Hi Gang, I noticed my battery seemed to discharge pretty quickly over 2-4 days or so. So I thought I'd try a method for finding shorts I had been told was simple and reliable. It seems logical at least.
I disconnected my Neg. Cable and out a Test Light between the cable clamp and the neg. post. The light completed the circuit and the light lit. I hoped I could pull fuses one after another thinking the Test Light would go out when I got to the right one/circuit.
I checked each fuse next to the steering wheel, then then did the same for the fuses in the engine compartment. Unfortunately, the light bulb never wend out!
Now I'm wondering if there is a constant Parasitic Load on the battery? I know there are a number of computers but I thought if the car was turned off, the parasitic lost would be nil. This doesn't seem accurate though -
Is there a better way to test for a short? Please be specific, electrical stuff is not my strong suite. ;-)
Thanks ya'll --

The next thing to do is disconnect the alternator B+ wire. This could indicate a shorted alternator diode.

Would I still be looking for the test light to go out; like when I tested the fuses?

Thanks!

So after I disconnect the B+ wire, what do I measure that could indicate a shorted diode?

Thanks!

, I think he means disconnect and see if your current draw is gone

Thanks! (duh!)

A couple of key points, doors must be shut. Or flip lock in doors, same with hood , current draw is lower when alarm set (doors locked),with inductive meter connected to battery, let's say your reading 3,000 milliamps, it will drop after 20minutes indicating the car is starting to shut down consumers,so after 20 minutes say it drops to a lower figure say 1,000 ma , measure at lowest figure then try pulling fuse, or disconnecting radio ,etc, thats how you will find your draw, YOU HAVE TO WAITevery time you disconnect,to find the offending circuit

Quicker way. Hold a piece of steel ( like a screw driver ) against the alternator pulley or back of the alternator, if it sticks the field is staying on or there is a shorted diode for the stator. Actual shorted not what some call any electrical fault.

Thanks! I'll do this today! (love these kinds of quick tests!)

Ok, charged the Battery and hooked it back up - turned the Ign Sw til the dash lights came on, then I put a small screwdriver blade against the pulley and body and there was zero magnetic attraction

Hope I don't need to start the engine, but I can probably get my hand close enough to the pulley to test it with the engine running if I need to?

Does me not seeing a magnetic attraction mean my alt has no shorted diode or a
stator?

Thanks for the help.... When I am able to, I'll disconnect the B+ wire and will try that test if needed.

LarryT
06 E350AWD

To do a parasitic draw test on this car its best to use small jumper cables and a Digital multimeter.

firstly get a starter pack or jump cables from one car to this one from the posts in the hood so you dont lose any saved settings etc.

now disconnect your battery negative and connect 3 small jumper leads to the battery and the terminal to complete the circuit. You can now remove the jumpers from the front of the car.

push in the door latches or tape the push button switches on the door pillars along with latching the trunk and hood and taping up any push buttons there too to simulate the car having all doors closed.

leave the car alone for 30 minutes to an hour.

take your DMM and select it to amps, change the leads position on top too (if your DMM requires)

remove one small jumper cable from battery post and connect it to one lead of the DMM, remove another cable from the battery terminal of the cars wiring and connect it to the DMMs other lead.

you should see some reading on the DMM proving that your circuit is complete. Now finally remove the last jumper altogether from the battery post to the terminal - this way all your amps are now flowing through the DMM, at this time your reading should be less than 50 milliamps.

if its more than this - find your fault by isolating the circuit at fault - on the W211 I start with the power seat module under the driver seat.

Best way is to measure voltage between the pins of each fuse with a new separate DMM, any live circuit will show some volts as voltage loss happens when any circuit is loading up. The voltage should start at some microreading and drop to zero. If it doesnt drop to zero. thats your fault. You can then isolate that fuse/circuit and see the DMM hooked to the battery and terminal. The problem with just disconnecting fuses on modern cars with can and lin bus is that this action can awaken/reboot some modules rendering your testing useless.

If your DMM starts to beep the autoshutoff feature (if so equipped) - then immediately plug back the 3rd jumper lead you removed from the battery post and terminal, cycle the DMM on/off to restart the autoshutdown counter in it. and remove the 3rd cable again.

tip: - buy a new generic battery terminal and attach three regular thin wires to it - 16 gauge is enough, attach three small alligator clips to the wires, these wires are now jumper wires - Now you have a handy tool that makes life/work easier.

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Order an inductive amp meter from amazon to measure milliamps, car should be at rest at 37 tO 70 MAXIMUM, start with front sam feed , then rear sam then interior, in the trunk you will see next to the spare agaist the sheet metal 4 single10 gauge wires that feed car sams , test each of 4 legs , onceif you find the excessive draw try the fuses first , but after you remove each fuse and plug it back you must wait for "CAN"BUS to go to sleep,or take it to the dealer, they can connect to the bus and see which control unit is on for further diag, usual culprits are radio,teleaid if you have it , switches near upholder (when drinks spilled) etc

About how long should I wait to be sure the CAN has gone to sleep?

I can get an inductive device to attach to my DMM, but which wires should I be testing?

Thanks for your suggestions!

Sincerely,

If you get an inductive amp meter, be sure it measures DC as the lower end ones only measure AC. Extech and Tenma made decent general use test equipment.
Be sure to get one with small tongs as industrial level meters are too big for most automotive use.

$ 50 for a lower end but still useful DC clamp ( on sale from $ 100 )
https://www.newark.com/tenma/72-9480/digital-multimeter-clamp/dp/76T4530


When using a general use amp clamp, the last few miliamps can be difficult to chase due to the meter floating around.