Be careful of those digital volt meters that fit in the cigarette lighter socket... if your car doesn't turn off power to the socket when the key's out, they can contribute to parasitic drain on the battery. Mine was pushing drain past the 50mA limit on my BMW, so I'm now taking it out while the car's parked.

I am always surprised on threads like this where it takes days [in this case 10] for the OP to come back with battery voltage static and running.

This is an easy test with what can be a $20 USD volt meter that you should have at your disposal. You should do this first thing.

Always suspect the battery first and then see if the Alternator is charging.

Some info in here

Sluggish battery is measuring 13.25V while engine is running.
http://forums.pelicanparts.com/porsche-911-technical-forum/1032433-sluggish-battery-measuring-13-25v-while-engine-running.html

I have this in my cigarette lighter...

https://www.amazon.com/Jebsens-Charger-Battery-Monitor-Voltage/dp/B01N00I4TM/ref=mp_s_a_1_5?keywords=cigarette+lighter+usb+char ger+voltage&qid=1570153844&sr=8-5

You can test the ground strap on the transmission by measuring the voltage from a chassis stud to the engine case while running. It's an easy test and should yield zero volts.

OP, did you find and fix the issue yet?

"zero volts" in this case = voltage drop. Circuit path components and connection points should have almost nil voltage drop (voltage loss via circuit resistance). Connect voltmeter leads on each end of the test path, then observe voltage drop with the circuit ON.

Sherwood

Never quite understood voltage drop.
When I measure battery voltage, am I measuring voltage drop?
I put the dial on VDC 20 (exactly as in photo)
And then touch the leads to each battery post.
When I get a reading of 14V, is this voltage or voltage drop?

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Have your alternator and voltage regulator tested or test yourself. You need over 14 volts, as others have mentioned. 12.6 or less is not enough.

Another thing you can do to make sure you don't have some huge voltage drain is to test at each fuse one at a time to see if you may have a seized fan or something that is sucking up massive voltage. Front blower motor, air conditioner fan in the front trunk, etc. may be shorted or seized, etc.

Voltage and voltage drop are the same thing. With the multimeter you basically measure voltage from one point to another. With charging systems and batteries one usually measures voltage from chassis to battery positive or D+, DF, D- on the alternator to chassis.

I think of "voltage drop" as the reduction in voltage from the source (battery or alternator) to where you are using it. For instance, if you ran a cable a quarter of the size of the one Porsche uses from your 911's battery back to the starter, activated the starter, and measured the voltage from the starter + terminal to ground, it would be less than the voltage you measured across the battery (even discounting the fact that the starter load may well drop the battery's voltage too, but that is a sort of special case - nothing else draws as much current). The drop is due to the resistance of the cable. If you have corroded connectors between the battery and something, they introduce a resistance, so there is a voltage drop. You want to avoid or at least (with suitably large wires) minimize voltage drop.

14 volts across the battery is pretty good, at least if it is under load. Mine don't show that unless the engine is running.

Clarification: A voltage drop test in a circuit does not include the designated load (accessory) in that circuit, only the circuit path components between the VM leads. A voltage drop test measures excessive resistance in a specified section of a circuit (switch, connections and/or conductors) that might reduce the available voltage to the load or to ground. Circuit must be activated.

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For example, a circuit that provides only 10V to a 12V light bulb due to circuit resistance will result in the bulb either not working or provide less light than designed. If a motor (e.g. starter motor), it may rotate slowly or not rotate at a threshold voltage, again due to either excessive voltage drop or insufficient voltage (discharged/faulty battery).

Sherwood

Plus, if I have my Ohm's Law right, with a lower voltage, the motor, for instance, will draw more current (amps) to try to make up for the lower voltage (its resistance is the same) and still do the same work (Watts). This will result in more heat in the motor, which isn't a good thing.

But a bit off topic, which seems to be why Sugarwood isn't getting the voltage from his alternator that he should.

