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

I just noticed today that when I switch on the ignition, I hear the fuel pump start. I'm not sure this is right. I thought the pump only ran when the fuel
injection 'balance' plate came off its rest stop as the starter ran and air inducted (isn't there a microswitch?). I know where the main inj pump relay is in the engine bay. Does this fail?, or is the microswitch in the injection body the culprit?. Is there any history with either of these items? What 'is' correct operation.
Lots of small questions sorry.
Its a 77 280E W123 with the M110 unit.
Thanks........... Rob.
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It's (I think) supposed to start to build up the right pressure, and stop when/if the pressure is good.

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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
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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
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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.
snipped-for-privacy@aol.com (Bill Ditmire) wrote:

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Checked over the weekend.
Its the microswitch in the fuel injection body (the one operated by the baffle plate). And guess what, you have to remove the top of the thing to get it out. Nothing's ever easy is it
............. Rob.
snipped-for-privacy@aol.com (Bill Ditmire) wrote:

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