911 1986 Alternator light came on but now is off

[3.2Targa]

Recently after driving my 86 3.2 in 50 degree weather for about 25 minutes I parked it for about 45 minutes and then started it up. I noticed the battery warning light had turned red but very, very faintly. After about a 1/2 mile of driving the light had gradually turned bright cherry red and the car bucked a couple of times lost power and went dead. I restarted the car and with the light glowing brightly managed to get another 1/2 mile down the road before the car bucked and died again. After a minute or two I turned the car back on and the light was not lit anymore. I drove another 20 minutes or so without the indicator light coming on and the car sounded fine with the engine pulling normally.

A couple of days later I checked the fan belt (1/2 inch deflection tight). With my handy dandy cigarette lighter voltmeter I saw 12.6 volts with the key in first position (battery) and 13.9 with the engine on at 2000 rpms (alternator). Checked the battery cables and they were nice and tight. I ran the car for about 15 minutes without any warning light and the engine sounded fine. Two days later I did the same tests with readings of 12.3 V with the engine off and 14.0 V with the engine at 2000 rpms. I turned the headlights on and they didn't flare or dim. I also ran the car around the block without any warning light or strange occurrences. The oil and battery light come on red in the 1st position and go right out when the car starts.

I'm not sure if what happened is a warning that the alternator (or voltage regulator) is going to quit on me soon or if there is a ground that's decided to ruin my day. It's also possible some other component is acting up but not sure that could set off the alternator warning light beforehand. I wonder if anyone else has encountered a problem like this and could point me in the right direction.

[Mark44]

How old is your battery? I usually get 8 or 9 years out of a battery, but people who live in more extreme climates get significantly less. A reading of 12.6 volts is not very good, and 12.3 volts is very low. A chart I have puts 12.6 volts at 50% and 12.2 volts at 25%, but these percentages might be low, according to other tables I've seen.

From your readings, it appears that your alternator is working fine. You said the battery connections are tight, but are they clean? Undo them and check that the inner clamp surfaces are clean. If you don't have one, get a battery terminal cleaner. They're pretty cheap, and any parts store would carry them. Is there any corrosion showing on the battery cables? Is the grounding connection nice and clean? The ground cable wants to be bolted to shiny metal.

[PeteKz]

I have found that the cigarette lighter voltmeters are not very accurate. Get a good meter on it to measure the static and running voltage. Fluke preferred, but there are other accurate brands.

[917_Langheck]

12.6 volts is nominal for a "6 cell" auto battery.

If the regulator is failing you would see the signs you described, but did not observe. If you do a high speed run you may smell some H2S "eggs" and your battery will be very warm. Your fasten seatbelt may come on randomly. And the lights will do as noted.

Check the major ground straps and the leads at the starter. There is also a ground wire from the alternator to the engine block under the shroud that becomes embrittled due to engine heat, so if there is isolation from the fan housing to the chassis you lose that grounding point.

[3.2Targa]

Thanks everyone for the quick replies. I'll put the focus on tracing the electrical grounding points.

[EC900]

A cheap Harbor Fright multimeter will suffice as 12.3v won’t even start a motorcycle.

Was there any slow or hesitant turning over, and then start up… if so then the battery is too weak and unable to hold a charge - even if using a battery tender.
Generally if the problem is intermittent then likely a ground connection. An faulty alternator/voltage reg that won’t charge a running engine and once battery is discharged, will stall the engine.

So I agree with other that the battery age and the main ground connections (often neglected) as being culprits, but at normal idle, the output should reach +/- 13.5 to 14.5 volts.

Instances when removing fan p, it can cause the ground wire there to break on reassembly or just from age as they turn brittle.

[proporsche]

i would just focus on you alternator regulator.It possible you regulator brushes are going bad.

Ivan

[john walker's workshop]

Yup, brushes barely touching the commutator due to wear. Sometimes they contact, sometimes not.

[3.2Targa]

I'm inclined to think the alternator is the problem. The fact the alternator warning light began to glow before the problems must be a clue.

I'll clean the terminals, replace the battery, install a new alternator(regulator) and hope I fixed the problem.

Thanks everyone!

[Mark44]

Originally Posted by 3.2Targa

I'm inclined to think the alternator is the problem. The fact the alternator warning light began to glow before the problems must be a clue.

I'll clean the terminals, replace the battery, install a new alternator(regulator) and hope I fixed the problem.

Before you replace the alternator, take care of the easy and cheap things first -- cleaning the battery terminals and making sure the battery cables don't show any green corrosion, and that the battery cable connections to ground and to starter solenoid are clean and corrosion-free. I believe that the alternator warning light could come on if the battery connections aren't good, even though the alternator is putting out its nominal voltage.
I'm not saying that your alternator doesn't need to be replaced, but rather that would be the last thing I'd do.

[proporsche]

Originally Posted by Mark44

Before you replace the alternator, take care of the easy and cheap things first -- cleaning the battery terminals and making sure the battery cables don't show any green corrosion, and that the battery cable connections to ground and to starter solenoid are clean and corrosion-free. I believe that the alternator warning light could come on if the battery connections aren't good, even though the alternator is putting out its nominal voltage.
I'm not saying that your alternator doesn't need to be replaced, but rather that would be the last thing I'd do.

Not the alternator ,most of the time is the regulator ,very inexpensive way to fix the problem...
Ivan

[john walker's workshop]

The regulator brushes often wear deep channels in the commutator, so when you remove the reg from the back of the alt, you can decide what to do.

[tsuter]

My alternator diodes went bad four or five years ago. $2.00 part. Made the red light flicker and glow.
Went ahead and replaced the bearings at the same time with some nice German FAGs when I had it out. 42 years and running fine....

[3.2Targa]

Finally got around to removing the alternator. As several members thought the culprit might be the regulator I've posted a pic to show the wear.

I've also taken a picture of the alternator rotor.



I must admit I don't know what "normal" looks like but I believe rounded pickups on the regulator and groves on the rotor mean a new alternator (with new built in regulator) is in order.
Does anyone second the motion?

[911obgyn]

Plenty of brushes left, normal looking contact rings, it may have got hot at some point. I would burnish contact rings with 400 sandpaper, while spinning the alternator, check for play in the bearings clean it up with contact cleaner. Re install and monitor voltage.
I have replaced my ground wire to engine block with a larger gauge wire. Check transmission ground strap.