First put the right generator back on. No generator will develop more than the electrical system demands. What you have done is added more capacity to the system. You won't get more current except under only one condition. That is during a full-load output current test with a professional load tester.
The second law of AC generators is they are physically incapable of developing more current than they are rated for. If your car came with a 100-amp generator from the factory, there is going to be a fuse link wire spliced into the output circuit going back to the battery that was sized according to the size of the generator installed. By installing a replacement generator with a higher capacity, if the electrical system needs 43 amps, for example, the largest generator in the world is going to develop exactly 43 amps, except during that brief output current test. That is intended to run the generator at its maximum output for a few seconds. A 140-amp generator will develop close to 140 amps, and that is going to burn out the fuse link wire. Newer vehicles use large bolted-in fuses for this circuit. Older vehicles like what I drive still use those fuse link wires. In some cases those are hidden in the wiring harness, and when you do find it, they are quite time-consuming to replace.
What you have going for you is the engineers at GM redesigned their generators for the '87 model year and turned them into a real pile. The '86 and older unit is, in my opinion, the world's second-best design. There's two common things that can cause the symptoms you described. The first is a slipping drive belt. Checking that is easy on GM and Ford vehicles. They use an external fan blade right behind the pulley. Of course you will do this with the engine not running! Use your thumb to push on the fan blade. If you can get it to slip under the belt, the belt is too lose, or it is worn down too far, or it's the wrong belt. If your generator still uses the reliable V-belt, those need to wedge into the pulley so it will grip on the sides of the belt. You can get a skinnier belt, or one that is worn down, then it will run on the bottom of the groove in the pulley. There's no grip there.
To add to the misery, all generators require three things to work. That is a wire, (coil of wire), a magnet, (we use an electromagnet), and most importantly, movement between them. As a result, all generators are very inefficient at low speeds. Even though the belt is slipping, it is still running the pulley faster at higher engine speeds. That could be one reason you only see the low charging voltage at idle.
The second common problem that only pertains to GM generators is a defective diode trio. Current flow through the dash warning light gets this system started, then once it's producing current, a tiny sample of that is tapped off and goes through the diode trio. That develops a little current that keeps the electromagnetic field energized until you stop the engine. The voltage it develops has a second function. That is to turn the warning light off. The clue here when one of those three diodes fails, is it only turns the light off two-thirds. The often-overlooked symptom is the warning light is still lit up very dimly. Quite often you can only see it glowing at night.
The issue here is that is the current that runs the generator, and it is cut by one third. Under that full-load output current test, which you need a professional load tester for, your 140-amp generator will only be able to produce close to 90 amps. That standardized test is run at 2,000 rpm. At idle, that output current will be even less, and if it can't keep up with demand, system voltage is going to slowly drop as the battery takes over to make up the difference.
Another part of the tests can actually confuse this problem. That is measuring charging voltage. You can do that with an inexpensive digital voltmeter. With the engine running, you must find battery voltage to be between 13.75 and 14.75 volts. When you have a defective diode trio, the internal voltage regulator sees that dip in voltage when current is supposed to be coming through the failed diode. It responds by bumping up charging voltage a little. That can offer a clue on some cars. You might find battery voltage to be closer to 15.0 - 15.2 volts. That's not enough to boil the water out of the battery, but we want to correct that.
When you find charging voltage in the acceptable range, that only means it is okay to perform the rest of the tests, but that requires a professional load tester. There's another chapter related to low output current. All AC generators use six main diodes to turn the alternating current into direct current that can be stored in a battery. When one of those diodes fails, you will lose exactly two thirds of the generator's output capacity. That is less affected by lower engine speeds, but 35 amps from your original 100-amp generator is not enough to run the entire electrical system under all conditions. Again, the battery will have to make up the difference until it slowly runs down over days or weeks. You may also see this as low voltage at idle. The professional load tester checks for this. Also, with one of these failed diodes, "ripple voltage" will be very high. A few testers make a printout and they show a specific value for ripple voltage. Most testers just show it on a digital relative bar chart. On the newer generator design, replacing any internal part is a lesson in frustration, and it is not practical even for an experienced mechanic. With yours, these output diodes are replaced in two blocks, each with three diodes. The diode trio is a very inexpensive part that is just bolted in once the unit is apart. I can post photos if you need to tackle that job.
For my last comment of value, do not take a generator to a parts store for testing. At most, those have a one horsepower motor to run the generator, and they can't do that at a raised speed that is necessary for proper testing. On the engine, it takes a good five or more horsepower to run a generator. When it can't be run under full load for a few seconds, ripple voltage will not be as high as we need it to be to know it is bad. Complete on-car testing of the charging system only takes a minute or two when it's working properly, and usually just a few additional minutes to diagnose the common problems.
You can find more dandy information here:
https://www.2carpros.com/articles/how-to-check-a-car-alternator
Sunday, March 31st, 2019 AT 10:05 PM
