Alternator-battery connection

This is why I often tell people to use a test light instead of a digital voltmeter. If you stop reading right here and retest what you did already, but with an inexpensive standard test light, you'll find different results.

You inadvertently stepped over the best clue in that you found different voltages at the battery's positive post and the generator's output stud. Those two are tied together by a fat wire, so there has to be the same voltage at both ends. You'll notice the starter solenoid is in that circuit too. Don't pay any attention to that. It is only there because GM often uses that as a convenient tie point so they don't have to run two separate wires back to the battery. That solenoid is not part of the charging system.

At some point the end of the wire at the generator became grounded, either by accidentally shorting it to ground with a wrench, or because two or more of the generator's six internal diodes were shorted. That burned open the fuse link, shown by my blue arrow in this diagram. That is why you see a higher voltage at the generator than at the battery. The generator can be thought of as a pump for electrical pressure, (voltage) and the battery is the storage tank, like the water tower for your city's water supply system. As long as the voltage is higher at the pump, current will try to flow into the battery, but it can't get there because of the blown fuse link. GM's voltage regulators are inside the generator, so the entire system is working properly and regulating at an acceptable 14.8 volts. It's not supplying the electrical system though. That's why the battery is running down as you drive.

The fuse link wire is usually located near the starter solenoid. You'll see at least one piece of heat-shrink tubing on it about four to six inches from the terminal. Tug on that wire to test the fuse link. If it's still good, it will act like a piece of wire. If it's burned open, it will act like a rubber band.

You can buy new fuse link wire from any auto parts store. They may even have special ones for GM vehicles that have the terminal already crimped on one end. You'll need to splice the other end to the red / black wire, then seal it with heat-shrink tubing with hot-melt glue inside. That will seal out moisture. These are typically about 12" long. You can cut them into two or three pieces to make multiple repairs. The length you install is not important. It simply needs to be a smaller diameter than the other wires it protects. That makes it the weak-link-in-the-chain, just like any other fuse, except it is a slow-acting fuse.

Fuse link wires are selected by color which denotes their current rating. Don't try to use a regular piece of wire. Fuse links have insulation that will not melt or burn.

There's one confusing thing related to fuse link wires. That is they form a coating of carbon on the inside of the insulation where arcing occurred as it burned open. The generator's output current can't get through that carbon, but when no current is trying to flow through it, no voltage is dropped across it either. For all practical purposes, a digital voltmeter doesn't rely on current flow through it to take a voltage reading, just like a pressure gauge on a compressed air line doesn't require air to flow through the gauge to take its readings. You start with 12.6 volts at the battery, drop 0 volts across the fuse link, carbon, and rest of the wire, so you end up with 12.6 volts at the generator, as you found.

Now try to do that with a regular test light with an incandescent bulb inside, not one of the new fancy ones with all kinds of electronics. The bulb requires current flow through it to light up, and that current can't get through the carbon track. It will correctly show 0 volts at the generator. Another way to see this is to take the reading with your voltmeter, then while you're seeing the 12.6 volts, touch the test light there at the same time. When you do, the voltage will drop to near 0 volts.

So are you saying the system is working now? I suspect it is.

As for your observation of different fuse link wires, this is one item that needs a mention, especially for the benefit of other who might see this when researching a similar problem. Very often a car model could come with one of multiple choices in fuse link wires. Depending on factory-installed options including cruise control, air conditioning, high-output or rear heater, and rear-window defroster, those packages usually include an optional larger generator. A smaller generator could come with a 12-gauge fuse link wire, while a larger one would come with a 10-gauge fuse link. The wire has to be large enough to handle the full output of the generator for a short time. In the rare event two diodes were to short inside the generator, that would cause a dead short, and any fuse link wire will burn open.

Where it's possible to get into trouble is many of us think if big is good, bigger is better, and they bolt on a higher-output replacement generator. I must also point out the generator is physically incapable of developing more output current than it is designed and rated for. Lets say that's a real common 90-amp unit. The next important detail is they will only develop as much current as the vehicle needs, and no more. With head lights on, and heater on a lower speed, a typical car might need around 30 amps including a little to recharge the battery. Now if you replace the generator with a 120-amp unit, it is still going to develop only 30 amps. This would equate to a municipal water pump that can pump 100 gallons per minute, but since the tower is full, it's actually going to pump just as much as the people are using, and no more. If they install a 150 gallon pump, it isn't going to pump any more than the old pump.

