Alternator low voltage intermittent issue
 

So I have recently experienced an issue on my `84 Carrera where my alternator randomly starts only putting out 12V. This read from the volt meter in the ciggy lighter, but also confirmed with a DMM at the battery.

I have a rebuilt alternator that has been working just fine for the last 2 years or so, and this problem just recently happened.

I say this is intermittent because it may start at 14V, and then some short time later only put out 12 V. Then some times it consistently puts out 14V. Other times it consistently puts out 12V. For whatever reason, at 12V my alternator light does not come on (maybe it's not yet low enough to trigger the light??)

I checked all connections and they are clean and tight, ground straps are new.
Alternator belt is tight and not slipping (also new Porsche belt)

I decided to try a simple VR change to see if that improved things as this part is cheap.
I took a long 2 hour ride after the VR change and all is great, 14+V for the entire trip.
Then today I start it (@14V)...back out of the garage, drive down the block and then the sucker dropped to 12V. Back home I go to take my DD instead.

So at this point I guess my alternator is no bueno but perhaps there's something simple that I missed.

Any other thoughts on things to do before I go and get this rebuilt or buy a new one?
The cost of a new alternator on these cars is stupid high but the thought of dealing with another crappy rebuild that fails in <2yrs is not all that enticing.

Anyone have a SoCal shop (Ventura area) that does good solid alternator rebuilds or should I suck it up and get a new one?

Hopefully there is a shop in SoCal. I think John up here in San Jose died or somehow stopped doing business out of his house. I got the alt. for my 89 3.2 rebuilt less than a year ago. John put in new bearings/rings/volt reg. Should be good for another 30 years.

Check the 14 pin connector in the left rear engine bay for good connection. 14 volt is charging. 12 volt means battery power only. It sounds like a loose connection that you have not yet identified. The alternator exciter is supplied by the blue wire at pin 11, as is the alternator light. It may be loose. With the engine running and a voltmeter where you can see it, try moving the 14 pin connector and the associated wires. The other possibility is that your alternator light bulb is loose or failing, maybe with an intermittent connection. The circuit through the filament is required to excite the alternator.

Circling back here....things move slowly with 2 little guys under 5 to occupy me.

Over the past few weeks I could start the car intermittently and in situations where the car started, I had low voltage (~11.8v).

I was only measuring 9V at the yellow wire on the solenoid when cranking the engine,so that narrowed the scope for wire checks because I was expecting somewhere closer to 12v. I had no visible corrosion on my battery, but I took off all the red wires and scrubbed them shiny again with 220 grit sand paper.

I haven’t remeasured the voltage at the solenoid again since I did the clean up on the wire terminals but it fires up every time and has 13.8v at my cig lighter voltage meter when running whereas I was previously at 12 or lower.

I love these small wins. Time to crack another [emoji481]




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in my case it was a poor connection at the alt warning light in the dash that led to intermittent charging because the circuit wasnt being completed to the exciter

I did that one for good measure as well. This makes me want to accelerate my fuse box upgrade.


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I was having the same issue on my 87. It was the voltage regulator going. Replaced it, the bearings and the ground from the alternator to the engine. Working like a champ since then.

Scott

Question for the collective brain power - Does the exciter function that the light bulb in the dash plays only required when you initially start the car or is it needed the whole time for the alternator to output power?

Working with a buddy on his 911sc and he starts off with decent charge voltage but it starts to dwindle down as he drives. You can sometimes get it to raise a little by putting more load on the system and then removing the load. But that is short lived and it soon returns to ~12.2-12.5V.
We have replaced the ground strap, added an additional ground strap from the fan shroud to the engine mount bolt (the small 13mm bolt that isn't rubber insulated), bench tested the alternator (tested great). The car is only a few years out from its complete restoration so most items are basically brand new.

Since the car is a 79 but it has an internal regulator alt in it, i'm wondering what the symptoms would be if the light bulb wasn't changed out for a 4watt and didn't get the additional resistor added. Would it simply not charge at all or could it be intermittent?

Pre 82 cars need either the 4 watt bulb or the 91 ohm resistor to stimulate the VR to charge when changing to
An internal VR.

Actually, the resistor can be from 82 to 110 ohms. The key value, though, is the power rating for a key-on and engine not running.
Assuming 14 volts and 100 ohms, the power rating of the resistor should be at least 2 watts. Once the engine starts, the regulator
voltage is supplied by the diode-trio in the alternator.

What is the result of not having a 4w bulb or resistor? Will it charge intermittently or not at all?

That would be the case.

The use in that application (starting the alternator) would exceed the power rating of the resister, not having a power rating greater than the application.
The power consumption of a resistor (91 ohms) is; Power = V^2 / R = 14 X 14 / 91 = 2.2 watts