Hi guys; I'm back from a trip to Missouri. The first issue is you're right in the middle of when GM was switching over from a mechanical voltage regulator mounted on the firewall to an internal regulator inside the generator. The older type had a two-terminal connector on the back of the unit, near the center bearing. They were parallel to each other like on the legs of the letter "H". The newer design has the two terminals on side of the housing right next to the back. They are right next to each other like the letter "I" cut in half through the middle.
For the older style, a 60-amp generator was the largest that was typically installed at the factory, based on options on the car but a 78-amp was available for special applications. You didn't list which engine you have so it's hard to know what came on your car from the factory, but the smallest was a 42-amp on the 250 c.I. 6-cylinder if it didn't come with air conditioning. If you want to go larger than the 78-amp generator you'll have to switch over to the newer "SI", (System Integral), generator, and that is fairly easy to do.
If you have the newer design already, in my opinion it was the world's second best generator ever built. The 78-amp unit was the largest used in 1970, but the design was used all the way up through 1986. (At that point GM came out with by far the world's worst design, so don't even think about doing some kind of modification to use that). In 1986, the largest unit was the 78-amp for passenger cars and trucks, which was way more than necessary for the day, but there was a 108-amp for special applications like police cars and ambulances.
There are some important considerations to be aware of when looking for a replacement. The first one sounds like nit-picking but it involves a key discussion point later. That is that these are called "AC generators", not "alternators". All of these, regardless of brand or manufacturer, are generators that develop alternating current. Since direct current is the only electrical power that can be stored, the generator's output has to be "rectified" or changed into direct current, and that is the job of the diodes, which are one-way valves for electrical current flow. Chrysler developed the AC generator for use on their cars in 1960 and they copyrighted the term "alternator". Obviously everyone will know what you're referring to when you say "alternator", but to stress using correct terminology in the classroom, we use "AC generator".
The second point of interest is an AC generator is physically incapable of producing more current than it is designed for, while maintaining at least 13.75 volts. If the car's electrical system requires more current momentarily, it will be supplied by the battery. If you were to run the power windows, power locks, heater fan on "high", wipers, and all the lights, you still would be unlikely to exceed the generator's capability because the manufacturer took all of that into account when they chose which unit to install at the factory.
Related to that maximum output, building a generator capable of more current only requires adding a few inches of wire, so at first the additional cost would seem to be real low. All generators need three things to generate current. Those are a wire, a magnetic field, and most importantly, movement between those two. We increase the capacity by using many wires to "induce" the output into, and that simply means we coil the wire so the magnetic field interacts with all of the loops. The magnetic field we use is an electromagnet because it is so easy to control and adjust for the needs of the electrical system. The movement comes from spinning that electromagnet with the engine's belt and pulleys. There's lots more exciting theory related to the output current, but that's not what you asked about.
The important point I'm leading up to is a voltage is magnetically induced into each loop of wire in the "stator" winding. That's the stationary coils you can see on some generators. They pick just the right number of loops so they develop a total of around 14.5 volts. You add another group of those coils to add to the total amount of current they could develop. Add another group, ... Add another few potential amps. This is where adding a few inches of wire will increase the maximum current the generator could potentially develop. Of course "bigger is always better", but you have to remember that to add ten or twenty amps to a million cars could require buying 100 million more feet of ten-gauge copper wire, and that would be for cars and trucks that are never going to be able to take advantage of that extra capacity. Ford used to obsess over saving twenty cents per car by leaving off four five-cent grease fittings, so you know no one is going to put more into a generator than is necessary. That extra generating capacity can easily translate into an additional twenty to fifty bucks on the cost of the car by the time it passes through all the hands that want some profit from it.
On the other hand, when aftermarket rebuilders and suppliers get involved, they have fixed costs involved in labor, but only a couple of more pennies to add those few inches of wire to the stator. In return, they can price their finished products similar to the optional stuff from the car manufacturer. They will make a huge additional profit by pushing those higher-amp generators on the consumers, and convincing you that you need one.
Now, we have to go back to my comment about the diodes. Every diode has a reverse voltage it can withstand and a forward current it can handle, and those are designed in. Besides adding just a few inches of wire to the stator to increase capacity, you must use diodes that are capable of passing that higher current without overheating and shorting. THAT is where most of the additional cost comes in when you buy one of these from the manufacturer or new-car dealer.
To continue this wondrous story, all AC generators put out three-phase output current, and each phase uses a pair of diodes. When just one of those diodes shorts, you will lose exactly two thirds of the current-producing capacity. That means during a load test, you will only be able to get 20 amps from a 60-amp generator. On some cars that can be enough to meet the demands of the electrical system. On others, the battery will make up the difference until it slowly runs down over days or weeks. You need a professional load tester to measure the full-load current, and it will display "ripple" voltage. Output voltage, (electrical pressure), is very steady with three-phase output, which is why it is used, but with a shorted diode, one phase will be missing. Every third current pulse, (and voltage pulse), will be missing, and that makes ripple voltage go very high. (That's the difference between maximum output voltage and minimum output voltage). We can't measure that with any other equipment but it is also what causes a lot of whine in the speakers when listening to AM radio. Low maximum output current and high ripple voltage are what your mechanic uses to determine there's a failed diode.
The point of all that is if you're having an insufficient-current problem, have the generator tested before you automatically go looking for one that's bigger.
