Hi everyone,
I've been working on my brother's 73 Scamp and we've been having a charging problem. We've replaced the alternator and VR, checked for good grounds for the battery and the VR, and we still can't get the battery to charge. The car's ammeter always shows a constant discharge, and when I connected a voltmeter between the alternator's B+ terminal and a ground, I read ~40 volts ( with the VR in place ), which isn't good for a 12 volt system. Any ideas or troubleshooting suggestions, and does anyone have a wiring diagram of just the charging system and how it relates to the ammeter?
Thanks

Something doesn't make sense - if you have 40v your overcharging the battery not discharging.

You sure the VR has a good ground? Make sure you scrape the paint off the back of it and you can try running a seperate ground wire between its body and negative battery terminal. If there is a short somewhere, or your voltage regulator isn't getting a good ground, you can see higher voltages then usual.

And just to make sure its not your volt meter on the fritz - try measuring a known working battery.

Be really careful here though - if you subject a car battery to 40v continiously it will go boom soon enough.

It seems that I am having several problems. One, the VR isn't regulating anything, and two, the 40 volts from the alternator isn't getting to the battery at all. I've used several meters, and at the battery I find no charging, while from the alternator I'm recieving 40 volts, which means a wire has fried somewhere in between. Is there a fusible link somewhere that I'm missing that would affect the VR?
I'm going to try swapping the alternator and VR from my Dart into the Scamp and see what happens.

So voltage at alternator measures 40 but at battery reads something nominal like 12? Check the ammeter itself under the dash and make sure the connections are tight and not corroded - you can try bypassing it all together by hooking the wires on it together and insulating them. The wire starts at the alternator, branches off to all the circuits, the ammeter, then ends up at the battery. Are all your other accessories, lights, horn, etc working? Check connections on starter relay and engine ground as well.

The only fusible link is under the hood just at the bulkhead connectors. If this was bad though you'd have other problems not just charging. There are no other fuses or protections on the charging circuit.

I think your battery charging and 40v @ alternator problems maybe 2 different things.

If you have an open or nearly open connection between the alt's BAT and the rest of the car, you'd get those symptoms. W/o a load, the alt can go up to ~50V. That much voltage can do a lot of damage; I'd check the alt after you fix everything else for burned diodes.

The regulator thinks the system voltage is too low, so it turns the alt on full tilt, and there's noplace for the current to go due to the open, so the voltage at the alt goes very high.

W/ the car off, hook a test light (or better, a headlight) to the BAT term at the alt - it probably will be very dim or not light at all. You should have less than 1V between the alt BAT and the battery -less is better - at around 5-10A (~0.1ohm).

Good places to look for the open are the little white cylindrical connector in the line to the alt; at the bulkhead at the firewall; and at the ammeter. Sometimes a joint or wire can fail inside the harness too - rarer, but it happens.

Such a dramatic drop should be easy to find with just a test light. With the car off and a headlight at the alt BAT terminal, a testlight will quickly spot the break.

Here's another interesting symptom, when I unplug the VR, I suddenly get no voltage reading from the alternator, which makes absolutely no sense. I'll head outside and do some poking around with a test light and see what I can find.

The '73 system works by the regulator sinking (~4A) current from one FLD to ground; the other FLD is connected to +12V whenever the ignition is ON. The alt's BAT terminal should always read battery voltage, even if the ignition is OFF. The regulator decides how much current to sink based on the voltage going to it from the ignition feed.

If you unplug the regulator you should measure nearly full voltage at both FLD connections and the BAT termnal, the system shouldn't charge at all, but you should still have full voltage at the alt's BAT terminal.

It's pretty clear from your symptoms that the problem is an open between the BAT connection as just about everything else- power is not getting anywhere from the alt. The 2 most likely spots are that little white connector and right at the bulkhead. Unplug the bulkhead connector and take a good look at the pins with the heavier wires running to them - one is quite likely to be partially melted or badly corroded.

W/ the car off, use your test light to check the 2 heavy wires at the bulkhead - both should have full voltage.

If doubt you ran 40V to the battery; at idle there isn't enough power available to do that. A typical 12V battery will take a lot of current at 40V and start getting very hot and boiling and will quickly fail (or explode) if you really give it 40V, so I really think you'll find an open between the alt and the ammeter.

Sure it was 40 VOLTS and not 40 AMPS?? The highest I've ever seen out of one of those alternators was 16.something, with the VR full fielded. (temporarily bypassed for test) Disconnect the VR connector and ground the "off to the side" terminal; DON'T accidentally ground the terminal in the center; its "hot"; dead short!

volaredon - full fielded but still connected to battery. Full fielded but not connected to battery.....

You really don't wanna run with the batt cable disconnected even the old
"disconnect the battery cable to see if it dies" test for a few secs, you can
create a spike that can ruin what was a good alternator.

Thanks for the help everyone, after poking around I found a bad connection at an electrical block near the rear of the valve cover. Fixed the connection and everything works great now. I'm also sure it was ~40 volts, when I switched the alternators and VR's I was getting nearly 50 volts with my set. Thanks again, now to fix the broken /6 in my dart...