'78 alternator question

[FastWoman]

Hi all,

My alternator stopped working yesterday. When I wiggled the L/S connector, it started working again. I had assumed it was a corroded contact at that connector. Then I started thinking about it. The L is simply the alt error light signal, which did correctly cause my alternator and brake lights to glow (no bad connection there). The other is the sensing line, which provides voltage feedback to the internal regulator. If that were momentarily disconnected, wouldn't the sensed voltage be zero, and therefore the output of the alternator at its maximum? At the time I was incorrectly thinking of the S as a field wire and thinking, "Ah hah! No field, no output!" I would test this hypothesis by running the alternator with the L/S plug unplugged, but I really don't want to risk over-volting my system.

I'm thinking there's a bad connection on the voltage regulator board and that my wiggling the plug momentarily settled that internal connection. Any thoughts?

In summary, what happens with the S wire doesn't make contact? Is it maximum alternator output, or is it no output at all?

[FastWoman]

Er... That should be "what happens WHEN the S wire doesn't make contact?"

[Zed Head]

Seems like you would lose voltage regulation without the S wire connected, and get maximum output.

You could connect a rheostat to the S wire and vary the voltage it allows through while watching a voltmeter. That would avoid damaging spikes and just look like lots of load to the regulator circuits.

When my OReilly auto parts rebuilt alternator was going bad, the charge lamp started to glow very dimly, even though the voltmeter showed normal voltage output. Eventually, the charge light brightness increased with RPM, like some sort of "RPM by brightness meter", while the voltmeter needle sat steady at ~14 volts, which made no sense at all, by typical "how things work" knowledge. While I was being fascinated by the weird behavior I think it destroyed my ignition module.

[FastWoman]

Thanks, Zed!

Well, it finally occurred to me to simply disconnect the alternator from the battery (i.e. pull the maxifuse on that circuit) and then unplug the L/S wire to see what the running alternator would do. (Duh!) Much to my surprise, it put out NOTHING. Then when I plugged back in the L/S plug, it put out about 15V. So maybe my S connection wasn't making contact after all?

I guess I won't wander too far from home and will just see how it goes. Do you have any idea how far I can drive on a fully charged battery to get home, should my alternator fail? I'm glad I get the brake light when the alternator goes down, because that's hard to miss (being bright and being on the spedometer).

[Zed Head]

Maybe when you removed the L wire you lost the excitation current through the windings and it quit charging. I've read that some of the cores used in various alternators have enough residual magnetism to work without the L wire's excitation current, but some don't. Guess you'd have to run without each, S and L, separately to know for sure. Two variables.

Just though of another possibility for odd behavior. If the L wire shorted to ground, instead of through the regulator circuit, the light would come on. Did the voltmeter stick at 12.6 while the light was on the first time?

No idea on the battery. Depends on it's capacity, I believe.

Must be fun to have a new problem to work on.

[Walter Moore]

On nearly all cars that have an alternator light on the dash the "L" wire is the initial excitation source. Once the alternator is running and producing power the alternator's output voltage is higher than the signal from the idiot light and the light doesn't glow. (There is a blocking diode in the alternator for this purpose.)

I don't know how far a fuel injected Z will go running only on the battery, but other cars that I have owned left me stranded after about 10 miles.

(In fairness the battery may not have been fully charged.)

[FastWoman]

Interesting! Well, the light came on while I was driving, so the alternator was already excited. I take it, then, that if the L/S plug had come undone in with the alternator in its excited state that the alternator output would have gone to its max value, instead of zero. I'll try replicating that tomorrow (with the output disconnected from the battery/system, of course). If that's the case, then I indeed have something else going on -- probably a bad voltage regulator board. Fortunately my local AutoZone has enough confidence in my abilities to take my word when I tell them the alternator has an intermittent fault. Gotta love small towns.

[FastWoman]

Yup! It turns out the L wire gets the alternator excited, and then when I unplug the L/S connector, the alternator still puts out. That means when I wiggled the connector, what really made the alternator start working again was my wiggling some internal part. The alternator went back today, and I'll have a replacement Tues.

On a side note, I really seem to be going through alternators, each time for a different reason. Just before I bought the car, the PO had his Nissan dealer install a new/rebuilt alternator. I don't know whose it was, but I assume it would have been a quality rebuild. The alternator lasted a few months before the voltage regulator started regulating over 16V. So I got an AutoZone lifetime warranted alternator (yes, I know, cheap stuff) that died a month later. It was rebuilt in Mexico. The next replacement was rebuilt in the US and lasted 2 years before this intermittent problem surfaced. Maybe the third time is the charm?

[Zed Head]

So back in Post #4, maybe the removal of the plug wiggled it to the "bad" no output state? Just trying to get the full picture, it sounds like your second test gave different results.

I've come to the conclusion that old used Nissan alternators are better and will last longer than "new" auto store rebuilt alternators. Just an opinion. My old used Nissan factory alternator puts out more voltage at idle than the rebuilt auto store alternator I had on there for a while, that died after nine months (~10,000 miles).

The literature in the rebuilt alt's box said something along the lines of "all parts are tested and those that fail are replaced." In other words, the old parts are re-used if they pass the function test. In even more other words, parts that could be on the verge of failure are re-used. I think that's why so many rebuilt alternators fail quickly.

[FastWoman]

Zed, I think the key fact in the diagnosis was that I was driving with an excited alternator that suddenly (without my having stopped the engine) lost output. So even if the plug had come loose, that wouldn't have accounted for the problem.

I suspect you're right about old/used vs. rebuilt. However, I do know that part of the rebuild, at least on these alternators, is a complete replacement of the old voltage regulator and diode board with new -- probably made in China or Mexico. (That's probably where the problem is.) I wonder whether I'd fare better with an external voltage regulator style of alternator.

[FastWoman]

Grrrrrrr.... I think in its last hurrah, the failing alternator must have overvolted my system. My power amplifier and power antenna both seem to be kapoot -- pending further diagnosis, that is.

Edited November 28, 2011 by FastWoman