The next issue refers back to what I said about an AC generator being incapable of developing more current that it is designed for. You must also realize that with GM's internal regulator design, up to a maximum of about three amps goes right back into the generator to run the field winding which is what develops the electromagnetic field. That doesn't get included in the measurements. If you're real lucky, you might squeeze out 80 amps from the 78-amp generator, but any more than that and you'll start to draw the voltage down too low. The generator's voltage has to be one or two volts higher than the battery's voltage if you want to convince some current to go into the battery to keep it charged.
So the generator can keep up with the electrical system's demands, the diodes can handle the highest current the generator will ever develop, and the generator will never develop more current than is needed at that instant, but there's one more tidbit that no one ever considers. The diodes block current flow in the reverse direction, and they are wired, in effect, in pairs. There are two groups of diodes with three diodes in each group. I explained that with one shorted diode you lose two third of the output capacity, but in addition, if one diode in each group were to short, there would be a direct short to ground. Something is going to smoke and burn up somewhere. Every mechanic, at some point in their career, will come across a generator with the output terminal charred and falling apart, and the wire burned off. That is the result of bypassing some type of fuse device. Today almost all vehicles have a very large fuse just for the generator's output circuit, bolted into the under-hood fuse box. They're bolted in because simple slide-in / plug-in terminals can't be trusted to pass that much current without overheating. On older vehicles, however, fuse link wires were always used. Those are a few inches long, and spliced into the rest of that piece of wire running from the generator's output terminal back to the battery positive terminal. The secret is though, that short piece of wire is smaller in diameter than the rest of the wire, and its insulation will not melt or burn. It is the weak link in the chain and is the fuse. Chrysler put that near the battery right in the wire harness and they used one of up to three different sizes, depending on which alternator they installed on the vehicle. GM uses the larger "battery" terminal on the starter as a convenient tie point for multiple circuits, and that's where you'll find their fuse link wires, including the one for the generator.
The issue that is always forgotten when installing a generator with a higher capacity is that fuse link. Suppose, for example, your car came with a 60-amp generator. (I'm guessing at the numbers here so don't use them for reference). The car might have a 12-gauge fuse link wire. An ambulance might have come with a 78-amp generator and a 10-gauge fuse link. Under normal conditions your car will likely never need more than perhaps 30 amps. If you install a 78-amp generator, the car is STILL never going to need more than 30 amps. Remember that while the generator has that higher capacity, it will never develop more than what the electrical system needs. The problem comes in when you take the car to your mechanic to have the charging system tested. The two main tests are ripple voltage and full-load output current. During that test, the charging system is loaded to force it to produce its maximum current for a few seconds. If you have a fuse bolted into the fuse box, it is going to blow as soon as its rating is exceeded. Fuse link wires take some time to burn open, but that might be only one or two seconds. Your mechanic might get away with performing the test once, then to bring you into the shop and verify his findings for you, he might do the test again and burn that link open. Now you have a totally dead charging system that you blame on his test, but it really wasn't the fault of anything he did. It was due to the fact that someone previously increased the capacity of the generator but not that fuse link. The car's electrical system never called for that much current. It was the combination of the generator with the higher capacity and the higher load from the tester that caused the problem. The fix is to install a larger fuse link wire, and most of the time the rest of the wire in that circuit is already large enough for the job.
Fuse link wire insulation is colored to denote its current rating, just like spade-type fuses are. You can buy replacements from any auto parts store, and you'll get a piece about a foot long. That's more than enough to cut into pieces to do three or four repairs. The length has nothing to do with protecting the circuit. It's the diameter of that piece of wire that's important.
One last comment of value, unlike with Chrysler engines, on GM's, you could find the generator in one of a number of locations depending on engine size, car brand, car model, year, other optional equipment, paint color, ... Well, maybe not paint color, but because of that, they needed different mounting ear locations on them. Through 1986, there were three different variations of mounting ear / rear mounting bolt hole combinations. If you have the generator you want to use but the front mounting ear is in the wrong orientation, remove the four long bolts that hold the front and rear case halves together, then spin the front cover to the orientation you need. Don't allow the halves to pull away from each other because the spring-loaded brushes will pop out of their holder. It's not hard to reseat them, but that's for another story. You'll hear them click if they pop out. Just rotate the case and put the bolts back in.
Now, ... To finally answer your original question, there is no list of typical current draw ratings but you can look at the fuses in their circuits. Your fuses are going to protect multiple circuits so take that into account. Also, fuses are over-rated to provide a safety margin and prevent nuisance blowing. For example, if a heater fan is expected to draw up to fifteen amps, the circuit would likely have a 20-amp fuse. To look at it a different way, if you want to know how much a wiper motor might draw, look at its fuse rating and calculate roughly half to three quarters that amount. A single brake light bulb draws close to one amp. A tail light bulb draws around three quarters amp. Figure five amps for one low beam head light and six amps for one high beam. GM radios draw more than most others at two to three amps. For a single dash light bulb, interior light, shifter light, etc, figure half an amp for each bulb. The ignition system is real low. With breaker points, expect a maximum of less than two amps.
What kind of accessories are you adding? If it's a loud, obnoxious stereo system to destroy your hearing, there is no generator that is going to solve any problem because those suffer from the "voltage drop" in the wires due to those wires' resistance. You need to have steady voltage right at the amp and to do that, you need to add a small car battery right there on the 12 volt feed wire, or one of the outrageously over-priced capacitors the manufacturers sell to kids with too much money. If you're adding something that only draws current once in a while, the battery is there to make up momentarily what the charging system can't supply. You don't have to go to a higher-output generator for things like that.
Saturday, July 16th, 2016 AT 2:30 PM
