Mercedes-Benz: Is the fuel pump supposed to run when I switch on the ignition? (123 280E)

It's (I think) supposed to start to build up the right pressure, and stop
when/if the pressure is good.

"Rob. Smith" <ac.uk> skrev i meddelandet
news:com... 

Hi Rob,

My '97 E320 does the same - humm when key is first turned to ON before
cranking engine.

Apparently normal.

How's your 280E running?

Ready for winter?

Tom

>I just noticed today that when I switch on the ignition, I hear the 

The pump should run for one second, then stop. It will then run only if the
engine is running or if the starter is engaged. .What shuts it off is a switch
in the mixture regulator that grounds the fuel pump relay when the airflow
plate is at rest.


Bill Ditmire
Ditmire Motorworks,Inc.
425 White Horse Pike
Absecon,NJ 08201
http://www.ditmire.com
609-641-3392

Many thanks

I poored over the circuit diagram and I agree that it looks like it
should work as you say.
I pulled the relay in the engine bay and checked that. It's OK. I
checked the voltages at the relay coil pins and found that with the
ignition on, there is not 12V across the coil, so the relay is off,
and the pump therefore on.
I must say, I am suprised that the ground connection from the switch
on the injection air flow plate actually has to operate the fuel pump
relay to switch off the pump. Still, it works. I'm not sure you'd call
it fail safe. Rather fail working. Maybe I'll get a shock switch off a
moder car and put it in series with the pump.

I'll have to pull the air filter to check the switch on the air flow
plate.
If that switch is faulty, is it easy to change be undoing the screw in
the middle of it? The thought of having to take the top off that air
flow chamber implies it isn't a 5 minute job.

Thanks.......... Rob.

com (Bill Ditmire) wrote: