That fuse is shown as a 30-amp fuse, so it will be green.
The next step is to check for 12 volts at the plug on the back/side of the generator. For the best accuracy with this type of problem, that reading should be taken with the connector plugged in. Back-probe through the back of the connector on the orange / light blue wire. You must find full battery voltage there all the time. If it's missing, check fuse F1.11 under the hood. That is a yellow 20-amp fuse. Typically that fuse blows when the voltage regulator is shorted. We used to replace them separately, but if it's too difficult to get to, replace the entire generator.
The blue arrow in the second drawing is pointing to that fuse. Drawings 2, 3, and 4 are for the first version of the "Battery Junction Box, (BJB) under the hood. The last three drawings are for the second version. I didn't add an arrow, but it's fuse # 11.
The next voltage reading is at the large output wire bolted to the back of the generator. That one must also have full battery voltage all the time and it must always be the same as what you find right at the battery. If you find 0 volts there with the engine not running, and possibly real high voltage, in the order of 15 volts or more when the engine is running, the fusible links are burned open. Those are wires spliced into the circuit, with special dull-colored insulation that is designed to not burn or melt. I doubt you're going to find a problem with them because they also feed the starter relay. If they were burned open, you'd be diagnosing a failure to crank, not a charging problem.
The last voltage reading is on the white / black wire shown way down at the very bottom of the first diagram. Back-probe that one with the engine running. When the generator is working, you'll find very close to half of system voltage, or 7 volts. It's that voltage that tells the voltage regulator the system is working, and to put 14 volts back out on the light green / red wire. With the same system voltage now on both sides of the dash light, the light turns off.
We also need to know what symptoms or observations led you to believe the charging system is not working. The best way to do this is to measure the battery's voltage with the engine running. If the system is working, you will find between 13.75 and 14.75 volts. If you find that, but the "Battery" warning light is on, and that is the only symptom, there is likely one failed diode of the six main diodes. Eight are used in your generator to bump up the maximum output current a little, but if one of the six is defective, you will lose exactly two thirds of the generator's maximum capacity. Common generators for this time period had maximum current ratings of around 90 amps. With one failed diode, 30 amps is not enough to run the entire electrical system under all conditions. The battery has to make up the difference until it slowly runs down over days or weeks.
These generators develop three-phase output. One phase is missing when a diode has failed. That shows up on the white / black wire, and that is what tells the voltage regulator to turn on the dash warning light. Charging voltage will usually look okay, and on some systems the regulator responds to the dip in voltage during the time that missing phase shows up, and that results in system voltage actually being a little high. This was common on '86 and older GM vehicles that used the world's second best generator design.
If you suspect a failed diode, that can quickly be verified with an output current load test, but that has to be done by your mechanic with a professional load tester. Besides the most output current you can get is only one third of what it should be, the tester will measure "ripple" voltage. That will simply be shown on a relative bar chart as "high" or "acceptable". A few testers actually measure the amount of ripple voltage and those can make a printout, but few of us know how to interpret the voltage. We normally just look for the "high" indication along with the low maximum output current.
Let me know what you find with the voltage tests. If they all look okay, except output voltage doesn't increase to at least 13.75 volts, the generator itself is the best suspect.
Images (Click to make bigger)
Friday, January 22nd, 2021 AT 11:36 AM