For your car, that means no matter how big the generator is you install, it won't matter to that fuse link wire, ... That is until your mechanic comes along to perform a charging system test. Professional load testers have us dial up a heavier and heavier load on the system until system voltage drops no lower than 13.75 volts. The voltage regulator will see that and in response, run the generator harder and harder to keep up with demand. When it reaches that maximum output is when we take the current reading. It's also important to note that there's no such thing as a weak generator. It will develop very close to its rated current or, if it has one defective diode, very close to exactly one-third of its rated current. The point of interest is if you have a 90-amp generator, during this test is the only time it will ever develop 90 amps. The fuse link was chosen at the factory to handle 90 amps for enough time to make it through the test.

Now if someone installed the 120-amp generator, and your mechanic runs this charging system test, it's going to develop close to 120 amps for a few seconds, and that can burn open the fuse link. Now you'll have a no-charge condition. Of course the mechanic gets the blame, but he had no way of knowing this modification was done.

At this point, if you happened to notice the color of the fuse link, and it's the same on the old and the replacement cable, everything will be fine. Even if it is different, it's going to work fine because every fuse link is a smaller diameter than the wire it's spliced to and is protecting. It's still the weak link in the chain.

No need to worry about the size of the generator. The engineers look at all of the possible loads that could be turned on at the same time, which could be as much as 50 - 60 amps, then they install a generator with plenty of extra capacity. That's so it never gets run continuously at near that maximum rating.

As a point of interest, it's the stator windings that determine the output capacity. That's the stationary windings that you can see through holes in the housing. If you'd stretch those coils out, the copper wires would be about two feet long. To increase the output capacity, all that's needed is to add a couple of inches to those three wires. Additional cost might be 50 cents worth of copper, and they charge dozens more dollars for that generator.

As for the heater fan, my only comment of value, is at least on older models, that current came through one section of the ignition switch, and that current is hard on those little contacts and on the corresponding connector terminals. On a model as new as yours, heater fan current often comes through a relay that handles the high current. The people who have the most trouble are those who have a habit of turning the ignition switch on and off while the heater fan is set to one of the higher speeds. That results in a lot of arcing across the contacts. That results in resistance, and therefore heat build-up, and that can transfer to the connector terminals. This applied to my mother. She had ignition switches burn up and melt the connector body on two vehicles. Can't yell at your mother, so I always made sure to have a few good used switches on hand.

Happy to hear you solved your charging problem.

The engineers have added computers to systems that never needed computers before. In the '60s, Chrysler had ignition switch lights that stayed on for 20 seconds after the doors were closed so you could see to put the key in. That was done with a two-dollar thermal switch that had a 0 percent failure rate. Today they must have dome lights that fade out gradually. That's critically important, but it can only be done with an electronic module.

Some over-educated politicians want us to all drive with our headlights on all the time. On cars where that's automatic, they turn on the head lights but not the running and tail lights, so now every night I see three or four cars leaving the parking lot and they forgot to turn the lights on. For those of us who forget to turn the lights on, that could be done with a ten-dollar relay to do what the head light switch normally does, but instead, GM's engineers developed a daytime running light module that turns the high beams on at 80 percent. That also requires an electronic module, and now the person in front of you has to switch his "day / night" mirror to the "night" setting during the day because the yellowish bright lights are blinding him. Two hundred dollar computer module to do what a ten-dollar relay could do more reliably, and dimming the mirror defeats its purpose.

In Wisconsin motorcycles must have their headlights on so they'll stand out and be noticed. When all cars also have them on, no one notices the occasional motorcycle since they blend in and look no different. Hence, the law of unintended consequences. Motorcyclists are in more danger while accomplishing nothing for car drivers.

Electronics is very intolerant of heat, vibration, and moisture, so we pack way too much of it right where those conditions exist most. Very disappointing that no one makes one car model that's still simple and easy to repair. Oh well. I can still walk if I have to.

Good luck with your other problems. I'll pop in to dispense wondrous advice if asked and if necessary, but we have other experts who specialize in the areas you've mentioned.