Help - Voltage Regulator, Alternator, Other?

[Sharks]

I pulled the engine in my wife’s 1983 911 SC about a month ago to fix oil leaks at the back of the engine. Everything went well, but when I started the car the battery/low voltage light on the dash came on. I figured the battery was just week and it would eventually go out after driving for a while, but it didn’t. I purchased a new battery as I didn't know how old the battery was as it came with the car. No luck, light is still on.

Low voltage light does not come on with key on, but comes on after start and stays on while running. Light goes off at key off.

Done So Far:
New battery voltage and cigarette lighter voltage 12.3 V
Cleaned ground strap and added second strap from body to trans
Cleaned ground strap and terminals next to battery
Cleaned 14 pin connector in engine compartment on driver side
Pulled alternator and checked connections (Red - 10mm nut bottom, Brown - 8mm nut bottom, Blue - 8mm nut top – Picture below)
Pulled voltage regulator, cleaned shaft and checked brushes
Attached red, brown, and blue wires to back of alternator so I could run the engine and take the following readings

Running with Low Voltage Light On
Red to Brown = 135 mV
Blue to Brown = 1.7 V

If I rotate the key to the off position briefly while the engine is running, then back to on, the light on the dash goes off and stays off after the engine stumbles, then continues to run and I get the following readings

Running with Low Voltage Light Off
Red to Brown = 0 mV
Blue to Brown = 11.7 V

I’m trying to figure out if this is a coincidence that this low voltage light came on after the engine install (never had a problem before engine removal), if the voltage regulator is bad, or if the alternator is bad. Any suggestions/help appreciated thanks.

[Discseven]

Good morning Sharks. Why you were not responded to is unsettling... I'm bumping.

Sounds like a voltage regulator but I'm an electrical idiot. Better that someone with smarts in this area responds.

[E Sully]

A picture of your alternator wiring would help, but the Red wire connects to the battery, the brown wire is ground, and you should read the batteries @12.3v between them with the engine off. Some of the early SC. did not have the ground from alternator to engine, If you do not have the large brown wire from the alternator to the engine it should be added.
Here is a picture of an alternator with internal regulator.

I posted Porsches wiring update in this thread if you want to read it over.
alternator wiring help please

[rick-l]

I'm stumped but welcome to the forum.

Right now there are several experts on the forum who take it personal if you don't immediately take their advice. You seem to have a base in electronics so filter through the good and bad advice.

[mreid]

Since you didn't touch the alternator and it sounds like you have the internally regulated alternator, check the 14 pin connector and make sure that the blue wire is properly connected. It is odd that the light doesn't come on when you turn on the key. It is clear from your voltage readings that the alternator is not charging the battery. Also check the connection at the starter and your main ground straps. Open grounds can also cause this problem.

[rick-l]

Next time you are out in the garage could you check from red to engine case and red to body? Red should be at battery voltage all the time.

[timmy2]

As others have indicated there should always be 12vdc at the alternator red wire.
Power from the battery goes through the black cable to the starter and then from the starter on a red cable directly to the alternator. You may have a bad connection at the starter or a failure of the cable from the starter to the alternator.
You should always have 12 VDC in the 14 pin connector pin 14 as well.

[47silver]

I think you should remove the alternator, and have it repaired as it is cooked.

Before you start the car recharge the battery.

Any time you let a car sit for more than a week put on a battery float minder (10 bucks harbor freight) or you will get into the endless cycle of dead battery, cooked alternator, dead battery, cooked alternator.

The reason I think that this occurs is just simple math:
my 75 with a 75 amp alternator uses about 25/30 amps when all systems are running. since the the alternator is rated at 75 amps, it will cycle on and off at about 33% of the time. This cycle off time gives it a chance to cool down. if you increase the load then the alternator will operate longer...and essentially shorten its life.

If you battery is dead then the alternator is running 100% of the time,,and the alternator will burn out trying to charge the battery and to keep the car running. if a battery is 300 amps and dead then a 75 amp alternator will have to run for ? 4 hours to charge it at 100% plus trying to keep the car running. all this at 200+ degree f.

[wwest]

Originally Posted by 47silver

I think you should remove the alternator, and have it repaired as it is cooked.

Before you start the car recharge the battery.

Any time you let a car sit for more than a week put on a battery float minder (10 bucks harbor freight) or you will get into the endless cycle of dead battery, cooked alternator, dead battery, cooked alternator.

The reason I think that this occurs is just simple math:
my 75 with a 75 amp alternator uses about 25/30 amps when all systems are running. since the the alternator is rated at 75 amps, it will cycle on and off at about 33% of the time. This cycle off time gives it a chance to cool down. if you increase the load then the alternator will operate longer...and essentially shorten its life.

If you battery is dead then the alternator is running 100% of the time,,and the alternator will burn out trying to charge the battery and to keep the car running. if a battery is 300 amps and dead then a 75 amp alternator will have to run for ? 4 hours to charge it at 100% plus trying to keep the car running. all this at 200+ degree f.

Alternator "runs" 100% of the time, supports FULL electrical load plus keeping battery charged. Battery must fill-in only when engine RPM isn't high enough.

Automotive alternator/VR systems never charge the battery beyond 60-70%, safety measure since battery SOC cannot be detected/sensed with parallel electrical loads.

The only real "wear" items are the bearings, slip rings, and brushes.

[timmy2]

Sharks, do you have 12 volts to ground (brown wire or engine case) at the alternator red wire with the battery positive and negative cables connected and the ignition turned off?
Safer to test at 14 pin connector female terminal 14 or even the rear fuse panel that has a constant hot.