Mercedes-Benz W123 gauges drop to zero when braking
Why do the W123 gauges and turn signals drop to zero when braking?
The most common cause is a worn fuse: the ends of the fuse have corroded even though the fuse looks intact from the outside. Braking and the turn signals load the electrics, the voltage drops and the sensitive gauges react first.
How can the fault be traced?
1. Remove the fuses and check the condition of the ends; looking at them is not enough. 2. Clean the fuses and holders; in old torpedo-type fuses the holders can be worn. 3. Check the connection of the 18-pin connector behind the instrument cluster before taking the cluster apart. The hazard lights do not run through the fuse box, so whether they work does not tell you the condition of the fuses.
What else can be the cause if the fuses are fine?
On a car with ABS the brake light switch is a two-part one, and the fault can be in its earth or in the switch itself. If the gauges do not work at all, clean the fuses and contacts in the engine bay and check the engine earth cable.