Originally Posted by rhodes2010

Looking for help on how to diagnose the slow battery drain on 1980 300D.
Could use some help.

After owning this car 28 years I am having a slow battery drain that is killing the new battery in 24 hrs.

Having read many posts on this forum regarding this I am starting a new one that will give a clear step by step to diagnose , locate, repair so that others may hopefully be able to use this as the final answer to this odd problem.

A 1980 123 MB does not yet have the small electrical block on the right frame like the 1982 and later cars. Keep that in mind.

Many possible causes of slow battery drain are power antenna or other accessories and the alternator. Possibly the starter or solenoid but I found
no conversations on this forum that pointed to the starter.

Also, many posts talked about the expected load on the battery from the clock and radio memory. This has been stated by some as possibly .01
milliamps, but hopefully someone can correct me on that and the proper
tools and how to use them for testing draw.

I begin:

Before the disconnecting described below I did a AMP test by disconnecting
the negative cable only, placing the positive lead of my digital volt/ohm multi meter in the 10amp connector and clipping to positive battery terminal and cable.
Then I connected the negative lead from the meter to the negative battery cable of the car.
This gave a reading of zero, so according the instructions with the meter I then moved the positive meter lead to the milliamps range and set dial to 200ma.
Now I get a reading of .1 when set to 200ma.
Disconnecting the fuses one by one and unplugging the alternator did not change this reading at all. Odd, because I thought at least the clock fuse or radio fuse would do something.


New battery, reasonably new alternator that does produce above 13 volts
with engine running. Car starts and runs, but if left for more than one day it will have a dead or near dead battery. Clock has never stopped yet.
After doing simple tests on some accessories I decided to take a disconnect everything approach.

1. Disconnect:
battery from both + and - terminals.
Power Antenna at the lead in the trunk.
Removed every fuse from fusebox under hood .
Alternator at the single plug at back
Glow Plug Relay.

At this point I would expect little to no load and infinite resistance between
the pos+ and neg- battery cables connected to the car.

But I get zero resistance using an analog (needle type) volt/ohm meter.

Now I am ready to pull the starter out completely and run some resistance tests again on the starter and the car pos and neg leads.

I suppose I should mention that the positive and negative cables were replaced more than a year ago with heavy gauge molded end cables (not the clamp on ends).

Before the testing I am now in I also removed the complete ignition switch last week to test it and look for corrosion or other problems. All looked good so I reassembled it and ruled that out.

Step 2: I do not know what to do next.

Any ideas ?
Would be curious if anyone has any advice on how to use analog and digital meters to track down the problem. And if anyone has some solid advice on the load testing and what to expect for good or bad numbers.