I have a 1966 F-100 custom cab with a 390 FE that a previous owner installed w/ c6 auto trans.

My original ALT Gauge shows a range from -45 to +45 amps (I think) with "0" in the middle.

Do the following readings indicate that my charging system has a problem, or seems normal (assuming the meter is wired correctly - which it may not be):

• When idling in "park" (higher idle than drive): Needle at approx. 0
• When idling in "drive" (rpms drop) needle at approx. -30
• when starting from a stop needle goes up to about +30 then levels out at approx. 0
• needle at approx 0 while driving most of the time

What concerns me is how the needle drops to -30 when idling in drive. Is that normal? It seems like there is a drain on electric: with headlights, turn signal, and brake lamp on at a red light tonight it seemed like the turn signal slowed down and lights dimmed. Once the light turned green I made it home no problem and that was an hour drive with about 6 red lights. ALT meter dropped to -30 or so at every stop while in drive. Putting it in park levels it out to 0 . . .

This is not my daily driver . . .

 

That sounds like an ammeter. They pretty much should stay at 0 mark all the time. You'll want to measure your voltage at the battery. At idle with no lights or accessories on want to see approx. 13.8 to 14.3 volts. Colder temps a little higher, hot temps a little lower.

One tool that takes some of the guess and by golly out while driving, plugs into cigar lighter and about ten bucks:

 

The fact the ammeter shows a charge (+30) right after starting the rig and normally runs near 0 says that the charging system is working correctly.

The discharge (-30) showing at a slow idle, with the loads you describe, seems a bit high, however some discharge could be normal depending on the condition of the electrical system.

The slowing of the turn signals and the dimming of the lights would indicate a low voltage condition, possibly a weak battery, an alternator that isn't up to snuff, or both.

As Tedster9 said, check your battery voltage. A load check is even better, it will show the condition of your battery.

Factory gauges are not known for their accuracy.

Good luck.

 

Never had an ammeter in a truck, but in one sense they may be more accurate than many OEM voltmeters, though they don't tell much. Thing is you can drive a long way on just battery power so long as no headlights are used or other high current draws.

The fact that you made it home suggests the alternator hasn't failed but the fact that everything went dim at times is .. Wait. You sure you have an alternator, and not a generator, lol? In any case the alternator is supposed to carry 100% of the electrical load. The battery is basically just for starting the engine.

Sounds a little like an alternator going south, but if the idle RPM is on the low side and the lights and accessories (heater) are on, an old school alternator would be taxed pretty good. The lights normally shouldn't dim with an alternator though. Check resting battery voltage today, clean/replace cables. Charging systems don't like resistance in circuit. Alternator grounds to engine via brackets and hardware etc.

One quick test is measure voltage at battery at a high idle ~2000 RPM, connect a heavy jumper cable from negative battery post to clean unpainted portion of alternator case.

 

An ammeter is nothing more than a electricity flow meter, showing the rate of flow in amps. In the case of our trucks the ammeter is showing the flow of electricity in and out of the battery. A (+) flow says electricity is going into the battery and a (-) flow says electricity is flowing out of the battery.

While totally different in design and function, in some respects alternators are somewhat similar to generators. They generate electricity, they keep the battery charged and they don't produce full output at an idle. An alternator however does produce more power at an idle than a generator, that is why the manufacturers changed to them.

As the electrical demands of cars and trucks got greater, the need for higher output alternators followed right along. Look at the 3G alternators of today, 200+ amps, a far cry from the 45 amp unit that came on our trucks.

Your alternator may not be charging to capacity, but since it is showing a +30 amp charge after starting your truck, I don't think it is the main culprit.

As I said earlier, an alternator doesn't produce full power at an idle. With heavy electrical demands, especially at idle, such as headlights, brake lights, heater, etc., the battery will have to carry part of the electrical load. If your battery is weak, this condition can cause low voltage. This is more obvious with an older, tired battery than it is with one in top shape.

If you glance at your ammeter as soon as you leave the stop light I bet you will see it slightly to the (+) side for a moment or two.

 

So, I have not had a chance to test under load, but that 1 hour trip (each way) was last night. The car sat all night, and all day today and I just took a quick reading of the battery with a voltmeter with the engine off . . . 12.77 volts. Since the battery is fully charged after 2 hours of driving I am thinking that it's OK. I'll test under load to see what I get this weekend. I am guessing that the ammeter is just kind of worthless. Thanks for the replies.

 

X2 as Roger
Sounds like your system is working as it should. I would still run a load test on the battery just to see how good it is.

 

Don't think lights should dim at idle though, what size battery is in there now, how old is it, etc.

Old trucks always have corroded cables and ground points and big voltage drops, restored trucks often have thick coats of paint leading to same. Clean, tight, low resistance electrical connections are key to happy electrons.

 

The battery is old, but it tested very strong when I brought it into carquest this summer. Just hooked a voltmeter up to it and it read 12.77 volts at the terminal and on the cables coming off the terminals. I think you might be onto something with your mention of connections. Some that I've seen here and there were so beat up that I snipped them off and replaced the crimped on connectors. I would not be surprised if there were some issues like that.

The car was never restored properly and it sat for a long time before I bought it. Been making upgrades and repairs all summer. There is a long list of items I want to address so I'll keep an eye out for any more questionable electrical connections. I'll also make sure ground straps are not hindered by paint.

Thanks again.