Pretty sure it's the alternator, but to confirm...
 

Alternator light comes on with key turned, before engine on, engine starts fine, light is full bright red at idle (900 rpm), goes to about 1/3 glowing above 2000 rpm, stays at that level regardless of increased rpms. Drove home last night about an hour and a half, no problems - headlights, stereo on. It's glowed very dim for quite a while now - you could only see it at night, but yesterday was the first time it's been full red at idle.

So just to confirm, this can't be anything BUT the alternator, can it? The voltage regulator (external) couldn't cause this, could it?

Thanks.

Sure, your VR could be dying.

Could be high-resistance connection somewhere in the blue wire circuit.

Most likely a loose fan belt.

Easiest way to check is to measure voltage at the BATTERY with engine off and with engine running. Post your results for instant diagnosis!

I would think the voltage regulator plays a part in this. It controls the output of the alternator. It is part of the circuit for the indicator light.

+1 don't guess; pull out a meter!!!

Ok, will do later this evening. Thanks guys!

My experience is that a VR failure usually result in overcharging as opposed to under. Be sure to get voltage with the car off, starting, running at idle, and running at 2k.

Pull Alternator out (20mins) easy job. Take to alternator repair/rebuild shop have it tested. Then you have no doubts as to it's operation.

Ok, here are the #s:

off: 12.75
idle: 12.36
load (3k rpm): 14.77

#s look normal to me, no?

I vote loose or worn belt.

+1

Just checked the fan belt. It looks fine, and tension feels normal. I can understand a fan belt stretching with time and gradually slipping or somehow not spinning the alternator properly, but all of a sudden?

Either way, I'm pretty sure I've got a spare in the tool kit in the trunk, so I'll swap it out tomorrow and see if it makes a difference.

I vote that the resistor in parallel with the indicator bulb is open.

Gonna have to google that one! :)

With 14.7 at 3K? I don't understand your diagnosis.

So open, in fact, that it was never installed until after 1982!

Early cars use a 2W bulb. The later cars use a 4W. When converting to an internal regulator (which I don't recommend) you can use a 68 ohm resistor in parallel with the generator light bulb to increase the current flowing through the "blue wire" or D+/61 circuit.


Let's take it step by step.

12.75v with engine OFF-- GOOD! That indicates a fully charged battery.

12.36v with engine running at idle-- BAD! The ignition system and other ancillaries are pulling the battery voltage down. The alternator is not charging. The warning lamp should be glowing-- you have more voltage in the battery than is coming from the alternator trio, so current flows from the battery to the alternator, lighting the bulb.

3000 RPM, 14.77v. GOOD! (Sorta). What is happening is that the alternator has woken up (technical answer is it's operating above "zero-ampere RPM" and is now trying to recharge your battery by basically dumping all of the alternator output back into the field, a 100% duty cycle for the regulator. Things can get pretty hot when that happens, and hot is not good.

OK, so your first order of business is to

1) remove the battery negative strap so you don't electrocute yourself or burn the car down;

2) GENTLY, and I mean gently, remove your oil pressure gauge from the dash. It should wiggle out with finger pressure, if you must pry it out, use something plastic that won't scratch the dashboard or the gauge.

3) On the back of the gauge you will see the bulb holder for the warning lamp. This has one blue wire going to it and one red wire with black stripe.

4) GENTLY pull both connectors off. Don't lose them in the spaghetti behind the dash. Now take a piece of emery paper and clean all the oxidation off the male contacts. If you can get some electrical contact cleaner, the kind that you buy at Radio Shack that comes in a spray can, spray some inside the FEMALE terminals attached to the wires. These were originally shiny brass, now they have the dull patina of brown. GENTLY put both connectors back on. Before you replace the gauge, look around for any chafed wires - if any wires are showing strands through worn insulation (a common problem) STOP immediately and report back here.

5) Back on the electrical console in the engine compartment, find the voltage regulator, it's a big rectangular can. There should be a three-terminal plug stuck in the bottom of it. Now, is that plug connected to another rectangular box below it? That's a radio noise supressor. If the supressor is installed, you can remove it for testing-- take the plug off the supressor and plug it into the voltage regulator directly, it has the same pin orientation.

If no supressor, clean the voltage regulator contacts as above.

What we're trying to do is increase the level of excitation current to your alternator at low RPM by reducing resistance in the circuit. When corrosion builds up you get a high resistance.

What is PROBABLY your problem is that the alternator brushes are worn, but try fixing your blue wire circuit before you pull the alternator.

Good luck!

(for further reading search for "Early_S_Man" and "zero-ampere rpm.")

I finally got a chance to get out to the garage and clean up the contacts as suggested. They were actually fairly clean. I remember cleaning the voltage regulator contacts when I upgraded all my ignition system last summer or the summer before, so that was no surprise, but I was surprised how clean the contacts on the bulb in the oil/pressure guage were. I cleaned off the positive battery post, too, and made sure the cable was on nice and tight when I replaced it. Still the same thing - full light at idle, drops to maybe 1/3 when I hit 2000 rpm. :(

Bad brushes in the alternator, time to pull it!

If you want to try installing a big ceramic 1 watt 68 ohm resistor in parallel with the bulb contacts that may help but if the brushes are bad it's probably more trouble than its worth.

I'd rather fix it properly, no problem to pull it, and from what I've read here, it might be a DIY repair, if I can find the parts. Am I stupid to keep driving it, at least in the short term? Probably won't be able to get to it until next weekend.

Thanks for your help!

I don't see that you'll hurt anything driving it. Never pull the leads off the battery with the engine running, you'll fry the alternator instantly, unlike American and Jap cars.

This actually happened to me. Negative lead came off while driving. Fried voltage regulator.