Voltage rising then dropping? (67 Barracuda)

[bloodyholly]

Good evening...

I've been having ongoing issues with my electrical system in my 1967 Plymouth Barracuda.

Recently there has been a ton of symptoms but I believe it has to do with the alternator.

If the headlights are on, the lights will start dim, getting brighter, brighter and brighter only for them to drop seconds later. The alternator is also making weird whirring noises and even the heater (if I turn it on) spins faster and then drops. It all coincides with the sounds coming from the alternator. At night, the dashboard lights also get brighter and then eventually drop again, too.

We changed the alternator already, along with the voltage regulator AND the battery. We scraped paint of the firewall to have a better ground, too and originally we got the car working again. But now it's acting up again.

The car does run... here's a video. Please check it out and listen closely. The first 4 seconds are a prime example.

If you have any insight, please share it with me.

 

[67Dart273]

Check for voltage drop first in the harness and then the ground circuit. To do this

With key in "run" but the engine stopped. stab one probe of your meter directly into the top of the POS battery post. Stab the other into the IGN terminal of the voltage regulator, or if you can't get into that, the "key" side of the ignition ballast resistor. You are looking for a small reading, the lower the better. Anything over .3V or so (3/10 of one volt) you need to look for the cause. The bulkhead connector, the big red and big black ammeter wires, the "run" wire where it comes through the bulkhead, connections at the ammeter and the ammeter itself, and the ignition switch terminals and the switch itself.

Next run the engine at high idle to simulate "low to medium" cruise RPM. Do this test first with all acessories off, and again, with lights, heater, etc powered on

Stab one probe of your meter directly into the battery NEG post. Stab the other into the mounting flange of the VR. Stab hard, to get through paint, rust, chrome, etc

Once again, you are looking for a low reading, lower the better, and zero is perfect.

If you need or want to improve the ground from battery to body, look at the pass side front of the head. Those holes, unused, are also on the driver's side, REAR of the head. You can attach a Ford style "starter cable " (eyelet to eyelet) to the head, and bolt it to the firewall, maybe the master cylinder mount studs

 

[413]

Scraping the paint off the firewall does nothing. Don't do it. It will rust there over time and cause you more trouble.

The automakers don't scrape the paint off the firewall of a new car, and they build millions of cars this way and warranty them for years. WHY and HOW does this work? Because the ground is through the fastener threads and head of the fastener to the part. So make sure it is snug in the sheetmetal and not stripped. Now use a star washer under the head of the fastener. Done, now you have a good ground.

 

[413]

Do you have stock parts? Like a mechanical voltage regulator? Single field alternator? Then that is normal to the system at idle.

You check a charging system at 1200 rpm. Raise the engine speed to 1200 rpm. How are the lights and voltage now?
Steady = Normal
Not steady = a problem. Like high resistance in the wiring or a bad component.

That noise is not right and if coming fron the alternator then go exchange it another one.

This condition of the lights going dim and bright and the fan changing speeds is normal for this system at low rpm. New cars did this and customers didn't like it, and there is no fix with stock parts. It is what made Chrysler corp change to the dual field charging system for 1970.

 

[67Dart273]

"Normal." No, it's not. It's a feedback loop in the wiring/ connections causing a cycling event. If you are using an old electro-mechanicall regulator, and it is not in the harness, then it's the VR out of adjustment, or needs replaced.