Starter/alternator short to ground?

[retireduper]

My 80 911 SC has developed a short which is draining the battery in a few hours. I pulled the negative battery cable off and measured the voltage between the cable and the negative battery terminal and got 12.47V. I then measured the current draw between the terminal and the cable and got 17 A. I pulled the (3) red wires from the + battery terminal and measured the current, and it was about .25 A. I suspect either the starter or alternator, but it is difficult to trouble shoot any further without tearing the alternator out. The car stated and ran pretty good this last weekend, as I put about 70 miles on it. I never stopped for very long so didn't notice any problem with starting, or running. When returning I parked outside my garage and left it for about 3-4 hours. When I returned the battery was dead. Does anyone have any further checks I could make .

Thanks
Chuck

[T77911S]

17A is a lot.

did you do any work on the car before this started happening?

you need to pull fuses one at a time to see what is draining the batt.
you can also use a test light between the post and wire, makes it a bit easier to see from a distance.

the voltage is not important, just the current. you will have battery voltage even at the .25A.

starters dont short and drain batteries, not likely an alt either, diodes usually open. the windings could short, but that would give you a whole lot of other issues. if the alts output is fine, rule that out.
if the fuses dont leed you anywhere, check wiring problems.

again, have you done any work on the car?

[retireduper]

I have not done any work on the car, but it had been stored in the garage over the summer. I have pulled all of the fuses from the front fuse block as well as the fuses in the engine compartment. The only way I can eliminate the voltage across the negative terminal to negative ground wire is to remove the 3 red wires from the positive battery terminal. The single red wire removed only does not eliminate the problem, but removal of the paired 2 wires does minimize the current draw to .35 A., with the single red wire attached to the positive terminal. My problem is I don't know which componenet the paired red wires goto. The strange part is that the car continues to start and run. I just have to remove the negative battery connection whenever I leave the car for more than 2-3 hrs.

[cmonref]

Are you using a good DVM for the testing?

Disconnect the negative battery terminal. Hook the DVM from the terminal to the disconnected terminal wire. If you see the ~.25A drain, remove each of the fuses, one at a time, and note the drain. When it is zero, THAT fuse's circuit has the short to ground that is causing the drain. Reinstall each fuse as you go. Do the engine fuse box also.

If you find a circuit with a drain, drag out your wiring diagram and start chasing connections in that circuit. Classical culprits include the door closer switch and the interior light switches.

If all circuits at the fuse boxes are OK, disconnect the battery ground. Remove the alternator. Disconnect the alternator's ground wire. Connect your DVM across the alternator ground wire to an engine ground. Reconnect the battery ground cable. Note if the DVM reads the drain amperage. If YES, then the alternator needs repair -- new diodes or identify and fix a short in the alternator's wiring.

Good luck. I found my drain in the alternator.

Brian

[Bob Kontak]

Yikes. My multi meter will only allow 10 amp draw max. What are you using?

[86 911 Targa]

Originally Posted by retireduper

My 80 911 SC has developed a short which is draining the battery in a few hours. I pulled the negative battery cable off and measured the voltage between the cable and the negative battery terminal and got 12.47V. I then measured the current draw between the terminal and the cable and got 17 A. I pulled the (3) red wires from the + battery terminal and measured the current, and it was about .25 A. I suspect either the starter or alternator, but it is difficult to trouble shoot any further without tearing the alternator out. The car stated and ran pretty good this last weekend, as I put about 70 miles on it. I never stopped for very long so didn't notice any problem with starting, or running. When returning I parked outside my garage and left it for about 3-4 hours. When I returned the battery was dead. Does anyone have any further checks I could make .

Thanks
Chuck

Chuck, just to be certain about your measurements, here's a page out of the Bentley:



My multimeter only records up to 10A.

You may want to perform the internal meter fuse check.

pm me as needed.

[boyt911sc]

Originally Posted by retireduper

My 80 911 SC has developed a short which is draining the battery in a few hours. I pulled the negative battery cable off and measured the voltage between the cable and the negative battery terminal and got 12.47V. I then measured the current draw between the terminal and the cable and got 17 A. I pulled the (3) red wires from the + battery terminal and measured the current, and it was about .25 A. I suspect either the starter or alternator, but it is difficult to trouble shoot any further without tearing the alternator out. The car stated and ran pretty good this last weekend, as I put about 70 miles on it. I never stopped for very long so didn't notice any problem with starting, or running. When returning I parked outside my garage and left it for about 3-4 hours. When I returned the battery was dead. Does anyone have any further checks I could make .

Thanks
Chuck

Chuck,

I saw this post yesterday and kept reading it till today. I'm a little confuse on how you obtained a voltage reading of 12.5 volts (???) between the negative pole and ground cable. Could you confirm this by using a test light? Thanks.

Tony

[T77911S]

tony, the 12.5v would be correct, but useless. voltage readings mean nothing when looking for a drain.

you said you removed all the fuses but still had VOLTAGE, you cant troubleshoot checking voltage. you need to do this again checking current or either use a test light.

if the same results, you still a big current draw, remove the battery cable at the starter, this powers the alt.


you only need a very small amount of current to read 12v. i dont know how small, but even .25a would measure 12v.

(i am an electronics tech for the FAA and i worked at an auto electric repair shop for a short while).

[retireduper]

Thanks guys for all the information relating to the electrical short. First, the current drain should have read 1.7 amps not 17, but the short was still there. I finally traced all the wires from the positive terminal of the battery back to their source, and discovered a short in the ignition switch itself. I found that I could pull the key out of the switch while it was still running, which led me to look at this component more closely. I have ordred a new switch and should have it by next week. After installation I'll rerun the tests again, and hopefiully not have the same results. More later, and thanks again for the help.

Chuck

[DRACO A5OG]

Yes, please keep us posted and you sure it is not the door switch relay that keeps the power to the windows until door is open, right? Heck, just in case it wasn't posted.

[T77911S]

you sure it is the switch? not to say you are wrong and not knowing how you came to that conclusion, i am a bit skeptical, just based on experience. (i hate to see people waste money)
i have only changed one switch and it was on a newer 911, but the electrical part and the mechanical part are not related.