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Ask Away! with Jeff Smith: When and How to Update (and Upgrade) the Charging System in a Classic Car

I just bought my first car, a ’64 Chevelle with a 283 small-block and a Powerglide trans. The engine runs well, but I’m concerned that when I put the engine in gear at idle, the red “Charge” light comes on. What’s the best way to cure this problem? My friend says I should install a bigger alternator but what did guys do back in the day before 100-amp alternators? Thanks


Jeff Smith: First of all, congratulations on stepping up to buy a 50-plus year-old car as your first car. You didn’t mention what kind of shape it was in or whether it has been modified. Because it’s still running a 283 and a Powerglide, I’ll assume it’s not been highly modified and that the engine is still equipped with the original alternator.

Let’s start with the basics. Voltage is electrical pressure. Think of it like the pressure pushing water through a hose. Think of current–or amperage–like the volume of water travelling through a hose. A larger hose with the same pressure (voltage) moves more water–or amps. In electrical terms, a larger wire will flow more current with the same voltage compared to a smaller wire with more resistance. As more current passes through a wire, heat increases. This also increases the resistance.

GM converted from generators to alternators in the early 1960s. This was a time when cars demanded very little electrical power. For a typical Chevelle like yours with a radio and heater and no A/C, apart from the current draw from the starter motor, the single biggest electrical draw on the charging system was the defroster fan when set on high speed–it draws something like 12 to 15 amps. All the electrical components running at once (lights, wipers, defroster, and radio) probably is around 40 to 45 amps, which offers roughly a 10 to 15-amp cushion with a 60-amp alternator. As the electrical load increases, this demand lowers the available voltage, making it difficult for the alternator to maintain the system voltage of 13.5 volts. The problem with these early alternators is that you had to increase engine speed up to around 2,200 to 2,500 rpm to get maximum output from these alternators. Late model alternators are now designed to generate nearly full amperage at idle or just off idle speed, making them more efficient.

Now let’s fast-forward 50 years and we’ll assume the stock charging system is still in place and working. The first thing to do is to check the voltage output of the alternator. If your car is equipped with a voltmeter in the cockpit, don’t rely on it. Often, these meters are inaccurate due to poor grounds or that they sense voltage from under the dash rather than at the alternator. More on that subject in a moment..

With the engine idling, use your multimeter to check voltage at the output terminal on the alternator. Maximum voltage should be around 13.4 to 14.0 volts. Let’s say your alternator reads 13.6 volts. Now check the voltage directly at the battery touching the actual battery posts and let’s say the reading is 13.0 volts. That means that the battery is charging with only 0.50-volt more than the 12.5-volt pressure necessary to maintain the battery. Now with the car still running, turn on the high-beam headlights, the heater fan on high, and also run the wipers and the turn signals. This will be close to maximum load.

Now check the voltages at both places again. Let’s say that at the alternator the voltage is 12.9 volts and the reading at the battery has fallen to 12.0. Can you see the problem? With maximum electrical load, the alternator cannot maintain voltage and the battery is actually discharging while the engine is running. That’s why your “Charge” light is illuminated. The classic response might be: “Okay, a bigger alternator would fix that.” Yes, that’s one solution, but it’s not the best and the charging system will still be limited due to resistance in the circuit. Let’s look at what’s really happening.

Note that under maximum load that there was a rather substantial voltage drop between the alternator and the battery of nearly 1 volt (12.9 – 12.0 = 0.9-volt). Also look at the voltage drop with only minimal load on the charging system. With minimum load, the drop between the alternator and the battery was 13.6 – 13.0 = 0.60-volt. The reason there is a greater drop when the system is loaded is because additional current flow also creates heat and heat is resistance and that causes the a greater voltage drop.

An acceptable voltage drop between the alternator and the battery with a load applied is around 0.40 to 0.50-volt. As our test revealed, our car had much higher numbers. This means we have resistance between the alternator and the battery. You can also assume that the rest of the electrical system is also suffering from low voltage due to this increased resistance. The first place to look is at the connectors. If you inspect the wiring and connectors, they probably are corroded. Each time that alternator charge wire makes another connection, look for corrosion to reduce the voltage.

In the case of your Chevelle, our guess is that the original charging wires are still in place. A quick fix would be to remove the charge wire between the alternator and the horn relay, which is the main connection (electrical people call it a buss bar) to the rest of the car. It would also be a good idea to replace the wire between the horn relay and the positive post on the battery. I would suggest a minimum of a 10-gauge wire but 8-gauge is even better. This will minimize the resistance. Make sure all the connections are good with no corrosion (including the battery terminal) and then check the voltage again.

I did this upgrade on my 1966 Chevelle a few years ago. We ended up with a 0.45-volt drop between the alternator and the battery with a load of over 40 amps yet the charging system voltage was still able to create 13.5 volts at the battery at idle speed of 900 rpm. You should be able to create something near this voltage with new wires and solid connections. We mentioned earlier that the voltmeter inside the car may read lower than what you see at the battery. This is because there is a main feed wire that runs to the fuse box through the firewall. Older GM cars suffer from corrosion at this spot because the connectors were not coated to prevent corrosion. Often what will happen is when driving down the road, the car will suffer a complete loss of all electrical power and the engine will die. Then perhaps a minute or so later, it will just as quickly come back up again. This is caused by an intermittent connection at the fuse box for this main power feed wire. You can disassemble the fuse box and clean the corrosion and that will generally solve the problem.

