No luck finding a battery drain w/ a multimeter

[Brad Beck]

An amp meter is only measuring voltage drop over a known resistance and converting to amps. With a draw as low as you might have (say a bad alternator diode or clove box light) you can put your meter in DCV mode and connect in series with your neg cable and neg battery terminal, if you have a draw it will show a voltage reading. I actually prefer using an analog meter for this, the needle will react much faster as you disconnect fuses and such. Also disconnect your clock wile testing, I'm not sure on Porsches if has a windup mode or not but since its not a quarts clock I'd assume it does.

Good luck!
Brad

[Deschodt]

It is the circuit for inner lights, glove box, and clock. That was dead until
I had all the instruments redone. Ironic...

From 48 mA down to barely 3 with that fuse pulled. Will check door switches too...

Conclusion: Next time don't doubt yourself, doubt the equipment ;-)
I found fuses for the MM at home depot...wonders never cease.

[Decolliber]

Originally Posted by jsaucedo

No fix for the multimeter, but a common drain (happened to me) is the door-triggered light switch. Sometimes the rubber gets old and the switch gets sticky. There are a few threads about it. Solved my drain problem believe it or not.

81 911SC

I have drained my battery twice, as a result those lights. I keep meaning to remove the bulbs ...

[T77911S]

nothing like buying a MM to check your MM.

you can also connect the MM in place of the fuse.

you cant use DCV to do this. it only takes a very small amount of current for the MM to read batt voltage. you HAVE to either use a MM on amps or a test light.

[uwanna]

Originally Posted by Deschodt

It is the circuit for inner lights, glove box, and clock. That was dead until
I had all the instruments redone. Ironic...

48 mA to 3 with that fuse pulled. Will check door switches too...

I had your same symptoms a while back on my Pcar. Drove me nuts!! Turns out it was the
sliding switch (just above the door) for the glovebox light was in the ON position, slowly draining the battery.
It just never crossed my mind to check the glovebox light, because it seemed to work
whenever I opened the glovebox just like it should! The problem is, you close the glovebox door and never realize the LIGHT DOESN'T GO OUT!!! LOL

[T77911S]

glove box lights, hood lights and trunk lights were always first checks for drains. you cant tell they are on.
another good one was ciggarette lighters in the rear doors. kids like to put coins in there and it blows fuses.

[emac]

Greg, as I stated in my post they checked my battery not once but twice and each time it showed good. I even waited and watched the second test. At this point I demanded a new battery as mine was only six months old and this solved the problem. Try replacing your battery with a know good one and see what happens.

[darrin]

my drain was caused by a bad driver's side door switch -- along with turning on the interior light when the door is open, in the closed position the door switch leaves the power window circuit operational (drawing current) so that you can roll up windows after turning off the engine but before opening the door.

If the door switch fails, your interior lights won't turn on when you open the door and, more importantly, the power window circuit will remain energized, draining your battery.

Easy to check - if your interior lights aren't coming on when you open a door or if you can still use the power windows after opening the driver's door with the key out, you've likely found your culprit. Note -- be sure to leave the passenger door closed when testing.

[uwanna]

Originally Posted by darrin

my drain was caused by a bad driver's side door switch -- along with turning on the interior light when the door is open, in the closed position the door switch leaves the power window circuit operational (drawing current) so that you can roll up windows after turning off the engine but before opening the door.

If the door switch fails, your interior lights won't turn on when you open the door and, more importantly, the power window circuit will remain energized, draining your battery.

Easy to check - if your interior lights aren't coming on when you open a door or if you can still use the power windows after opening the driver's door with the key out, you've likely found your culprit. Note -- be sure to leave the passenger door closed when testing.

Good to know info, but the OP's cars are listed as a '68 912 and a '72 911T. Doubt
either one has power windows. Helpful info though for other folks!

[dad911]

Originally Posted by Jim Richards

^^^this.

I used a DC clamp-on ammeter to troubleshoot a current drain on my 964. It's great because you don't break the circuit to do your testing, which may lead to the offending circuit being disconnected (i.e., reset) until the next time it's activated.

Sears sells the DC clamp-on ammeters.

Yep, I have a Harbor Freight one I use all the time, best way to see if charging/discharge, etc. It was the first reasonably priced one a few years ago. They don't sell that model any more, but Sears has one for about $60.

If you buy one, make sure it's a DC clamp, many inexpensive AC clamp-on meters available, few DC.

I had a heated discussion with an EE a few years ago, who claimed DC clamp on meter was impossible... it was fun to prove him wrong...

[aoncurly]

Had a similar problem - installed a new battery, drained in a week, took battery back, replaced, second battery drained in a few days. Turns out to be the hood light - I leave my hood unlatched to put my car cover inside after use, and you cannot tell the light is on. I disconnected the hood light wire (tape it because the wire is hot), and that seemed to have solved my problem. I also got a Battery Tender after that, and now I never have problems with my battery going dead between drives.

[Deschodt]

Originally Posted by emac

Greg, as I stated in my post they checked my battery not once but twice and each time it showed good.

The battery retained a charge overnight (12.70 down to 12.65V) when disconnected from the car. When connected, it was down 0.5V in about 4 hours. My problem as indicated earlier was the door light/glovebox/clock circuit, which I found out easily (2nd or 3rd fuse I pulled) once I figured out my damn multimeter hasd blown fuses that looked pristine and unblown ;-)

Yes, buying a MM to diagnose another MM was an exercise in irony, but the new one came with useful alligator clips so it's not all bad. I have not fixed this, for now I'm simply leaving the fuse out. This is the 69 912, my wife's Friday ride, it does not need *anything* on that circuit, really (daytime driving)... I will get to it eventually, when I'm bored.

This is either the clock that was recently repaired after 10y or being stuck at 5PM, or more likely the door switches as you guys pointed out. I will buy 2 new ones, I had noticed the interior lights had stopped working, so it could well be those... So glad it's not Generator/ VR related !!

I really don't know what I did to blown both fuses in the MM, but hey, $5 fix, including spares ! Thanks all !

[HawgRyder]

The most common reason for having a blown fuse in the multi-meter...leaving the meter on "amps" and then checking for voltage.
This puts the full voltage through the meter...and since it has no resistance to reduce it...it draws full current that the battery is capable of.
That's why the fuse guys!
Bob