Mercedes-Benz W123 fuel pump not working
The W123 fuel pump does not run. What is wrong?
Often the fault is in the pump relay: its internal solder joint may be burnt or melted open. The relay is in front of the fuse box cover inside a tall housing, and the plastic cover is lifted off. The solder joint can be repaired and the contacts cleaned, or the relay can be replaced. The pump also gets its signal from the starter circuit, so worn start contacts can prevent the pump from working.
How do you test the W123's electric fuel pump?
Remove the relay and connect terminals 7 and 8 of the relay socket together. If the wiring and the pump are in order, the pump should run. On cars built before 8/1981, connect terminals 1 and 2. Bridge the terminals only for the test, remove the wire immediately afterwards and watch out for fuel leaks and sparks.
Should the W123's fuel pump run continuously with the ignition on?
No. The pump should run for about a second after the ignition is switched on. In older models the relay is controlled by a switch on the air flow sensor flap shaft, and later the relay has electronics.