Electrical short from radio? or bad battery? 300D W123

Hi All, This is my first posting on the message board. I own a 1977 300D I have come across a weird problem where my battery has been slowly dying on me. I was told that it most likely is an electrical short, I
have been going through all my electrical circuits in my car testing out to see if any of them draw power with my multi-meter. So far the only thing drawing power is my aftermarket Clarion DB345MP radio. My meter only reads up to 200mA and it reads overload whenever I have the radio hooked into the cars circuit. The car is off when I test and I put the multi meter between the cars ground cable and the negative terminal of the battery.
To prevent my cars battery from draining out I have been disconnecting the positive terminal of my battery up till today. Its an extremely slow drain, autozone says the battery is fine but when I leave the car overnight it will not crank at all.
Any suggestions? I am wondering how much power should my radio be drawing while the car is off, it does this to keep all the settings and the clock on the same time. The manual doesnt give any specifications for how much power this unit should draw while off.
Thanks!
-James
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Hood, trunk, dome, door, or glove box light will do it. Pull all the fuses, put your ammeter in series with the battery and then start reinstalling them while watching the meter. When it goes off scale you'll know the cricuit and you should be able to find th e cuprit, bulb or short.
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Hank WD5JFR
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Hood, trunk, dome, door, or glove box light will do it. Pull all the fuses, put your ammeter in series with the battery and then start reinstalling them while watching the meter. When it goes off scale you'll know the cricuit and you should be able to find th e cuprit, bulb or short.
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73
Hank WD5JFR
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If it goes dead overnight , that is not such a small drain.. Is the battery the right size for the car? I had all kinds of battery trouble with my 77 300D, but replaced both the battery with the correct size, and the battery cable, as I found the ground cable itself wasn't grounding where it was mounted on the fender wall. I grounded it right to the motor, under the breather and haven't had a problem since that was 100K miles ago. Mine started going dead overnite and by the time it was done was going dead by the time I drove to work. Jim
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All of the above are great suggestions.
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Well I did some more testing on my car. I disconnected all the fuses in my fuse box and added one-by-one as I had the multitester hooked up. The only thing drawing power is the radio, and that is to keep its settings as most radios do. The radio draws about 15mA, this is the only thing using power when the car is off. As of right now there is no short in the car and I have come to the conclusion that it is the battery. I am just going to buy another one despite what autozone says about the condition of the one I have now.
Thanks a lot guys!
-James
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Oh and I forgot to add. The battery is of the correct size, Its 1000 cranking amps and does 850 cold cranking amps. I have thought about adding a second battery though because of my experiences in the winter time. Anyone ever have a second battery on their mercedes diesel?
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I had a 78 300CD for 7 years in Denver, Colorado and only had trouble starting when it went to minus 20 deg F. With synthetic oils like Mobil1 I don't think that would be a problem. I think it would be a waste of time and money to install a second battery. Lets assume Autozone is right and the battery is OK and something is draining it since its OK when you disconnect it. Something may be turning on when you're not around. Since the drain is substantial you could put a light, or buzzer/horn in series and when it goes off you know that something came on or perhaps an intermittent short. Or get a simple 12volt DC relay and wire a remote light or buzzer inot the house. good luck
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Hank WD5JFR
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I've had great luck with wal-mart batteries,my roomates battery kept discharging overnight, so i took it in and they didn't even try to charge it, they just hooked a voltmeter to it and said it was dead and gave me a new battery, no reciept,previous owner puchased battery ,i think the trunk light was staying on.then a battery i purchased for my car died after about a year and a half,all nighttime driving with the belt loose most of the time, they put the voltmeter on it, dead, new battery. no questions. no hassle. mark
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i have heard of batts draining back through the ALT because of bad <open or closed> diodes . check the output of the ALT.
the case, minus a few cans!
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