Hey there --

This is a 280ce Euro from 1978

I can hear the fuel pump come on if I press on the metering plate. BUT it doesnt come on if I turn the ignition. Nor, inexplicably, can I find the relay.

It's not under the dash, nor is there an orphan end to the wiring harness.

Can anyone describe the fuel pump circuitry? Are there two circuits, so to speak, one triggered by the ignition and another triggered by demand?

I am flummoxed!!

Thank you!!

Fuse box? or rear in trunk like the w140?

__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

looking at a w123 shows relay at fuse box 1st one at top left.

__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

or kick panel drivers side.Some have fuseible links too.

__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

bm:

From your description, it seems that your pump controls include a safety switch. It is mounted in the top of the airflow meter housing, just forward of the fuel distributor, and is connected with a single wire. When the airflow meter plate is pushed down, the safety circuit is opened. Pulling the plug off the switch also opens the circuit.

As mentioned above, the pump relay is in the fuse box. With the box cover removed, push down on the airflow plate while listening for the click of the relay in the fuse box.
The relay supplies power to the pump when: 1) the engine is running, and the safety circuit is open, and 2) when the engine is being cranked by the starter motor.

what is strange, though, is the black relay is nowhere to be found. I even removed the dash to hunt for it. Its not in foot well, nor under the hood, now under the dash. I was begiining to think it could be in the concole but theres not unused wiring there either.

Plus, the fuel pump isnt coming on when the engine cranks so I have to find it somewhere or find a way to re-wire (I have another relay).

Because the fuel pump is heard to run when the airflow plate is depressed, you may conclude that the relay is operating. It may take the assistance of a helper, but by repeatedly cycling the plate up/down, while listening in various places around the car, the clicking of the relay will reveal its location.

It can hide, but it cannot run away.

yes. Thank you. Ill try that!

I can hear the ice cube relay clicking by the way

but I have also noticed that the fuel pump does not come on when the engine is cranking...