Slow Battery Drain How To Test and Repair 1980 300D

[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.

[Codifex Maximus]

Look for whunter's post #7 in this thread:
No charge light 1981 300sd

There is a good electrical isolation procedure.

[pimpernell]

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.

I had the same issue with my 96E300d. Battery disconnected, no problem, battery connected, dead in about 10-12 hours. Turned out to be a bad diode in the alternator. Easy to check. Disconnect all the wires to the alternator, set you multi-tester to the continuity mode, put one lead on the battery terminal of the alternator, and the other one on the case. If you have continuity, you have a bad diode that is draining off juice via the alternators mounting to the engine. Hope this helps.

[funola]

you said you got 0 amps when doing the current measurement. try reversing the polarity of your meter leads and see what kind of readings you get.

[Diesel911]

This does not solve your problem but the Battery Cutoff Switch allows you to easily disconnect your Battery and does not cost much. Cheaper at Harbor Freight than the regular auto part stores.

I had an intermittent short in my Door Lock Wires on my Volvo Diesel caused by deteriorated insulation on one of the Wires. It took a Year before I found it by accident working on the Power Windows.
In the mean time the Battery Cutoff Switch allowed Me to use the Car without totally killing My Battery.

[cullennewsom]

1. hook up an ammeter at the battery (fully charged, key off)
2. note current
3. pull 1 fuse
4. note current, compare with current in step 2.
4. a. no change in current? replace fuse and go back to step 3.
4. b. current reading change to zero? congratulations, you found the problem circuit. Troubleshoot the circuit.
4. c. current changed, but not zero? you may have more than one faulty circuit, or a non-trivial problem. Post results, maybe the forum can help more. good luck.
5. gone through all fuses with no change observed? Disconnect alternator, note current.
6. Still no change in current noted? Find electrical schematic to your vehicle and post in this thread, along with readings from the above tests.

[scottmcphee]

If you see 0.1 Amp draw that is not enough draw to kill a good vehicle battery in 1 day.

Something else is going on.

Different modules have timers in them. Like when you connect a battery for the first time, the alarm system module will draw harder, ... my car shows a "spike" of 0.25A draw or so for about 30 seconds right after connecting a battery then quiets down to about 0.050A steady state.

Now, 0.25A will kill a battery in a few days.

Maybe there is a certain "time" of your drive cycle when you park that you have a really high draw (several amps) then goes quiet to 0.1A.

You can test current draw through the negative side of the battery too. Just insert your amp meter between any line you choose to open and measure it. You can insert your meter between the fuse contacts that you choose to remove and test that.

[compu_85]

Get a low wattage test light. You could even make one yourself, just get a lamp that's a few watts and connect it between the + terminal and the car (all power has to go through the test light). If the light lights up you know something is drawing power. You can lift fuses one at a time to find where the draw is... when the light goes out you've found where the problem is.

-J

[cullennewsom]

Originally Posted by rhodes2010

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.

Assuming you've eliminated every possible load, this measurement may be indicative of a short in your wiring harness. You might want to confirm the elimination of all loads with a proper schematic before tearing into the harness.

[my123ca]

Originally Posted by rhodes2010

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

The only time i experienced this was when my glow plug relay wont shut off.

[funola]

Originally Posted by my123ca

The only time i experienced this was when my glow plug relay wont shut off.

Very easy to diagnose. Just connect a light bulb across one of the glow plugs.

[rhodes2010]

All right - THANK YOU for the excellent list of suggestions.

NEW QUESTION - on the starter, should the small wire show a connect to ground ?

I am really testing every single thing I can for resistance and the small wire on the starter
does go to ground, wire is still attached to starter. Suppose I should disconnect and check that.

Can tell you that with large wires disconnected from the starter I get a 3k ohm to ground with alternator plugged in, 100 percent resistance (no ground) when alternator unplugged. This is good.

I have been using the 300SD as a control group sort of, disconnecting and testing on that car as these two are virtually the same setup. Still not enough clear information
to blame anyone part or circuit.

Still have everything disconnected.

I may start hooking everything back up and just keep testing along the way.

[rhodes2010]

All fuses removed.
Charged battery.
Let checked to see if it starts. Waited 24 hours tested again.

Added minimum fuses to operate vehicle - low beam headlights, left front window, brake lights, turn signals.

Waited 24 hours, tested again - still starts.

Added more fuses. Aux Fan, Clock and Dash Lights, front right window.

Waited 24 hours, battery too dead to start.

Took back out the added fuses , hooked up battery charger.
Started checking current draw on those empty fuse slots.
Only two fuses appear to have current with key off.
1. Aux Fan (b I think - top right)
2. Clock/Dash number 3 - bottom left , 2nd fuse.

The Aux fan drew zero.
The clock , when ammeter across the fuse terminals,
pulsed just like the second hand would pulse.

Small charge was enough to start car.
Check voltage at battery and got 13.5 to 14 so I presume alternator is charging.

I am now thinking of pulling battery and returning for replace under warranty.
Would like to have it tested as I hate random part replacement,
prefer to find the culprit.

This car is our spare and I want to know that the car is dependable.

Any suggestions on this slow battery drain are appreciated.

[Codifex Maximus]

Continue with your diagnosis:
Put one of the two fuses back in and see what happens. If all is well with one, switch to the other fuse after taking the first one out. See what happens.

You may very well have a weak battery but... you may also have a circuit that is nominal with the key off but when you turn it on it's pulling enough current to drag down the weak battery.

[mbzjag8090]

FYI- I have only browsed this post so if this has already been suggested my apologies.

I have heard that the rear seat overhead light can frequently be the source of battery drain. Easy to find a replacement and snap it in to see if there is any change.