battery problems - PeachParts Mercedes-Benz Forum

gogotd · #1 08-10-2003, 09:34 PM

Its been 3 months now that I've owned my wagon. After about a month I went to start it, after it had sat for a week, and it wouldn't start and when I tested the battery it was dead. So I got a used battery and it worked fine till this week. Again it has been about a week since I ran the car and when I went to start it the new battery was dead. So I tested the battery and after a full day of charging the volt meter read dead. While I was having this battery problem I was also experiencing problems with my horn and heater. I found that the fuse to them was blown. Could there be a short in this circuit that would cause my 2 batteries to be completely dead? And if it is a short, I have no manuel for my car, so where can I get an electrical schematic for it?

One more thing my Becker tape deck plays both sides of the cassette at the same time. Any ideas how to solve that?

Thanks for all your help!
Veronica

psfred · #2 08-10-2003, 09:58 PM

Get a new battery, then connect a test light between the battery and positive lead after you replace the blown fuses. If the test lamp lights up, even dimmly, you have a "leak" somewhere -- that is, something is drawing too much current. From there, you will need to pull the fuses one at a time til the light goes out, then track down the defect in that circuit.

Don't know about the Becker tape deck, I would guess the head is stuck partway down rather than being up or down -- it has to move to align with the tape. You might try switching directions several times rapidly, this may jolt it loose and make it work.

Peter

JimSmith · #3 08-10-2003, 10:01 PM

gogotd,

I am not sure about the tape deck thing. Sounds like that might be a candidate for replacement. How does that sound, by the way? Playing both sides at the same time must mean one is playing backwards!

As for the battery issue, it may be that the charging system is not working correctly. Check the battery terminal voltage when the car is not running (12.5 or so Volts), when it is running with no accessories running (13.7 or so Volts) and then when you load up the accessories (13.2 Volts or so). If the initial battery charge is low, showing a low initial charge, the running value will be drawn down for a while as the system tries to charge the battery.

I think it is more likely if you read zero volts that there is a problem with the battery. Even two of them. Read some posts about problems people are having with batteries these days. Even Die Hards are no longer reliable.

If you are significantly low on voltage with the engine running there is likely a charging system problem. These are most often due to worn brushes on the Voltage regulator. Do a search on voltage regulators and you should get a lot of data on what to do.

As for the horn, I had a case where the horn, attached to a strut in front of the a/c condensor, had a wire chafed that was touching the strut or the condensor, can't remember which. It would blow the fuse whenever I used the horn. I fixed it with electrical tape.

Good luck, and hopefully someone else will chime in with some more specific experience to help out. Jim

P.E.Haiges · #4 08-10-2003, 10:57 PM

gogotd,

Even if you have a new battery, it will go dead if you have something that is drawing current when everything is switced off. Something like a trunk light staying on will eventually make a good battery go dead.

A test light might not be sensitive enough to detect a current leak. Better to use an ammeter. Remove one battery terminal and connect the ammeter between the battery post and the cable terminal you just removed. If the meter shows a current flo, start removing the fuses to see if any fuse removal causes the ammeter to read zero. The current leak will be in the circuit that made the ammeter read zero when that circuit was opened.

P E H

#5 08-10-2003, 11:42 PM

When you get a new battery, make sure it's not made by Excide, and it's not a Diehard (I have had so many problems with them).

psfred · #6 08-11-2003, 10:01 PM

PEH:

Radio and clock always draw some current, so getting to zero may be impossible. So do remote keyless entry, etc.

However, total draw should be less than 100mA, and that is enough to make a lamp filament glow, if only orange, not white.

The test lamp is a quick and easy way to tell, and not everyone has an ammeter of the correct range.

Peter

Brian Carlton · #7 08-11-2003, 11:36 PM

I'm with PEH on this one. If you use a test light and the bulb glows slightly, you really do not know whether you have a problem or not. It may glow slightly just from the clock and the radio. Or it may glow slightly more (200-500 ma) with a serious leak.

I had the same problem with the clock. It seemed that as the clock was going bad, it would draw excessive current and kill the battery within about five days. The draw varied between 60 and 100 ma with the needle swinging with each tick of the clock!

After disconnecting the clock, the ammeter dropped down to about 10ma, just enough to keep the radio presets

My guess is that the vehicle needs to be below 50ma to maintain the battery over a week. You can only be sure of this with an ammeter.

Brian Carlton
1984 300SD

gogotd · #8 08-12-2003, 12:53 AM

Thanks for all your replies. Now I'm off to borrow an ammeter. I'll let you know what my results are.
Veronica

gogotd · #9 08-12-2003, 12:57 AM

Jim, could you tell me where the a/c condenser is and what it looks like.

I have a lot to learn so have patience with my simple questions. Thanks for your help.
Veronica

JimSmith · #10 08-12-2003, 01:11 AM

gogotd,

The airconditioning condenser is the heat exchanger in front of the radiator. It is called the condenser because it is where the heat of compression and the heat from cooling the air in the car are rejected to the atmosphere. In the process of taking this heat away the refrigerant completes its phase change from gas to liquid. There is usually an electrically powered fan in front of this "condenser" and the horn(s) are mounted on a structural strut that the fans are also restrained by. Good luck, and I hope this helps. Jim