Will not start, battery new keeps draining car sitting?

SMOORE2318

MEMBER
1991 MERCEDES BENZ 500SL
AUTOMATIC
80,000 MILES

Hello, my husband's car listed above AMG has been sitting in the driveway for over a year. We've had someone come over to start it and keep it running, but after a while it wouldn't start. There was a new battery in it, and it wouldn't take a jump. We took the battery to auto parts that should be charged and was told it was a dead cell in it. So, since the battery was less than a year old, they replaced it with a new battery. So the car has started up since, but now it won't start at all. The battery was dead after it wasn't started for a few months. We took the battery to the auto store and it did take a charge, but when we brought it home and put it back in the car it still wouldn't start. Someone told me that it might be a fuse to the battery, but I'm not sure. I'm not capable of doing anything to the car, I have a friend that's been helping me as my husband is out of town. Could you give me any kind of ideas as to what might be the problem. It was starting up fine previously but went totally dead. My husband seems to think the radio is draining the battery, so we were going to get it running and disconnect the radio as a shop installed the radio and he thinks that they did something and connecting the radio to cause the battery to drain. Any assistance you can be would be greatly appreciated. I will try to get my friend over here to try your suggestions and see if things work. Thank you so much, Sue

3 Replies

BRENDON S

MECHANIC
651 POSTS

Good morning, SMOORE2318,

Starting it every few months may not be enough to keep the battery charged. My truck sits currently, and I start it every couple weeks, let it get up to operating temperature and shut it off. This way the battery will get a good charge every couple of weeks.

Here are a couple things you can look at to try and find out if something is draining the battery.

You will need a multimeter and a test light. I have added images of both for you.

I found these for you if you don't have them. They are pretty inexpensive.

Multimeter:
https://www.amazon.com/Neoteck-Multimeter-Multimeters-Resistance-Transistor/dp/B01NAVAT9S/ref=asc_df_B01NAVAT9S?tag=bingshoppinga-20&linkCode=df0&hvadid=80882879268668&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4584482455699132&psc=1

Test light:
https://www.amazon.com/JASTIND-Automotive-Extended-Replacement-Indicator/dp/B07Q3622FT/ref=asc_df_B07Q3622FT?tag=bingshoppinga-20&linkCode=df0&hvadid=79920806880888&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583520385693266&psc=1

First start with the multimeter on the battery. Put meter on Volts DC (I have added an image of what the symbol looks like) and check the voltage on the battery. If the battery is properly charged you should see 12 volts. If not recharge the battery before moving on.

Once battery voltage is restored or confirmed, take the test light and touch the clip to negative and probe end to positive on the battery to make sure the test light works properly.

Then you will remove the negative battery cable and clip test light to it and touch the probe end to the negative terminal on the battery. If the light illuminates something is drawing power.

Next step is to leave the test light connected and pull fuses out one by one until the light shuts off. This will tell you which circuit is causing what is called a parasitic draw. Something is staying on and killing the battery. I would start with the radio fuse like your husband suggested. A bad radio installation is a common cause of this issue. A security system is another likely cause.

If the light didn't illuminate when connected between the battery negative terminal and the negative cable, then I would check the fuses using the test light.

Here are the instructions for doing this test:
https://www.2carpros.com/articles/how-to-check-a-car-fuse

If needed here are some instructions on how to use a test light and a multimeter:
https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Here is a link to a similar vehicle with a similar issue I am adding just for some extra information for you:
https://www.2carpros.com/questions/1994-mercedes-benz-500sl-sl500-will-start-there-fuse-that-controls-ignition-l

I am also adding fuse box locations and diagrams for you.

Hope this helps.

Thank you,
Brendon