I don't really understand alternators

[G B.]

There, I've said it. Now I hope someone will help me.

The symptoms: amp gauge on '63 shows 5 to 10 amp discharge when headlights are on at idle. Otherwise, charging system seems to be working fine. If I speed up the engine idle from 650 to 800 rpm, the alternator puts out enough current to show a slight charge rate.

I've tried two rebuilt alternators (one a Delco 60 amp) and two voltage regulators (one a new Delco). The higher amp alternator showed a little less discharge at idle than the other. The regulators made no difference. I've checked the resistance of the wiring harness (zero), cleaned all the wiring terminals, replaced the headlight bulbs, and put in a new headlight switch. None of these changes improved the situation. I even hooked up an NOS ammeter to make certain the dash gauge was accurate. In frustration I finally put on a smaller alternator pulley. This gave me 50% more alternator speed at the same engine idle rpm. While this does keep the alternator putting out enough current to overcome the headlight draw at idle, I know it's just a band-aid fix.

What's the deal? Are alternators really struggling to put out significant current at low speeds? At what rpm are they rated? Is low charging at slow speeds a sign of a particular alternator problem like one bad diode?

[Ed Jennings]

Re: I don't really understand alternators

Jerry, I can't help much but can tell you that my 67 does not have any discharge at idle with the lights on. It is an AC car, however, so I'm ideling a little faster. If there is nothing amiss with the current draw, have you checked the idle speed with an electric tach to see if the idle speed is really what the dash gauge shows? I know someone with your experience can probably tell by listening if it were off much, but it might be a little lower than you are seeing on the dash tach.

[Gerry Proctor]

Re: Me either!

But I know enough about alternators and generators to see you're chasing a non-problem. Dynamo's don't put out sufficient energy until you start spinning them a little faster than idle speed. I can't say this for certain, but I believe they are poor power producers under 2,000 rpm shaft-speed. I don't know what the excited amp/rpm rating are, but your system seems to be O.K.

[Gerry Proctor]

P.S.

You can't adequately address the surity of an electrical system by measuring the ohms. You need to do a voltage drop test with your VOM. A resistance of as little as .02 ohm can cause as much as a .4-volt drop in a headlight circuit. Use the resistance scale only for checking continuity in the circuit.

[andy#34338]

Re: I don't really understand alternators

Jerry,

It sounds like you may have some resistance in your headlight wireing harness. I did notice very small improvement when I changed that 1/2 of the front wireing harness (front harness is in two parts). My issues was the alternator.

Some of the regulators are adjustable. I remember posts on that issue. You may want to search the archives. I adjusted my regulator by taking the cover off and turning an adjusting screw(it had a spring around it.

I do not think you will run into any problems keeping your car the way it is.

Hope this helps.

Andy

[Dave Henderson]

Re: I don't really understand alternators

I have the same problem when I depress the brake pedal, a 10 amp discharge. Naturally, the electrical starvation screws with the TI ignition.

I have a wire in the brake/tail light circuit which I believe is shorting somewhere on the frame, as this problem only happens with the headlights on when the brake pedal is depressed.

My spring project is to trace the wires from the brake lights through the floorpan to the fuse box and try to locate the short and insulate it.

This is the only condition in which I show a discharge on the ammeter. Running a stock TI ignition setup with rebuilt orig alt and VR, as well as orig Amp box and coil. Good luck

[Robert C.]

Re: I don't really understand alternators

Jerry, I thought all alternators worked that way. You have to have a higher idle speed to produce a + range. Thats why you should give it a little gas if you are jump starting another car etc.

[Duke W.]

Re: I don't really understand alternators

Alternators have a steeper current-speed curve than generators and GM sized the alternator and output characteristics so as to maintain system voltage with a nearly full load at idle as long as the battery is not severely discharged.

The ammeter works on the basis of very small voltage difference at a couple of points in the charging circuit, so any additional resistance such as connector corrosion can upset the readings.

