I have a 1994 E320 wagon that has developed an electrical drain the last month or so. It will drain the battery in 3-4 hours so it is a severe drain (battery about 2 years old, and holds strong charge). I keep the negative disconnected unless I need to drive it. Since it is such a strong drain I am guessing substantial power must be flowing through the fuse panel.

With the negative battery terminal disconnected I have complete circuit (test light glows brightly) from negative battery terminal when touching the following fuses:

C - rear roof light, trunk light, rear door lock sys, seat back arrest, central locking, door lights, reading lamps, aut antenna

9 - diag term 6, hazard, clock, front roof light, radio, vanity mirror light, relay comofrt circuit term 86

10 - heatable rear window

11 - vacant (has 16 amp fuse installed)

12 - vacant (has 16 amp fuse installed)

D - Auxiliary fan

G - window lift front (discharges, test light bright, but drains away to barely lit)

13 - low beam left (discharges, test light bright, and fades quickly to completely off)

Anyone suggestions on where to start? Thanks!

Thanks Arthur Dalton for making me realize my flawed thinking.

When connecting test light in line with negative terminal and negative lead finding:

190 mah drain

1.2 amp drain (no typo) -fuse C disconnected, and hear relay triggering when connecting meter to negative...meter reading decays then relay trips off again (relay in the relay panel attached to fuse panel), then nominal drain.

180 mAh drain - fuse 9 disconnected

1 mah drain when both fuse C and 9 disconnected

9 - diagnostic terminal 6, hazard, clock, front roof light, radio, vanity mirror, relay comfort circuit term 86.

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check convenience relay , under the carpet at left rear under the seat, unplug then check

Thanks Nulu, seems to help some.

Removed the convenience module under the carpet...drain shows 190 mah for 20-30 seconds then drops to 16-23 mah, in a spiking fashion. Going to pull fuses and measure again when back from work trip.

What is an acceptable drain on the W124?

16-23 seems ok, spike may be clock, or radio, report back new cars are 23-47 ma, older s class 50-75 ma, latest newest 20-40 ma,1000 ma battery will be low in less than 2 days, 400-700ma battery dead in 3-4 days, depends on battery size etc,

Sounds good, I'll test battery after a few days connected when I return. What are the chances used convenience modules work if bought used/in a yard etc? From my research seems like a common fail item. New is exorbitant.

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Not that common to failure , most yards offer guar of 90 days, took me three times to get a good used cruise control module, but way cheeper that new

I'll hit a pick your part place and hope to get lucky, maybe get two. Thanks!

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Crack open the module and look for electrolytic capacitors that have buldged or are leaking fluid. Leakage can cause all sorts of odd behavior.

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.

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I'd amp clamp the large battery wire and small trigger wire at the alternator. I've had a diode fail in a conductive state and drag the battery down. A quick test is ,with the key off, hold a steel object on the end of the alternator and see if it is attracted. If it is, the alternator is failing and acting as an electromagnet.

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.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

It's a long shot, but is the original radio still in the car?

__________________
'89 260e (212K Mi.), '92 400e (208K Mi.), '92 400e (not a misprint) (146K Mi.), '95 C220, '81 240D--Sold

It's aftermarket, and maybe failing. I'll try disconnecting if can't come up with anything else.

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From fuse 9 = 1.2 amp drain (no typo) - fuse C disconnected, and hear relay triggering when connecting meter to negative...meter reading decays then relay trips off again (relay in the relay panel attached to fuse panel), then nominal drain.

180 mAh drain at fuse C - fuse 9 disconnected

1 mah drain when both fuse C and 9 disconnected.

Have to investigate further, couldn't last weekend due to lemons race.

Thanks for the help

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