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1988-300SEL 1989-300SE
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382 Posts
Discussion Starter #1
sometimes i wont drive my cars for several weeks. 126 and a 124 300e. i have to disconnect the neg battery cable.

what is draining the battery on these? takes several weeks.
 

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3,783 Posts
It may be normal. Might be worth your while to get a trickle charger pr battery disconnect switch... or to start up the cars and let them run for 5 or ten minutes once a week. I've been doing this since I lost my license in march and haven't had a problem. One common drain is aftermarket radios.
 

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1986/1990 W126
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14,131 Posts
Could be stereos. Some have a low drain mode you can select. But both of mine are fine for ages usually. A couple months and they start right up. Does depend on the condition and charge of the battery though, your batteries could be just old.
Funny both your cars do it. Perhaps you should get a tester and see how much drain there is.
 

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1982 500SEL; 1987 Mazda B1600; 2001 Holden Vectra 2.6L
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111 Posts
If you want to narrow it down you could try disconnecting your battery negative cable and placing a circuit light tester in line between the negative cable and the negative terminal. If it lights up you know there is a parasitic load draining your battery. Remove your fuses one at a time and refit them until you find the one that kills the light, thats the circuit that you need to work on to fix the parasitic load.

Common problems for your issue are normally the aftermarket stereos or amps (as stated), trailer plug wiring issues, aftermarket alarm/immobilisers, aftermarket central locking kits, weak battery or poor alternator circuit or belts causing poor charge for your battery. But there could be short circuits in anything else.

I would recommend you try the light test to narrow it down a bit more.
 

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1988-300SEL 1989-300SE
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382 Posts
Discussion Starter #5 (Edited)
thanks for the replies. i have four cars so they dont get driven sometimes.

as for the light test. could i just hook a meter up the same way, and watch the voltage while taking out the fuses?? why not just read across the fuses??? wouldnt the voltage show
up there?

as for the radios, the aftermarket radio in my 300e does not work, so i dont think it is the problem. i disconnected the battery, charged it back up and when i connected the neg terminal i saw spark so i know that something in the car is
pulling some current even with everything off. is there some current draw that is normal or should there be none when the car has nothing turned on in it?
 

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1988-300SEL 1989-300SE
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382 Posts
Discussion Starter #7
It may be normal. Might be worth your while to get a trickle charger pr battery disconnect switch... or to start up the cars and let them run for 5 or ten minutes once a week. I've been doing this since I lost my license in march and haven't had a problem. One common drain is aftermarket radios.
think i will try the battery disconnect switch. looked at them on the net. anywhere from $7-14. easier than R/R of the neg terminal every time i drive one of them. thanks for that suggestion.
 

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1990 560 SEL with Euro engine, 1998 SL500 Sport
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2,957 Posts
You may even have a dead cell in the battery. It could be just bad enough to not hold a full charge. I had this issue until I bought a new battery.
 

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2012 CL550 4MATIC Coupe
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9,992 Posts
Both Rusty and Greg could be right. Take your battery out and take it to a good battery shop and leave it overnight and they will test it for deep cycle hold and let you know if there are dead cells. That was the case with my battery while I was rebuilding my motor. That battery was brand new right before the motor blew. But left sitting for 18-20 months??? So I had a couple of dead cells.

If the battery is good, then do as Rusty suggested. I could not have said it better, though I too would use my meter. But his method is completely sound.
 

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84 300SD (2 ea.), 98 E320 4matic Wagon, 51 Chev 1T pickup, assorted british motorcycles
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86 Posts
I have had a similar problem but worse. The parasitic load will drain the battery to the point where it won't star the car in about 4 days. I measured the current draw with a DMM at ~450mA. I pulled all the fuses on at a time and the current draw remained so its something that isn't fused. It's not the alternator diodes, I checked. I looked at the wiring diagram and one of the things that's not fused is the seat memory unit. I couldn't really tell what else as my eyes quickly lose focus on the lousy resolution I have on the on-CD manual. Anybody else know of another unfused circuit that's easier to get to than the seat memory unit? I'd have to take the seat out to get at it.
 

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1982 500SEL; 1987 Mazda B1600; 2001 Holden Vectra 2.6L
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111 Posts
thanks for the replies. i have four cars so they dont get driven sometimes.

as for the light test. could i just hook a meter up the same way, and watch the voltage while taking out the fuses?? why not just read across the fuses??? wouldnt the voltage show
up there?

as for the radios, the aftermarket radio in my 300e does not work, so i dont think it is the problem. i disconnected the battery, charged it back up and when i connected the neg terminal i saw spark so i know that something in the car is
pulling some current even with everything off. is there some current draw that is normal or should there be none when the car has nothing turned on in it?

The point of the light test is a quick go/no go to eliminate or confirm large current draw. You can hook up a multimeter but you will have to hook it up in series (like the light) and set your meter correctly to Ammeter, voltmeters only work when connected in parallel to the circuit being tested. You would expect around 0.2A as standard draw for clocks, radios, etc. 0.3A is my personal max allowable, and 0.5A or higher will drain the batteries quite fast. During the test make sure the key is off and all accessories like interior lights are turned off or you may blow your multimeter.

And Thomas looking at what you have said I would first aim for disconnecting the alternator from the circuit even though you have looked at the diodes already, they can be funny things at times and may reverse bias with 0.4A but not with your diode tester. Then if the load is still there try disconnecting the power feed to the seat memory.
 

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216 with everything. 2002 SL500 with everything. 2009 SL500 with everything.
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One of the most common causes on the 124 and 126 is the convenience box, take it out and look under it to see if it has been getting hot and burning up.

It should not be the reg pack as the only connection is to the exciter and this is out of the circuit when the key is out. it could be an alternator diode, but this would mean the short is there all the time
 
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