Introduction

It's important to check the fuses on your W123 car, especially if you haven't in a long time or if the car is new to you.

A blown fuse can often be the culprit for what seems to be larger electrical problems.

Additionally, there are certain kinds of fuses that do not work well in the W123 fuse box and these can lead to problems later on if not replaced now.

Learn to check your fuses and replace any that need it.

Begin by locating the fuse box. It will be on the far rear driver's side corner under the hood.
  • Begin by locating the fuse box. It will be on the far rear driver's side corner under the hood.

  • It is behind the large, round brake booster.

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The fuse box cover needs to be removed to view the fuses. It is held in place by a pair of knobs that can be unscrewed by hand.
  • The fuse box cover needs to be removed to view the fuses. It is held in place by a pair of knobs that can be unscrewed by hand.

  • It can be a bit tricky to get the fuse box out past the brake booster, hood hinge, rubber hood seal, etc. Take your time.

Also check the rubber gasket on the cover -- after many years they may dry rot, allowing moisture to get into the fusebox. Also, examine the lid carefully for damage caused by removal and replacement; this can also allow moisture to get in. Both my cover and gasket were in bad shape when I checked my fuses (and 11 of 16 were of the incorrect type!). Mercedes Classic Center carries a new cover/gasket (not sold separately) for a reasonable price.

ptbowman - Reply

Now that the fuse cover is off, you can view the fuses.
  • Now that the fuse cover is off, you can view the fuses.

  • There are three kinds of fuses on this W123:

    • White fuses, 8 amp

    • Red fuses, 16 amp

    • Blue fuses, 25 amp

  • Your car may have others; check before ordering.

  • Notice the numbering on the fuse box housing. This relates to the key shown in the next step. There are also a few lower case letters, before and after the numbered section.

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Now compare the fuse box chart on the inside of the fuse box cover. Ensure that the fuses installed in each spot are the correct fuse for that location.
  • Now compare the fuse box chart on the inside of the fuse box cover. Ensure that the fuses installed in each spot are the correct fuse for that location.

  • Then, visually inspect each fuse; look for burns on the fuse, splits in the metal section in the middle of the fuse, or other signs of problems. Any questionable fuses should be replaced.

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You can use a little plastic fuse remover/installer tool like shown to pull out any questionable fuses for further inspection and replacement. Or, you can use needle nose pliers.
  • You can use a little plastic fuse remover/installer tool like shown to pull out any questionable fuses for further inspection and replacement. Or, you can use needle nose pliers.

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This fuse, while not broken or burnt, was removed for inspection. The element was bent, and the end piece was loose and crooked. It was replaced on principle. Before replacing any fuses, check the housing connection points for corrosion. If you find any you can clean it up with a brass bristle brush
  • This fuse, while not broken or burnt, was removed for inspection. The element was bent, and the end piece was loose and crooked. It was replaced on principle.

  • Before replacing any fuses, check the housing connection points for corrosion. If you find any you can clean it up with a brass bristle brush

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As noted earlier, some styles of fuses are not compatible with the W123 fuse box and should be removed if found in the box during inspection.
  • As noted earlier, some styles of fuses are not compatible with the W123 fuse box and should be removed if found in the box during inspection.

  • These fuses are those made with aluminium, rather than brass. Shown here, you can see that these have a silver color to the metal used.

  • The aluminium fuse will lead to dissimilar metal corrosion between it and the brass housing, which will lead to a poor connection and a failed circuit.

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Conclusion

When done, test the devices that are attached to the fuses you replaced to see if they work as they should.

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Nicolas Siemsen

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3 Comments

Good point about the gasket and cover!

smata671 - Reply

I can personally attest to the dissimilar metal issue: previous owner of my 300TD had recently spent several hundred dollars replacing window motors and switches and the windows still worked intermittently when I got it. I replaced the aluminum fuses and haven't had a single issue since!

Wes Brown - Reply

I had a problem with my fuses so I went and bought a test light The Laurel Of fuses tested good but the upper row of fuses I couldn't get any power can you tell me what should I do either replace my fuse box or what should I do please help thank you my email address is ronell London 803 @gmail.com

Ronell London - Reply

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