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1979 300D - 108k miles/ 2014 E250
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220 Posts
Discussion Starter #1
I put a new battery in (the old one tested bad) recently. My 300D (1979) would constantly be dead after the car sat for a few days. The new one is doing the same thing. Something is draining it down - I took out the fuse for the radio and it still died. I am wondering if the clock could be draining it?

I am not sure if I did it correctly but I think my multimeter reads .1 amp draw with everything off and the engine not running. I pulled the fuse that runs the clock (#2 I believe) and it read the same thing.

The ACC system is not working either - at all, not sure if that is related or not.

Any thoughts as to where to look next? I am no mechanic but I want to learn as much as I can.

Thanks
 

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'85 500SEL -- Gandalf, the Anthracite
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189 Posts
Electrical Gremlins

Ahhhhh! Something I can actually help with instead of being a complete information leach.

First thing to do is to find the circuit on which the drain occurs. To do this, set up your ammeter (or multimeter set to read amps). Since you say that the drain is about 100mA, set your scale accordingly. I would remove the cable from the negative terminal of the battery and hook your meter up between the battery terminal and the cable.

With this hookup, confirm that you are still reading a 100mA drain. If you are then start pulling fuses. Don't put them back but check your meter after you remove each fuse. Eventually, your drain will drop to zero or you will run out of fuses to pull. If you find the fuse which causes your draw to drop to zero, you will then need to troubleshoot that circuit depending on what you find. If the drain is constant even with all of the fuses removed, I would suspect the calibration of your meter.

Once you have identified the particular circuit causing the problem, put the other fuses back one at a time checking the meter after each one. If one of these causes the drain to reappear, then you may be looking at a wiring harness issue where there is a cross-connection between two circuits somewhere. These are harder to find.

If you get all of the fuses back except the one you identified as causing the drain to occur and there is no drain, then you will have to start troubleshooting that particular circuit only and how you do this will depend on what circuit it is on.

Good luck. Electrical Gremlins are a real pain.
 

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1979 300D - 108k miles/ 2014 E250
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220 Posts
Discussion Starter #3
Thanks Dreamtym - I was doing that backwards, was using the + side not the negative side. Will try that again and report back to you in a few days. I have been around this car since I was 9. My dad bought it in 1981 and it had 56,000 miles on it. I had a couple of 220Ds (my first car was a '69 220D) but I always wanted this 300D. Dad died a couple of years ago but I am slowly taking it over from my mom. He loved this car - I don't think it has seen a drop of rain in 15 years.

After the battery drain is resolved I am going to focus on the ACC and then drive it more.
 

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'85 500SEL -- Gandalf, the Anthracite
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189 Posts
Good point. I wouldn't expect a 100mA current draw to drain a fully charged battery in a week. It may be a bad alternator that isn't keeping the battery charged properly or a bad or loose belt.

Either of those would be an easier fix than chasing gremlins through the wiring harness.
 

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238 Posts
Problem

Youre problem sounds like the mono valve is not going to "home" position and is leaving the power to the driver under the dash powered up trying to get there. I had the same problem an disconnected the mono valve and the problem went away until I replaced it...unfortunately, you may be able to fix yours. This makes the ACC work also.
 

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1979 300D - 108k miles/ 2014 E250
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220 Posts
Discussion Starter #9
Mr. Brewer - where is the mono valve located? That would be great if I can fix both problems at once! THANKS!:thumbsup:
 

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The monovalve is located just to the right of the battery, when facing the engine bay. It's got a little electronic plug on the top of it. This is literally under 20 min fix. I might not even put on shop clothes to do one again. :thumbsup:

Dave aka Ghan
 

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1979 300D - 108k miles/ 2014 E250
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220 Posts
Discussion Starter #11
Thanks to all - will work on that tomorrow night. Found a Bosch kit for$42.44 online so I will probably replace it, but I want to take if off first and see if it is working. As far as I know it it is the original one.
 

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W123 230E, W140 S320, W202 C220
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34 Posts
What is your driving style? Lots of traffic, A/C on? WHat is the current Amps output on the Alternator.

I had the same problem on mine (Especially after a couple of days traveling in traffic with the A/C blowing). The damn Alternator output of 55Amps, was just not enough. Upgraded to 125Amps and now all SMILES!!

:)
 

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1979 300D - 108k miles/ 2014 E250
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Discussion Starter #14
I don't think it is the alternator (even though I think it is the original!); battery died in 5 days without even starting the car once.

I have about decided it is the servo - trying to find one around here (North Carolina) without going to the dealer for it. Even a used one might be fine as long as it works. Rebuilt would be preferred though.
 
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