Fuel pump keeps running all the time on switch position 2. The relay I have is a five pin relay, I tried different relay still same problem, for some reason, the safety switch is not working. How does this safety switch work mechanically ?, wiring diagram show the switch is connected to the fuel pump relay. I checked the sensor plate; it does not show a big gap. Is there another relay connected to the safety switch ? Any suggestions !!

Thanks

Did you try siimply unplugging the connector to make sure the pump cuts off? In other words, did you verify that it's not a wiring short?

-CTH

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If you pull off the connector to the safety switch when the key is in pos. 2 does the fuel pump stop? Milt.

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Milterino
76 450 S.E.L.

First of all thank you guys for your reply. No the pump kept going on even when I unplugged the saftey switch.

I spend couple of hours today looking at electrical diagrams and researching this problem and I think I found the answer not only for my problem, but for those who differ on should the pump kick in on the Run position or the start position, I read various posts and I am one of those who claimed that the pump used to come on for few seconds on position 2 and then shuts off and kick in again on position 3. Here is my conclusion:

My two "second hand" Relays, at $2 each chepo me, that I had were at fault, in other words "stuck" and energized which explains the pump keeping on with or without the safety switch connector.

I tried a more expensive relay, ahim, at $3, Which worked exactly as it supposed to. The pump comes on only in start engine running and unplugging the connector turns the pump off.

For those of us who swear that the pump comes on for few seconds on position 2 and then turns off, My answer is this, I bet this happens either in the morning or after you leave the car off for a few hours, and the reason is that fuel pressure goes down which could allow the air flow sensor to open up "not in Zero" position and thus the pump will pressurize the system and bring the air flow sensor back to zero position and the saftey switch turns off the relay. Otherwise, there might be a major leak in system pressure "faulty accomulator" or check valve.

My problem now, the car is taking a little bit longer to start

BolBol: That was an interesting summary. I knew my pump did not come on in position 2 but had read others who said thiers came on in pos. 2. I just figured thiers was a different car. If your summary is correct that would explain the pump starting in pos 2. Milt.

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Milterino
76 450 S.E.L.

I "think" the electronic injection cars run the fuel pump for a few seconds when the key is turned to start. The CIS cars use the fuel accumulator to supply fuel until the engine starts.

My post, Wrenched on my 450SEL, covers my adventure in fuel pressure testing.

I used the Bosch CIS pressure tester from JC Whitney, which does not look anything like the picture.

I noticed that the control pressure rose from the 2.5 bar I was getting, (due to a dirty fuel filter or warmup regulator) rose to 3.something when I shut the car off and that it held pressure for quite awhile.

My car starts right up, or at least fires right away so I assume that my accumulator and check valve works but it sounds like yours may leak down after awhile.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120