How To Check Mercedes W123 Fuses Repair Guide

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How to check Mercedes W123 Fuses
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Introduction

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.

What you need

Step 1

              How to check Mercedes W123 Fuses

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.

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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Step 2

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.

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.

Step 3

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.

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.

Step 4

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.

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.

Step 5

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.

Step 6

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

Step 7

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.

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.

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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smata671 - Feb 25, 2016

Good point about the gasket and cover!

Wes Brown - Oct 5, 2016

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!

Ronell London - Jan 19, 2017

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

analogcd - Mar 16, 2018

A couple additional points are worth adding. Special equipment fuses are labeled A, B, C and D. These are held in pairs at the ends of fuse box. Some versions do not have either or both pairs (some Euro variants and US 230 and 240D have crank windows, manual sunroofs and may not use auxiliary fan). On earlier model years, spare fuses were stored in foam rubber inserts (above and below threaded inserts). These deteriorate with age and can be equated with rubber tubing or simple cardboard sleeve. Keep two pairs of white and red spare fuses in these locations, blue spares can be kept in toolbox or glove compartment, (they fail less often). When servicing fuse box, always check the 10 mm mounting nut to be sure it is snug.

Kevin Tillett - Jun 26, 2018

Thanks for a great site- very helpful! Last week I thought I dropped a fuse under the fuse box, so I disassembled it. When I put it back together, the nut is snug, but the top plastic tabs are inside the cavity, not outside like your picture shows. How can I maneuver the fuse box in order to get the tabs out and the screw in the right place?

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Introduction

What you need

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