Introduction

The glow plug systems on early Diesel W123 cars, mostly 1980 and earlier, have a glow plug relay that is underneath the dash within the cabin of the car. In these systems the fuse for the glow plug is located within a separate fuse box attached to the firewall of the car. The fuse can break if there is a short within one of the glow plugs; the fuse will need to be replaced after the glow plug is replaced. In other cases the fuse box itself can be damaged by excess heat or other conditions and so the fuse and the box will both need to be replaced. Note that on newer W123's the fuse is located inside the glow plug relay, which is located under the hood on the driver's side fender. This guide does not cover these cars.

The fuse box on early W123 cars, those with the glow plug relay inside the car and under the dash, usually 1980 and earlier, are located on the firewall back behind several of the soft rubber heater hoses. The location makes it difficult to photograph the fuse box when installed.
  • The fuse box on early W123 cars, those with the glow plug relay inside the car and under the dash, usually 1980 and earlier, are located on the firewall back behind several of the soft rubber heater hoses. The location makes it difficult to photograph the fuse box when installed.

  • To uninstall the fuse box simply locate it on the firewall and remove the two Phillips head screws that hold it to the firewall. These are the Phillips screws located on the upper right and lower left corners of the fuse box, outside of the fuse box cover.

  • Once the fuse box is removed from the firewall it can be gently pulled forward for easier access to the screws holding the wires and fuse in place inside the box. As shown, once you've pulled the box out from under the coolant hoses you can remove the two remaining Phillips head screws and then remove the fuse box and old fuse.

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With the fuse box removed it and the fuse can be inspected. This fuse box was removed because excess heat caused the fuse box plastic to melt. This allowed the copper fuse connection on the left to be pulled to the right within the body of the fuse box. Eventually the two copper sections would touch, bypassing the fuse. The old, damage box is shown on the left while the replacement box is shown on the right. See how the fuse is bent in the middle as it's left-side connection was pulled to the right.
  • With the fuse box removed it and the fuse can be inspected. This fuse box was removed because excess heat caused the fuse box plastic to melt. This allowed the copper fuse connection on the left to be pulled to the right within the body of the fuse box. Eventually the two copper sections would touch, bypassing the fuse.

  • The old, damage box is shown on the left while the replacement box is shown on the right.

  • See how the fuse is bent in the middle as it's left-side connection was pulled to the right.

  • In this case this was caused by the timer on the glow plug relay no longer functioning after the glow system on the older style glow plug system after it was converted to the fast glow plug system. The glow plugs stayed hot, with 12V going through the fuse, while testing the window regulators on this car.

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Replace the fuse box with the new one, and install a new 80 or 50 amp fuse (check to see which fuse to use, depending on year and model). Screw in the two Phillips head screws that hold the glow plugs wires to the fuse and fuse box, and then reattach the new fuse box to the firewall.
  • Replace the fuse box with the new one, and install a new 80 or 50 amp fuse (check to see which fuse to use, depending on year and model). Screw in the two Phillips head screws that hold the glow plugs wires to the fuse and fuse box, and then reattach the new fuse box to the firewall.

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Conclusion

To reassemble your device, follow these instructions in reverse order.

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Nicolas Siemsen

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One Comment

I just replaced glow plugs in my Mercedes Benz 300 CD 1979 with new ones( not pencil type). Now as soon as I turn the key glow plug fuse burnt out. What did I do wrong?

Idrejorb - Reply

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