Electrical drain - E320 W124 Wagon - PeachParts Mercedes-Benz Forum

oldtrucker · #11 09-11-2015, 12:04 AM

Quote:

Originally Posted by 2wheelsoff

Back to searching; the battery dropped from 12.91 to 8.60 volts in two days of being attached. Convenience module is disconnected. Battery holds charge for over a few weeks when disconnected.

If your battery is discharged within 2 days and the remainder is measured at 8.6 V, that is actually a deep discharge (may kill the battery). A car battery is considered to be discharged at 11V or slightly below (10.8V).

For example a 55AH battery discharged to ~11V within two days, would mean that a current of 1.14 Amps, was continuously flowing. This goes along with your measured 1.2 Amps.

I am confused about the battery and fuse box (disconnected negative terminal and having the tester light up.) That is not possible, unless there is another energy source installed in the vehicle.

The vehicle drives fine otherwise?
Anything visually different e.g. Dome Light not working, see if the door lights still work when any door is opened. These lights are wired into relays and other control circuitry that could have gone south. Sometimes, a contact shoe can fall off the spade making a slight contact to chassis. So if anything looks different or is not working can give you a hint.

Check in your ashtray, cigar lighter. Non Smoker like to use the tray for change storage. I have found pennies in the cigar lighter receptacle, causing just enough current flow as you describe.

Electrical control modules in this era, were assembled using a type of rivet on larger contact areas, soldering would require more heat and could damage other components. Aside, a rivet was much faster in manufacturing. Over time dust and other enemies creep into the enclosure or deposit dirt on contacts or in between, causing unwanted results. In general, the before mentioned does usually cause a malfunction the other way around, but could possibly cause a steady elevated current flow and thereby drain the battery.

It is extremely difficult to pinpoint the culprit, since there could be plenty of failure sources.
The only, somewhat faster way to troubleshoot is, with the ignition off, attach an Amp Meter in line with the battery (DO ABSOLUTELY NOT TURN YOUR STARTER MOTOR (400 AMP Load) UNLESS YOUR AMP METER CAN HANDLE THE CURRENT, otherwise, it will go off into smoke). Monitor the current e.g. the 1.2 Amp described earlier.
Now, take out fuses while monitoring the current and putting them back. If the current drops significantly, you'll know at least what fused circuit to investigate further. If none causes a drop, you have a problem in a non fused circuit.