The first thing you should do is check the actual battery voltage with the engine off and then with the engine idling.
A fully charged battery should read about 12.5 volts (engine off). Idle voltage at the battery (system voltage) should be between 13.5 and 14.5 volts. If not, the alternator output is suspect, the regulator could be out of adjustment, or the battery could have a shorted cell.

My suspicion is that the alternator is fine, but the ammeter is biased due to some excess resistance somewhere in the circuit. Cleaning the harness connectors on the cowl has helped many with electrical gliches as have cleaning grounds. It's not a bad idea on a 30 plus year old car to systematically inspect all the connectors and grounds for corrosion.

Assuming you have an OEM equivalent alternator and no extra power drawing accessories, the alternator should maintain adequate system voltage, even at idle. You appear to have fallen into the "re-engineering trap" by changing the alternator pulley , etc. rather than trouble shooting to find the actual cause of the "fault" and correcting it to bring the system back to OEM performance. This happens a lot with cooling systems i.e. low temperature thermostats, and "high flow" water pumps, when the problem is a clogged 30 year old radiator that no longer delivers design heat transfer performance.

The trouble with this approach is that few, if any, of us are really qualified to re-engineer the various systems. so we just meander around in the dark swapping parts, until we get lucky, or sit down and learn how to troubleshoot and find the real problem, which in your case may be nothing more that high resistance in the ammeter circuit, which is leading to false readings.

Take system voltage readings (across the battery) at idle, 1000, 1500, and 2000 and lets go from there. Also, make sure the battery measures about 12.5 volts and will hold this reading for several days without dropping.

BTW, Delco alternators are rated at 5000 rotor RPM. This is the "rated output" of the alternator as stamped on the case. Typical pulley ratios are about 2.5:1 on medium performanece engines and 2:1 on SHP engines, so full alternator output is achieved at about 2000 to 2500 engine RPM. Alternators will have higher output at higher revs, but the slope of the curve is very modest beyond the rated output speed.

Duke

[G B.]

Here's what I'll do next

Thanks for all the immediate (and intelligent) responses. I didn't expect this topic to be of such interest. I haven't noticed an idle discharge to be universal in mid-years, so I'm going to continue to assume that this particular car has something wrong.

I cross-checked the mechanical tach against an electronic tach last week. The idle speeds in my first post were already corrected for the 100 rpm error I had found. I have cleaned the firewall wiring harness connections thoroughly with a Dremel tool. I disconnected the battery while the car was running and showing a discharge on the ammeter. The engine died, confirming that it really was running off the battery and not the alternator at that time. This makes me think the gauge is reading accurately. The battery is a new Delco Professional (7-year).

Next I'll try chasing down the ammeter wiring for more terminals to clean. I'll also use the voltmeter to check the front wiring harness for voltage drop and the battery for voltage while running.

I wish I could fix this by working on the wiring. But frankly, I'm skeptical. I also wish I had longer legs. I guess life is full of disappointments.

[Bernard M.]

Re: I don't really understand alternators

Jerry, I can't claim to be an expert on alternators either, but there does seem to be a prevailing misconception about electrical circuits in many of the above posts. Decreasing the resistance in a circuit (given a constant voltage source) will INCREASE current, not reduce it. While I agree it's a good idea to clean connectors to reduce unwanted resistance in a circuit, everything else being equal, that will increase current draw. A short circuit is zero resistance and infinate current, an open circuit is infinate resistance and zero current. I would suggest load testing your battery, if it had an internal short it could (by reducing resistance) cause excessive current draw, when added to your headlamps, your alternator might not be able to keep up. I'm assuming you're using standard headlamps, not some high current halogen bulbs. Bernie Myers

[G B.]

You raise good points

I replaced the headlight bulbs although they looked like normal low beams. Both sets pulled about 10 amps with the engine off. The discharge at idle speed was unaffected by the bulb swap.

I'll have the Delco dealer load test the new battery.

I'll also replace the alternator once again. This time I'll use a known good unit from another mid-year. I know the likelihood of multiple bad alternators is slim, but I think I'm dealing with something out of the ordinary here. Maybe.