Disconnect the battery and charge it over night and load test it the next morning. A bad battery can cause everything you’re experiencing also.
I carried a brand of battery years ago and it took two or three to get a good one. The manufacturer said their failure rate was less than 5% and I told them to send me those because the ones I’ve been getting are crap.
Switched to interstate and never looked back.
True story
Tony

Hey Pelicans, thanks all for ideas and support on this. Sorry I haven't responded until now, real world and work got in the way.

Reminder its a 78 911 with an 86 engine swap and appears to have 89 monotronic box. recently installed the SW chip and can't wait to fully test its HP and driving gains

Update: I removed the alternator yesterday and took to an auto electric shop here in town that's been repairing starters and alternators for 70+ years. Old man tested it on the bench and said it was working perfect, no issue with voltage out or voltage regulator he said. I explained the circuit back to ignition and dash for the G light that wasn't turning on when key was turned to run, light would only come on when turned over to start the car as previously explained. he said that starter could be missing an accessory wire back to ignition.

Went home and installed the alternator all wired according to books and pics, normal start and run again like original but no charge light at dash. jacked the car up and inspected the starter. have a large black wire (ground?) yellow wire (not yellow and blue but its been spliced) and large red wire. see pics below

appears the starter is wired ok? ya'll experts chime in here please

I then disconnected the ground terminal at battery, before doing so i ran voltage on battery and is a solid 12.8V. contacting the cars ground and the battery ground it showed losing -2.25 volts steady. i think put multi-meter from battery ground to fuses one by one and all shows same -2.25 until i got to wear battery power ran to fuse box those matched 12.8V from battery test. i do have a random yellow wire in the front that isn't hooked up nor has power when turn to accessory or car running. as seen in last pic.

i installed the cigarette lighter voltage meter and its showing 12.3V with car on

Not good at this electrical game but am alright at checklists so please throw them at me, thanks in advance for continued help.

All ground straps appear new and good contact.

Something was boogered up on the engine swap and trying to chase down the mess of wiring.

Thanks

Jameshttp://forums.pelicanparts.com/uploa...1571164016.jpg
http://forums.pelicanparts.com/uploa...1571164016.jpg
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There should be a wire (red, fairly thick but not as thick as the black cable)with a ring connector on the bottom screw post in addition to the big black cable from the battery. The red wire goes to the B+ terminal on the alternator, and is how current from the alternator gets to the battery to charge it. I only see the black wire on that post, and the screw is longer than the ground screw post, I suppose so the additional connector can be there.

The bare spade lug is for the cold start/thermo time switch on the '78 motor, which I think your '86, with its Motronic EFI, doesn't have.

If I understand things correctly, your voltage at the cigarette lighter while the car is on the road is only in the low 12 volts, and not in the 13 range as it should be with the alternator working.

Whether or not this explains the behavior of the generator light, I don't know. But if I have it right, there is a fairly fat wire coming out of the back of the fiberglass shroud hump behind the fan housing which should go to the starter, but doesn't.

I think I barely see the be red cable from the alternator connected to the wrong large bolt. Both red and black connect to the same terminal as the black is currently on.

Correct there is a larger cable over on the other side. So that one needs to go with the other larger cable on the same post on the solenoid/starter?

Yahtzee! Wouldn’t have thought the red and black thick cables go to same post on the solenoid. Guess when whoever did the swap they wired her up wrong. Light comes on dash with key to run, light on start then goes off. Cigarette lighter shows 14.3V while running 12.8V with car in accessories switch. Thanks Pelicans for the support

The voltage measured at the cig. lighter should be the same at the fuse box. That's the available source voltage from the charging system. At high idle, with some accessories ON (fan, high beams, etc.), the alternator should be outputting about 14 volts. If it's only 12.3V, the battery is operating under a deficit, meaning normal system accessories will gradually discharge the battery.

It seems the alternator, for some reason, isn't producing the specified charging voltage to power the electrical system + recharge the battery (after engine cranking). The voltage regulator could be the culprit. Instead of being tempted to replace random parts, it might be best to find a good auto electrical tech to resolve your charging issues. Automotive electrical systems are fairly generic, save for where components are located.

BTW, low voltage = low electrical pressure = lower current flow. Put fresh batteries in a flashlight with old batteries and see the difference.

Sherwood