Funny Alt light behavior

[tevake]

I seem to have a charging issue in my 1980 SC. The alt. light glows dim at idle, then gets brighter as the revs pick up. am getting a low charge level also. 12.5volts coming to the batt. Does this light characteristic lead to any particular likely source of the problem?
I have been looking at past postings of this charging problems and haven't come across this warning light behavior. Any ideas?
Thanks Richard

[Jdub]

First, are you 100% sure of the condition and state of charge of your battery, and are both terminals and most especially the battery ground in very good shape? If you have ANY questions about your battery replace with the standard Group 44 battery prior to spending additional funds. In your part of the country (jealous!!) batteries fry on a very, very regular basis.

Now, it could be the alternator, so consider you may need to have that rebuilt or swap it out as core. By '80 there was no separate regulator so you will need to pull it from atop the engine. If you are good with your hands you can do this yourself. In doing so, get yourself a fresh belt for installation and mind the shims (count them) in the pulley assembly. Get a vacuum and brush to remove sediment and organics on the engine - good time to do this as well.

[Joe Bob]

What's the battery voltage at rest? Sounds like you may be having a regulator, alt meltdown or a bad battery. Are the gauges, specifically the tach, bouncing? Are you using an indash volt meter or a multimeter?

Does the voltage dive when the lights are on or dip and recover with blinkers or hazards? Have you fully charged the battery and done a poorman's load check?

Meter on the battery, start car......if it dips below 10v.....suspect the battery.

[tevake]

Good input guys thanks. This is the targa I posted earlier about that was in storage for ten years. So I am thinking the brushes may be stuck in the alt. Been using a multi meter at the batt. Batt. was replaced last year, so should be good. Am just getting the car back on the road. Been out for a few trial drives. So not a lot of background info.
Will check the terminals and ground points. Have a new Voltage Reg. on the way.
I will report back.
Mahalo Richard

[flatsixjunky]

Test:

1) battery engine off
2) battery engine idle
3) battery engine 2000 rpm

Post your results.

[Joe Bob]

Originally Posted by flatsixjunky

Test:

1) battery engine off
2) battery engine idle
3) battery engine 2000 rpm

Post your results.

You should see.....if not....alternator/reg

1) battery engine off - 12.5-8
2) battery engine idle - 13.0+
3) battery engine 2000 rpm - 13.5-14

[tevake]

I have had the batt. on the charger since early this morning. Once its done, I'll do the checks you suggest. I'll give it some time to settle after charging. Wasn't getting an increase of charge with more revs.

[MBAtarga]

My bet is regulator. BTDT - same symptoms.

[ClickClickBoom]

Alternator brushes.

[wwest]

The alternator/regulator circuit uses a 2 diode "OR" gate. Unlike a generator that permanently retains magnetism the alternator rotor will not. So when you start the engine the regulator must have a source of "trickle" current to magnetize the rotor. That source is the filament of the Alt. indicator bulb, sometimes with a parallel resistor, flowing through one of those two diodes through the regulator to the rotor.

Which is why the bulb glows BRIGHTLY with the engine not started.

Once you start the engine the 2nd diode ("internal" to the stator windings) should start supplying at least a portion of the current to energize, magnetize, the rotor. As the engine revs higher and higher the voltage level from the stator via the diode should become high enough to reverse bias the bulb diode and it should dim, not glow brighter.

You may have something as simple as a shorted diode, the one "to" the bulb, however unusual that may be.

If you start the "1,2,3" test sequence with a fully charged battery you will want to add heavy electrical loads, lights, etc, in order to see the battery terminal voltage rise with engine RPM.

And remember that even with one set/phase (of three) of rectifying diodes shorted many alternators will still supply enough charge unless the engine is left idling for long periods with a fairly high electrical load.

[tevake]

My parts have come in and I've had some time to check things out. Checked and cleaned ground points at trany, battery, and in the engine area. all seem good.
Did the 1 2 3 tests with the old regulator. Using multimeter at batt. and at the engine. I just get batt voltage what ever the revs.
Then changed out the voltage reg. installing the new one. this car has an external regulator. and repeated the 1 2 3 test. 1 batt. voltage 12.6, then at idle 13 volts, then at 2K revs 15volts. then at higher revs it went to 16.9 volts.
So I am still not driving the car, out of concern of burning out the CDI.
But it is clear now that the Alt. is charging.
I am coming to the conclusion that the new reg. is faulty. what else could it be?
Also the light in the dash is staying on and getting brighter with more revs, with the new V R installed.
No bouncing on the tach.

[Joe Bob]

16.9 is high....but could be a symptom of a low battery. Was the battery charged by an outside source?

Alternators are designed to maintain batteries, not charge them.....

[wwest]

You have a shorted diode in one phase of the alternator.

[tevake]

The Batt. was at full charge.
There is a quite intermitent buzzing sound in / under the dash.
How does the Diode help to manage the charging rate?
I thought that is the job of the Voltage reg.? Any tests to check the diodes with the Alt in the engine?
Thanks Richard

[tevake]

I was just read the My Tach is jumping thread. And thought I should add that I just had starter problems and found that the cable that goes from the solenoid to the starter was rotten, and did a replacement of that cable. Wondering now how that may fit into the other electrical issues?

[wwest]

The amount of DC, Direct Current, flowing through the Alternator's rotor windings, in conjunction with rotational velocity, determine the alternator's output voltage. Before you start the engine the ONLY source of that current is though the indicator bulb. Once the alternator is turning fast enough to supply more voltage, even slightly more, than the battery terminal voltage, the diode supplying the bulb current is reverse biased and the bulb extinguishes.

Your bulb not only doesn't extinguish, it glows brighter with rising engine RPM.

[CorsePerVita]

Tonight after putting in my fan and shroud and hooking the alt back up I am getting my battery light intermittently.

What is really odd is that i'll have nothing wrong at startup, the battery light will remain off. After almost exactly 1 or 2 minutes of driving, it turns on dimly. I checked my connection at the battery itself and the terminals.

I am headed out with a multimeter as we speak to go check and test my battery. I'm guessing something happened in the alt when putting it back on, or, something is loose, or, I have dirty terminals.

[CorsePerVita]

11.98 off
11.97 idle
11.94 2k

Seems my alternator is not making a connection, I must have something loose. Going to pull the shroud and check it out.

[Joe Bob]

Originally Posted by CorsePerVita

T, the battery light will remain off..

It remains off prior to start but with ignition on? Not good......

It goes off AFTER starting....good.


Coming on intermittently, not good....I agree, something loose.

[CorsePerVita]

Alright figured it out. Key on it was OFF. After running a minute or two light came on.

Tightened the battery terminals, cleaned again, same.

Took off the shroud and sure as hell, the POSITIVE wire had been backed off (the nut that goes to it) and was loose. Tightened it up, checked the charge, all is well in the land of 911 again. Drove it out around the block to celebrate.