Challenging electrical issue with 911 SC
 

Hi guys:

My '79 911 SC has a 12V draw that I cannot track down.

-Trunk light disconnected for testing.
-With the voltage meter checking charge at the negative battery post and ground cable, I've removed all fuses and relays individually and found no change in draw.
-I've disconnected the cable at the starter and find no change in draw.
-Nothing is "on" that I can find. Interior lights work, but door switches are not functioning. I have one loose wire -driver's side- for the interior light door switch, but get no voltage reading there.
-Note: I have two red wires running to my positive post on the battery and the car will not start without those attached. They do not appear to be stock.

The car had AC and an alarm that were removed after I bought it. Car runs beautifully with the exception of the draw. Everything works.

Does anyone have any insight? Electrical is not my first language at all and I'm stumped.

Thanks.

Why not try another battery in order to rule out a bad one? Perhaps your current one has a crack or something that is causing a ground.

Sorry, but after reading what you have already done, this is the only thing that comes to mind.

Ammeter setting........
 

JJ,

For current draw measurement, you need to set the meter @ ampere (current) not voltage. Do the test with a fully charged battery and are using a by-pass wire at the negative post?

Tony

Electrickery can be a real PIA to test! It has to be done right and then it's still a PIA. Tony is right, the meter has to be in line and measure amps.

If you don't have ammeter that can do this or the draw might be too strong, you can just wire a 12v light bulb between the terminal and the loose cable end. The brighter the bulb the bigger the draw.

both above.
I use a test light,

remove the fuses one at a time

Test light does not work properly as there is resistance in the bulb and it creates a current draw. Only true correct way is to use an Amp meter. Acceptable draw is less than 1/4 of the reserve capacity of the battery. (should be written on the battery somewhere on the label. Most are around 100, so you're looking for a draw over 25ma) If the draw is less than that and it still goes dead, then the battery is failing.

The bulb, if wired in-line between the cable end and the battery terminal will not create a draw. The only time it will light is when there is a draw/load on the battery. With electric being pulled thru the filament. Easy and cheep for this kind of test.

Not the best solution if you wish to know the exact draw in amps/milli-amps.

A bentley manual, i used to own said to test for a parstitc drain as follows ;

STEP 1) Use multi meter, in series with battery negative (i.e battery negative cable disconnected and mutlimeter connected to measure Milliamps inbetween end of cable and battery negative post) anything less than 60 milliamp was fine.

STEP 2) Use test light AFTER step 1 (no idea why) if passed both tests it was "fine"

Your stereo will suck some amps, try diconeccting that, and not every circuit is fused! do you get a big spark when you connect the neg cable back to the battery post? Can also chase it that way. Try pulling engine harness multiconnector from engine relay board, this section is largely unfused, as its power source is directly connected to the battery via the + on the starter motor

Luke

Sometimes a bad alternator will have a draw as well. It will still charge the car fine but draws when the car is turned off.

Yes, if one of the diodes shorts it will slowly drain the battery.

I just had a failed alternator. It killed the battery overnight. I thought i had just left the lights on, and put another battery in.

Later that day while driving, i pulled the lights on (with the other battery) and a sound like pistons hitting valves/chains screamed out of the engine.

It scared the life out of me. Lucky for me it, turned out to be the alterntor diodes had shorted out and it was only noticeable when the lights were pulled on, and it had to really up its output.

Took the belt off, spun it over, engine sounded fine. Replaced alternator, no more scary noises! lol.

If you have a power antenna on your car it is fed by a wire from the headlight switch that is always hot. The antenna can create a permenate draw on the battery. Disconnect this wire and see what happens. Ask how I know this.

It was an odd one in my SC :eek:

The car had a windscreen heater that was designed to switch on automatically to melt snow off, and it had kind of lost it's marbles (it has never snowed around here). It took an autoelectrician to find this. Actually he found it in minutes and cut the wire and it's been fine ever since.

do you have power windows? (at least on my carrera) power window circuit remains energized after shutting off car until door is opened (door switch is triggered) so that one can get windows up after turning off car. This results in a parasitic drain (that would kill my battery if not driven for a week) if door switch never signals an open door.

sorry but you don't know what you are talking about.
I use to do auto electric work for a living. we rarely used a MM. really only used one when working on the charging system.