Electrical short draining '92 300D battery - PeachParts Mercedes-Benz Forum

Dieter Palmito · #1 05-01-2015, 01:13 PM

I have identified a .3 amp drain on the battery in circuit 9, controlling radio, vanity light, hazard warning system, etc. An ammeter connected between the negative pole of the battery and the removed battery cable shows a .3 amp draw until I remove the fuse for that circuit, at which time it drops to zero.

I don't know what to do next except pull the fuse and do without a few bells and whistles or remove the battery cable when the car will not be driven for several days. It takes several days to drain the battery to the point it will no longer start the car.

Is tracing the short to the particular offending device something a neophyte can tackle?

Lycoming-8 · #2 05-02-2015, 01:26 AM

I have never owned a Benz diesel newer than the ’83 300D, so the suggestions will be generic. I would think that pulling the fuse would be the more convenient choice, as long as there are no safety items on that particular one.

As far as the check-out, first off I would look over the wiring to see if you can spot any area where a wire has worn thru the insulation. This is often a bit tough because of routing and/or the bundling of wires together, but do what you can. This is often where the problem lies.

Another thought is to check the vanity light area for the same sort of rubbing failure in the fixture. The same sort of approach can be used on the other items on this fused circuit.

jay_bob · #3 05-02-2015, 12:32 PM

Go to w124-zone.com, register, and download the electrical manual for your car. Somehow this site has a cleaner copy than the one on startekinfo.com...

With the diagram in hand you should be able to find it. It is a pdf file so if you have an old tablet lying around its the perfect application for it.

Hint: find a spare turn signal style bulb socket (either from the parts store or from a junker) that has the 2 leads for the shell and the base. Put clips on the leads and go either between battery - and battery cable, or across the fuse. When the bulb goes dark you found it.

ah-kay · #4 05-02-2015, 12:53 PM

I have a 87 300D so it is not an exact match. Check your fuse box label. The circuit on fuse 9 is live all the time without the key.

Diagnostic term - I do not think it draws current when off
hazard warning system - I do not think it draws any current when off
clock - on all the time
front roof light - look up and see, or switch to off position.
radio - on all the time to keep station settings
vanity mirror light - peep behind both vanity mirrors and check the light behind it.
relay comfort terminal circuitry - do not know exactly what it is. It is only relay so the current should be minimal.

0.3A may be the correct current for the clock and radio. Are you sure you remove fuse 9 and the car starts after sitting for 2 weeks or more? It could just be a weak battery. Good luck and report back.

jay_bob · #5 05-02-2015, 01:36 PM

300 mA is a lot of draw on standby.

I just went and measured the standby drain on my 87 300TD
Same electronic clock as your 92, Becker 780 one piece radio - you have the 3 piece radio with the tuner and amp in the trunk
I do have a third party keyless entry fitted, that probably accounts for a lot of the drain.

My car measurements:
5 mA (0.005 A) at rest with all doors closed
A 50 mA pulse about every 20 seconds when the clock ticks
150-200 mA with the drivers door open (due to the flashing fasten seat belt light)

A typical car battery has about a 100 Ah capacity, that's 100 A for one hour, or 10 A for 10 h, and so on
300 mA would completely drain the battery in 333 hours or 2 weeks
Of course it's not linear, as the voltage declines the current will change, but you get the idea, that 300 mA in 100 hours (several days) would put a serious hurt on the battery, to the point it would affect your ability to start.