getting. If you ground this wire, it will make the generator charge wide open. That is how we do the "full-load output current" test, and it could be what your mechanic did to verify the generator was okay. By grounding this wire, you're bypassing the voltage regulator, so that becomes suspect. Also, all generators are very inefficient at low speeds so you won't cause any damage if you do this test for just a few seconds. You DO risk causing more problems if you allow this to continue for an extended period of time, or especially if you raise engine speed. Proper procedure is to ground this wire for just a few seconds to verify whether battery voltage rises significantly. You can also turn on the head lights and watch them to see if they get brighter during the test.
My diagram also shows a double white wire at the generator. They are probably bolted to the output terminal on the back, and they will be pretty fat wires, ... I mean RATHER fat wires. You must always find the same voltage there as you do across the battery posts. If you find very high voltage on those wires, say 15.0 volts or more, while you find less than 12.6 at the battery, check for a blown 80-amp fuse in the fuse box under the hood. Fuses that large are often bolted in. The diagram only shows that something else feeds off that fuse on the same side as the generator's connection, so there will be other circuits that are dead if that fuse is blown, but they will work while the engine is running since they're being fed by the generator's output. Although the circuits will work, current won't be getting back to the battery to keep it charged. If that fuse IS bolted in, that's a good place to find an intermittent connection. Be sure both nuts are tight.
A second way to check that output circuit and fuse is to measure the voltage on the generator's output terminal, (double white wires), when the engine is off. You must find full battery voltage there. If the fuse is blown, you'll find 0.0 volts there.
If all these voltages are right, including you find battery voltage is between 13.75 to 14.75 volts with the engine running, there is one more way for the generator to let the battery run down. That is if one of the six diodes in the generator is defective. The only listing I found for your truck is a 75-amp generator. With a failed diode, all you will be able to get during the full-load output current test is exactly one-third of the generator's maximum rated current; in this case, 25 amps. That is not enough to meet the demands of the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks. You need a professional load tester to measure full-load output current. It will also measure "ripple" voltage. All AC generators develop three-phase output current which is very efficient and results in very little voltage variation, which is "ripple". With a failed diode, one entire phase will be missing causing ripple voltage to be very high. Some testers that provide printouts list ripple as a voltage. I don't know what "normal" is because I always used a tester that used a bar graph to show relative voltage between "low" and "high". Regardless, it's that 25 amps that is the clue we're looking for. If all the voltages are okay at the generator's plug, have the charging system tested again, and find out the value of output current.
Thursday, October 20th, 2016 AT 4:15 PM