More information on 300SD electrical drain - PeachParts Mercedes-Benz Forum

MS Fowler · #1 12-26-2006, 05:21 PM

I thought I had found the cause of the electrical drain on my 1982 300SD. I removed the fuse for the power antenna and the drain dropped to 0.02 Amps. I unplugged the power antenna lead.
The battery was dead a few days later.
More investigation....
I hooked up the amp meter from the neg battery terminal to the chasis ground. It said " 0.02 Amps" just as before. I left it on for a while and heard a dull click and the Amp Meter now read 0.48A. Pulling fuse 13 caused a click in the radio speakers. I reinserted the fuse and the draw was 0.02 for maybe 2 minutes--then the same dull click and the draw was 0.48A. There was also some indeciphereable characters on the radio display.
Does this now sound like a bad radio?
What causes the delay before it starts the 1/2 A draw?
Antenna switch position seems to have no effect.
Anybody been down this dark road before?
Mercedes didn't sub their electrical work to Lucas in 1982, did they?

yellit · #2 12-27-2006, 01:02 AM

I guess the next amp draw test would be the power leads of the radio to determine if the radio itself is causing the .5 amp draw .....Of course any lead from the radio could be the one that has the unwanted draw...but I would start with the + feed and work through the wires until I hit the wire that has the drain.....You may have to isolate all the ground connections except the one you are working on at the time to determine which ground source may be the leak....The delay click could be the radio trying to cut itself on.... (power up circuits)... but not completely.. resulting in the scrambled display due to low voltage from the incomplete switch....
The most sure test would be to put in a similar radio and see if the problem stays or goes.....Good luck...these are not fun to chase down but it looks like you are on the right path......

justinperkins · #3 12-27-2006, 01:14 AM

what kind of stereo do you have?

What's considered allowable draw? We've got the clock in the cluster, some aftermarket stereos (Sony, and maybe others) display a clock when not turned on. Maybe if you have an aftermarket amp, it's not switching off when it's supposed to. I know that a Sony stereo will turn itself on and make some beeping sounds if you don't remove the face, that could trigger other things (amp) to turn on again.

You sound pretty knowledgeable about electrical so this is all probably way beyond obvious.

MS Fowler · #4 12-27-2006, 01:22 AM

Thanks for the interest.
The radio is the oem Becker.

justinperkins · #5 12-27-2006, 01:27 AM

If it's not too much trouble, I'd pull the radio and disconnect it, then do your tests.

Don't know when Becker started putting the anti-theft in their units, but I know that on my 87 if you disconnect the battery power source (pull the fuse, disconnect the battery, etc..) it will become unusable until it is "reset" at the dealership. Could this be those whacky numbers you are seeing after you reconnected the fuse?

Without your ignition on, it seems odd that your stereo is even displaying anything (after you plugged in the fuse). Sounds bad to me.

MS Fowler · #6 12-27-2006, 03:19 PM

I removed the radio and now the draw is between 0.00 and 0.01 amps--even after 10 minutes.
It sure looks suspicious for the radio.

justinperkins · #7 12-27-2006, 03:22 PM

Quote:

Originally Posted by MS Fowler

I removed the radio and now the draw is between 0.00 and 0.01 amps--even after 10 minutes.
It sure looks suspicious for the radio.

that's good news

Johnhef · #8 12-27-2006, 03:24 PM

ditch that thing and go for an alpine

yellit · #9 12-27-2006, 03:52 PM

If you can get a schematic the problem will most likely be in the power on-off circuitry unless something down the line has shorted causing the prob.....May be as simple as a capacitor has dried out or a transistor is leaking....If you can find a schematic I can point out the most probable component failure...no garantee but sometimes you can actually see the problem if you can get the covers off and inspect the circuit board very closely....look for burned traces/components...capacitor electrolyte leakage or sometimes just something touching ground that has become loose....a lot of times you can even smell a burned area.....Some owners like to keep the car as stock as possible and this older radio should be fixable with common parts....It is at least curious enough to give it a visual once over...

MS Fowler · #10 12-27-2006, 03:58 PM

Thanks anyway, but I probably will install something that plays CDs as well as iPod compatible.

justinperkins · #11 12-27-2006, 04:13 PM

Quote:

Originally Posted by MS Fowler

Thanks anyway, but I probably will install something that plays CDs as well as iPod compatible.

All the new Sony stereos have an AUX input right in the front, makes it easy. Sure, you can't control the iPod through the stereo, but it's a more device-independent connection and much less expensive than those iPod to stereo cables.