Mercedes-Benz W123 turn signal faults
W123 turn signals stay on constantly or blow the fuse: what is the fault?
Common causes are stuck contacts in the flasher relay, poor earthing of the rear lights, and wrong wiring or moisture in the connections of the towbar socket. First check the towbar socket and clean the earths and connections before replacing the relay.
A W123 turn signal flashes too fast or one signal does not work?
The relay ticks faster when a bulb does not work or does not make proper contact. Clean the bulb socket and contact surfaces, bend the socket tabs slightly and replace the old bulb with a new one. Each turn signal takes a 21W bulb of the correct size. If the fault remains, replace the relay or clean the hazard warning switch.
Where is the W123 flasher relay, and what if the hazard lights work but the turn signals do not?
In the older model the relay is by the hazard switch; in the newer one it is separate, behind the gear lever under the plastic panel of the centre console. If the hazard lights work, the fault is probably in the relay or the turn signal switch. A stuck relay can sometimes be brought back to normal by using the hazard lights. Also check the fuse of the towbar socket.