I've noticed when I drive my '79 Catalina at night that the headlights brighten when I'm pushing on the gas and they dim down when I'm not moving, foot off the gas. In a modern car, I would suspect the voltage regulator in the alternator is going, since the battery was brand new and manufactured in April of this year. Before I go replacing the alternator, I figured I'd ask if there might be another reason a car with no computer would do this. The new belt is definitely not slipping, and the battery reads 12.7V, which is fully charged.
Likely the voltage regulator but it's also worth checking grounds, particularly the ground cable between the engine block and the frame.
I'll give that a look. Girlfriend's dad blew up a battery under the hood, which ended up with the car sitting as long as it did. The sulfuric acid corroded the horn cable connections and was part of the reason they didn't work. So that could also be this problem.
Worth checking, and you could also throw a voltmeter on the battery and see what the voltage is when you rev it. Voltage regulators definitely do go bad, had one go funny in the head on me several years ago and it was putting out nearly 20 volts! I think that car would have a Delco 10-SI alternator with built-in regulator, no problem getting a replacement if needed unlike the Motorola alternators on my old AMCs.
Worth checking, and you could also throw a voltmeter on the battery and see what the voltage is when you rev it. Voltage regulators definitely do go bad, had one go funny in the head on me several years ago and it was putting out nearly 20 volts! I think that car would have a Delco 10-SI alternator with built-in regulator, no problem getting a replacement if needed unlike the Motorola alternators on my old AMCs.
What AMCs do you have? I've always liked the old Javelins, and the Pacer, as hated as they were back then. Haha.
Well currently there's the '71 Ambassador I've mentioned before, a '67 Rambler Rebel convertible, and a '65 Marlin. (All those are V8s with automatic transmission - 360, 343, and 327 cid respectively.) These are not show cars, they were neglected unwanted vehicles in the 1990s that I picked up cheap and have kept mechanically in good shape. I don't care if they look a bit shabby. 😉
Honorable mention to my '99 Jeep Cherokee which although built by Chrysler was originally an AMC design. (The Jeep 4.0 is basically an updated Rambler six cylinder engine.)
Sounds like the voltage regulator is failing
That's what I was thinking as well. But given the simplicity of these old things, I figured I'd ask people with more experience, lol. The alternator isn't all that old, but it sat for several years, which probably wasn't great for it.
I took a minute after I got home from work and checked my voltages at the battery with my multimeter. The first thing I noticed is I don't get a good voltage reading by touching the multimeter to the screws on the battery cables. I need to go to the ground on the block to get a voltage read while keeping the positive lead on the positive terminal screw. With the engine off, it read 12.69V, pretty much fully charged.
With the engine running, the voltage jumps around from 12.3 down to 7 or 8. I also can't get a good voltage measurement only at the battery when it's running. I was not able to revv the engine and get the reading at the same time. I noticed the brightening and dimming is hit or miss, and the motor seems to idle a little poorly when it happens. The battery light wasn't on. Sometimes it comes on right after a cold start and goes out with a revv. I did notice it came on for a second today after it was hot. A quick revv in neutral made it go out.
Armed with that info, does it sound like a voltage regulator or something else?
In that case, Chuck may be right. Replace the engine block ground strap.