The best solution is to surrender to the fact that your car is 50 years old and that it desperately needs a new wire harness. The insulation on the wires is probably brittle and cracked and this also causes the wire inside to corrode. The solution is a new wiring harness available from several different companies. For ease of installation, it’s hard to beat a stock replacement harness from a company like M&H Electrical Fabricators. These plug right into the stock fuse box location and all the stock connectors are already installed. Plus, you can order a harness with modifications such as a connector to add a later model GM integral-regulator alternator of your choice or to upgrade to an HEI distributor. There are several other companies that offer stock replacement wiring harnesses so you might want to check them out.

You certainly can also upgrade your alternator to the aforementioned later model GM alternator. A popular conversion was to the ‘70s and ‘80s style 10Si alternator that is internally regulated. This eliminates the need for the external voltage regulator, but these chargers do not put out much more than the original alternators. A better move would be to the 12SI alternator used in the mid ‘80s cars like the Camaro. You should be able to find remanufactured alternators for less than $100 that will offer 80 to 90 amps of output. These will require an alternator connector/adapter available from several companies like Powerhouse or Painless. The connection information is a bit complicated to go into here, but the information is available online.

If you do a little research first, you should be able find an integral regulator alternator that will bolt right on and, with output of around 80 or 90 amps, will give you the freedom to upgrade to perhaps electric fans, or an electric fuel pump and EFI and still have sufficient charging power to sustain these systems with sufficient voltage.

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  1. Vaughn MacKay says:

    I had Prestige Motors build a 450 hp/383 sbc motor for me, I’m putting it in my 56 Chevy 2 door; my question is what’s the best correct way to upgrade my wiring? I had a 283 w/327 heads that I built in high school in 1974 so it has a ballast resistor, voltage regulator, 3 wire alternator; the new motor has an HEI distributor, 110 1 wire alternator, ac that I’ll convert w Vintage air at a later date. I’m also installing a Griffin cross flow radiator with condenser and electric fans. Aeromotive fuel tank w electric fuel pump and regulator.

    85551- MSD Pro Billet Distributor
    6425- MSD 6AL Series Ignition box W/Rev Limiter
    8207- MSD Blaster SS, E core HP coil. 40,000 Volt

  2. Craig Lechowicz says:

    That is a good discussion about time and its effect on voltage drop. However, the alternator light typically comes on when battery voltage is higher than alternator output voltage. There is probably a bad diode in the rectifier bridge assembly of the alternator.

  3. Roger Pendergrass says:

    First thing to check would be the idle speed. It could simply be that the engine idle is set so low it creates a low voltage situation. Problem only occurs when he puts it in gear and rpm drops.

  4. 1985 1984 350 Econoline I have put a new alternator new regulator and still its not showing that its charging I’ve went through two alternators to voltage regulators and still the same thing what can I do please help

  5. Victor Belmares says:

    I have a 2004 Honda Accord I’m on my third alternator purchased at auto zone when I put the alternator in everything works perfect lights lights don’t dimm everything looks fine after 30 to 45 minutes the abs light comes on radio turns off then all gauges turn off while it’s running. When I turn off the car and try to turn it back on battery is dead I’ve had the battery tested it passes it’s brand new one month old

  6. Well, Jeff Smith talks a long story. In short your car is fine. The charge light will flicker at times and even when sitting at a light. And in all likelihood it will say this in the owner’s manual if you have one. Two things to keep in check is your belt’s condition/adjustment, and the engine’s idle speed. If both are OK then you’re fine. Case closed.

  7. Pingback: Identify and Troubleshoot Automotive Charging System Problems

  8. Dan Hogge says:

    I did a ls2 swap in my 54 Belair. The engine came from a 05 GTO
    the alternator that came with it soon died after I got if running
    It has a modest stereo system operating find on a 10 amp fuse.
    It has a 16 inch radiator cooling fan that operates on a 30 am fuse
    It will blow a 20 amp.
    It has vintage Air A/C and the normal stuff.
    It keeps blowing the diodes in the rectifier.
    I went thru 3 of the Alternators were supposed to be for the GTO.
    I the switch to a one wire GM 120 amp alternator.
    It worked fine for about 6 months
    Now it seems to have killed the diodes in it too.
    Or at least if has the same symptoms. Voltage drops as additional load is added until it decreases to 12 volts or down to 10.5 at idle and all accessories operating.

    Do I need to go with a 180 amp
    Oh yeah I forgot it’s got power windows ,locks and cruise Controll too.

    Dan Hogge

  9. Angel Tejera says:

    i have a 95 toyota tercel dx i left the lights on one day and my battery was drained, i recharged the battery and found that a bunch of fuses had blown including The 100amp alt fuse, since then i have replaced all the fuses but my car keeps going dead thought it was the alternator so i bought a new one and a new Battery as well, still after replacing everything, my car keeeps going dead. any solutions ?

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