Parasitic drain !!

[Mercedes only]

Hello all.

Normal draw is ~0.20 A.

Removing Fuse 9 → 0.01 A.
Removing Fuse C → 0.01 A.
Removing Relay A → ~0.02 A.
Removing all the 3 = 0.00 A .

Disconnecting the aftermarket remote module reduced the draw slightly (~0.28 A → ~0.19 A) but did not fix the issue. With Fuse 9 + Fuse C + Relay A installed together, the drain returns. All other loads (central locking, seats, lights, modules) disconnected with no effect .

 

[gsxr]

0.20A is quite a bit. I assume when you say "normal" draw, you mean that's what you are measuring now, and it's draining the battery. "Normal" should be much less, maybe 0.02A? (I forget what typical is, just very very low.)

I'm confused why the drain only completely stops when all 3 items are removed.

 

[Mercedes only]

" assume when you say "normal" draw, you mean that's what you are measuring now "

Yes , this what I mean .

 

[rayhennig]

Don't know if this is any use but I had a 350-ish ma drain on my 300CE-24. It has Serpi Star alarm so I suspected that. But I narrowed it down to the convenience control circuit. Changed the small relay behind the fuse box - no change.

Probed around the convenience module under the rear seat. A little valve was hot. Unplugged things and no drain.

I installed a new valve and cleaned up the module relays and all well ever since.

Sorry I can't remember exactly what cured the problem. All this was around 10 years ago.

Drain now less than 50 ma, last time I measured.

RayH

 

[Surf Blau]

Could this be caused by a faulty ground? I have no specific knowledge of the problem presented here, but in my experience whenever "things don't add up" and seems impossible it is caused by a grounding issue.

 

[emerydc8]

Out of curiosity, what current draw do you get (if any) if you check the battery only, disconnected from the car? Check between the red and black.

 

[Mercedes only]

Out of curiosity, what current draw do you get (if any) if you check the battery only, disconnected from the car? Check between the red and black.

This what I did .

 

[emerydc8]

That’s measuring the car too. I mean if you disconnect the wires to the battery and just measure across the two posts like my picture below. The reason I ask is I had a bad battery from Autozone in my Camaro a few months ago and it was showing 1.52 amps when I did what you did in your pic and put the ammeter in the circuit. I was sure I had parasite drag somewhere and I probably spent six hours unplugging every fuse and relay. I replaced the alternator, starter and ignition switch. Traced every wire from the battery and checked all grounds. By accident I discovered that with the battery completely disconnected from the car the current across the battery posts was still 1.52 amps so it had nothing to do with the car. Autozone checked it with their machine and it was fine but I showed them in person what the battery was doing. I zeroed the meter before checking BTW. They replaced the battery and the problem was solved. I still don’t understand why it did this. What was really strange was that if I left the battery disconnected it wouldn’t drain down but if I connected both leads to the car the battery was dead the next day.

Some meters like mine have a key to zero out the reading before checking or you could start with the current being displayed and get an inaccurate reading.

 

[Mercedes only]

That’s measuring the car too. I mean if you disconnect the wires to the battery and just measure across the two posts like my picture below. The reason I ask is I had a bad battery from Autozone in my Camaro a few months ago and it was showing 1.52 amps when I did what you did in your pic and put the ammeter in the circuit. I was sure I had parasite drag somewhere and I probably spent six hours unplugging every fuse and relay. I replaced the alternator, starter and ignition switch. Traced every wire from the battery and checked all grounds. By accident I discovered that with the battery completely disconnected from the car the current across the battery posts was still 1.52 amps so it had nothing to do with the car. Autozone checked it with their machine and it was fine but I showed them in person what the battery was doing. I zeroed the meter before checking BTW. They replaced the battery and the problem was solved. I still don’t understand why it did this. What was really strange was that if I left the battery disconnected it wouldn’t drain down but if I connected both leads to the car the battery was dead the next day.

Some meters like mine have a key to zero out the reading before checking or you could start with the current being displayed and get an inaccurate reading.

I checked the battery itself. With the battery completely disconnected from the car, measuring directly across the battery posts shows 0.00 A. So the battery is good and there is no internal drain.

 

[Mercedes only]

A wire coming from Fuse 9 goes to Relay A (power seat relay), specifically to pin 2. This wire was causing the drain. When I disconnect that wire, the parasitic draw disappears and everything returns to normal.

 

[R129 UK]

A wire coming from Fuse 9 goes to Relay A (power seat relay), specifically to pin 2. This wire was causing the drain. When I disconnect that wire, the parasitic draw disappears and everything returns to normal.

Thanks for the update on the root cause.

What are you going to do next to resolve the issue?

 

[gerryvz]

That is, indeed, some nice detective work. Good job !

 

[whitehawk]

Great encouragement for me to find my minor parasitic draw - instead of continuously trickle charger use.