84 300SD Battery Drain

Anyone know how to diagnose a battery drain in a 126- 1984 300SD? I just bought this car a few months back with a new battery. I guess I know why the guy who sold it to me put a new battery in right before he sold it...... The car starts when jumped. The battery is 3 months new. I tested it with a multimeter. After sitting for the night the battery is around 10. After its jumped the battery reads 13.5. The alternator is fine as far as I can tell. Never died on me while I was driving and the battery charges while the car is running. So where do I start looking for the drain? What should I check first? Typically what is the most common drain in these models? Thanks in advance. Scott

Forums

Body and Interior Specific Aid

Chassis Model Number

W126

Use a 12 volt test light

Hi Scott, You can quickly isolate the circuit that is causing your drain with an inexpensive 12 volt test light. Remove the ground cable from your negative battery post. Clamp the wire end of the test light to the ground cable. Touch the pointed end of the tester to the negative battery post. If the lamp lights up bright, it indicates current is being drawn from the battery. (Note - If the lamp is glowing dim that is ok as most cars have a clock or radio memory that will draw very low current).

Make certain everything is turned off in the car and your doors and trunk are shut. If the lamp is still showing bright your goal is to start disconnecting circuits until the lamp goes out or just glows dim.

I recommend that you start with the alternator (common source of battery drain). Unplug the connector on the back of the alternator and retest at the battery. If the light goes dim you have found the problem. If no change move on to the glow plug relay. Remove the cover and unplug the two wire plugs at the relay. If no change at the test light move on to the fuse box.

Try to find a way to clamp the pointed end of the tester on the battery post. This will allow you to work at the fuse box and watch what is happening at the test light. Remove the cover on the fuse box and begin by removing one fuse at a time. If no change at the light, replace the fuse and remove the next one. When the lamp’s brightness goes away you will know you have found the fused circuit that is causing your drain problem. Replace the fuse. Read on the inside of cover to find which electrical components are connected to that circuit. Next go and unplug or disconnect each component until the lamp indicates you have found the source. Replace or repair the component as needed.

On the 126 300SD the most common drain culprits are the alternator, front dome light, and power antenna. Report back and let us know what you find. Kent

Thanks

I have some time tomorrow to find the cause. I will go purchase a 12 volt test light and follow your instructions. In the meantime I purchased your 126 fuse upgrade kit. I figure I might as well make sure the electrical system is upgraded while I am doing all of this. I will let you know what I find out. Thanks Scott

Update

Thanks for the instructions. I have found the fuse that was causing the drain. It was lucky #13, which controls: "diagnostic, hazard warning system, clock, central locking system, control unit memory of seat adjustment system, electronic radio" This narrows things down a bit, but I am unsure how to proceed from here. I noticed that my clock does not work, the radio works, the doors all lock, and I do not know how the control unit of the seat memory works, I have never tried it. I did have the seat taken out so that I could reupholster it, but I went back and checked all the connections to the motors that adjust the seat and everything is connected and is working properly. The car does have a factory car alarm, I am wondering if this is the problem and if so how to disable the alarm. I cannot simply leave the fuse out. When I tried starting the car without the fuse, lots of strange things began to happen such as a loud clicking sound, a muffled horn sound, my speedometer started bouncing around up to 60 mph and of course the car did not start. Any ideas on how to proceed from here? Thanks a bunch. Scott

Next step

At this point you will have to start unplugging or disconnecting one component at a time and re-testing at the fuse. I would first try unplugging the vacuum door lock pump. That is located inside your spare tire well.

If you can not find the problem easily I suggest you take it to a locate automotive electrical specialist.

One more question

Sorry Kent, I made a mistake in diagnosing the electrical problem. I installed all of the new fuses from your upgrade kit and they are a big improvement. I went through each fuse one more time and this time the #13 tested ok. It was the #15 fuse that is causing the drain. According to the fuse box #15 covers: "trunk light, roof light front and rear, illumin. vanity mirror, safety belt handover arm, door lights, autom. antenna, warning monitor term. 5" I look out every light and unpluged each socket from the wires for the trunk, doors, vanity mirror, roof light. I unplugged the antenna motor. I even unplugged the wires for the safety belt. Still showing a drain. I am positive it is the 15 fuse because when I remove the fuse the tester goes from glowing very bright to very dim. So the only thing I haven't checked (I think) is the warning monitor term. Can you tell me where that is and how I might check it? Is this common? Thanks again for your time.

battery drain

I suffered with this problem for over a year and won't bore you will all the time and money paid to mechanics for a supposed fix, all to no avail. I finally traced the problem to the same circuit as yours. It appeared to be that the trunk light would not shut off when closed. Finally my "shade tree" mechanic found the problem when his wife crawled into the trunk and opened and shut the trunk lid to find that the insulation on the wire to the trunk light at the back left hinge was cracked and it would touch metal periodically turning on my trunk light without me knowing it. It also explained why my alarm would go off inexplicably!! A little electrical tape solved the problem!! No more disppointing mornings with a "no start".