Troubleshooting an Electrical Problem

Unknown battery drain, electrical shorts, blown fuses - these can be difficult to isolate!

We receive numerous emails asking for help troubleshooting electrical problems. These types of problems are very difficult to diagnose via email. Shorts and open circuits are almost impossible to diagnose remotely. Many times it can be more than one problem masking the diagnosis. Often, just replacing a part will not fix the problem as there is usually an electrical reason why the problem surfaced in the first place. On the other hand it can be something as simple as replacing a fuse!

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Problem & Solution

Common among these chassis:

Classic Sedans
Ponton
W107
W108 W109
W110 W111 W112 Sedan
W111 W112 Coupe
W113
W114
W115
W116
W123
W124
W126
W129
W140
W201
W202
W208
W210

Electrical testing tools and wiring schematics may be needed as well as a good understanding of automotive electricity.  If you lack the above, we recommend you take the car to a good auto electrician in your area (in most cases it does not need to be a Mercedes specialist). 

If you decide to tackle the problem on your own here are a few additional recommendations:

  • Use the solutions finder to search for electrical problems, parts, and repair kits. Often Kent's description of the problem includes information on diagnosis. Almost every repair kit or part we sell explains the problem that it is fixing!
  • Check out Kent’s technical YouTube video tips related to electrical issues – especially the one on finding the cause of a persistent battery drain problem.
  • If you want to learn about the tools and techniques needed to diagnose difficult electrical problems refer to Kent's video manuals on electrical troubleshooting. 
  • If you are having problems blowing fuses, first learn every circuit related to that specific fuse. They are listed inside the fuse box on the paper insert. Start disconnecting or unplugging one circuit at a time until the fuse no longer blows. That won’t fix the short but will help you isolate its root cause.
  • If your heater motor keeps blowing fuses it could mean the motor itself is drawing too much current and will need to be replaced. A good electrical repair shop can test a motor for you.
  • If the problem is you are not getting power to a component or circuit, first check with a 12 volt meter that you are getting power to the fuse at the fuse box. Make sure you are testing the correct side of the fuse holder. Just because a fuse looks ok does not mean it is making good contact. Replace the fuse to be sure. If there is no power to the fuse in question, it could be your ignition switch is bad.
  • Low battery voltage can cause all kinds of problems especially on 1986 and newer models that use various electronic computer controls. Take your car into any good auto parts store and have them test your battery under load and voltage output with everything turned on and the engine running. They will do this free of charge.
  • If you are unfamiliar with basic automotive electrical tools and testing procedures then we recommend you watch Kent's on demand video on Automotive Electrical Troubleshooting.

I know finding the problem may not be easy. That is the nature of electricity. Hope this does help.

Kent Bergsma