1981 SC Electrical Issues (help)

[81SCTarga]

Hopefully I can explain this enough that someone could give me some pointers. I've spent all day today trying to figure this out but haven't gotten anywhere.

This morning I was trying to track down a drain on the battery / alternator charging issue. What I found is pretty strange. I have a battery cut off switch on the negative battery terminal. While the cut off switch is disconnected, I found that there is 12.6 V (the actual battery voltage) from the disconnected negative terminal of my battery to the ground point by the battery. So somehow my + battery terminal is connected to ground somewhere.

I checked a few of the ground points to ensure that when I put everything back together after my repaint a while ago that I didn't inadvertently connect something incorrectly, but I didn't find anything that made the voltage drop back down to 0V.

I also disconnected each fuse one by one but that didn't stop the 12.6V from showing up on the body of the car and anything that was connected to ground.

I have looked through a few wiring diagrams in the Bentley's manual, but I am at a loss here but I have run out of ideas.

Do any of you have any ideas of what else I could try to track this down?

Thanks in advance.

-Justin

Disconnected Battery Kill Switch


Disconnected Battery Kill Switch - to ground point on body


The side shot (just because)

[windsoreight]

You need to check amps on a draw not volts. You will always see battery voltage.

[81SCTarga]

I've tried checking the draw before, but for some reason I've never been able to get my multimeter to read amps it always reads 0A (even when it is set to do so). I have put it in series (not parallel) to connect the current flow, but it never has worked for me.

I never knew that the body of the car (ground) was connected to positive voltage if the battery ground is disconnected.

How / why is that the case?

Thanks again

[windsoreight]

You probably have a blown fuse in you multimeter. You will see battery voltage because you are completing the circuit with the meter.

[Oh Haha]

WHOA, freakin' deja vu!!!!!!

I just figured out the same problem.

battery drain----searched already

Let me pay it forward:

Put your multi-meter on DCA, the black probe to "common", and the red probe to the 10 A input on the multi-meter.
Connect the end probes to the neg. battery post and the negative cable. Be sure the cutoff switch is disconnected.

You SHOULD see a reading. This is what you need to know.

I did the same thing as you and got the 12volt reading.

I had blown the milliamp fuse several times before I figured out that I was gettingmore than a ma draw.

PM me if you would like to talk on the phone. I'm not an expert but all ofthis is still fresh in my mind.

BTW-it turned out that my headlight switch was wired incorrectly.......by me. duh

[81SCTarga]

Oh duh, I am not the sharpest tool in the shed, that makes sense about completing the circuit. That was a waste of a day, but at least I won't have to waste any more time trying to figure that out.

Now back to the drawing board on trying to figure out why my alternator isn't working and why there is a battery drain.

Thanks

[James Brown]

try just using a test light disconnect the + and hook the light up between the batt. and the + cable, if the light comes on, there is a power draw some where. start disconnecting (pulling fuses) untill the light goes out. There is the draw. If the light never goes out you might have blown some diodes in the alt. That will need a rebuild or a replacement. Do you have a internal regulator mod on your alt.? You should do that if you need a new alt. And there are some circuit on a Porsche that don't have a fuse, good luck!

[81SCTarga]

Yeah, I'll need to get a test light. I checked my multimeter and the fuse was blown as expected. I do have a 10A un-fused option on my multimeter but it still just reads 0.0A when it is put in series with the circuit. I'll do some more tests (pulling fuses to isolate the problem) once I get the test light.

I do have an internal voltage regulator (someone who owned the car before me did that update). I think I will probably have to pull the alternator and get it checked out and probably rebuilt. I have had some intermittent charging issues over the last little while.

When the car is cold the alternator charges fine (13.8ish at the cigarette lighter at idle and 14.2 - 14.6 across the battery), but once the car warms up the voltage drops to 11.6V or so (at the cigarette lighter) and continues to drop as the car is driven (battery not getting charged so it drains as I drive). It seems like the charging stops when the car reaches regular operating temperature, but that could be a coincidence.

I don't know if the alternator and the battery drain are related.

[T77911S]

you created an open with the switch. you should read 12v that way. look at it like this. you dont read any voltage across a good fuse, only bad ones.
use a test light and connect as you did before with the batt switch off to test for a drain.

a drain is something that happens when the car is off, not while running.
i see that you had 12.6 on the battery.
verify the battery is good first. have it load tested. advance will do it for free. tey will also chek your charging system to verify it works. sounds like it may be the regulator
always go by a multimeter at the battery not these plugin deals unless you can verify its accuracy.
clean and check all major connections, including grounds. dont forget the chasis to tranny gnd.