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Old 07-05-2013, 02:39 PM   #1 (permalink)
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Vehicle: 1984 280 SE
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How to bypass Fuel Pump Relay on '84 280SE M110.988

Apparently my fuel pump relay 001.545.43.05 KAE# 3.301.310 has crapped out. Is there a way to "fake-out" the system to think it's there? Jumping terminals 7 and 8 gets the pump running. It sputters then dies, so there must be more to it. I (think) I've attached a circuit diagram for the FPR. It's a 9-pin relay, with an awful lot of stuff inside the box. I've re-soldered everything twice now, and the problem went away for a while, but it's back. I'd like to be able to drive it to a repair shop, so I don't have to pay towing...
Dave '84 Euro 280SE M110.988
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1984 280SE M110-989 European Import
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Old 07-05-2013, 02:48 PM   #2 (permalink)
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When dealing with a relay, always look for terminals 87 and 30 if you need to bypass. Put a jumper between them and if that IS the problem, it should get you going.
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Old 07-05-2013, 02:50 PM   #3 (permalink)
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OOps! Sorry! It's a M110.989
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Old 07-05-2013, 03:04 PM   #4 (permalink)
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Been there, done that.
While dissecting the relay (again) it looked like pins 9 and 5 also need to be connected (microswitch circuit) whatever that's for. It has Tach signal, +12VDC, ground all where they're suppposed to be, fuel pump runs when I jump 7 and 8. Troubleshooting chart leads me to the relay NFG. But there must be a way to fake-out the system to get it to drive...
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Old 07-05-2013, 03:28 PM   #5 (permalink)
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I did the jumper between pin 7 (87) and pin 8(30) and the pump runs, but the engine just sputters but won't actually start. So obviously, something is missing from the equation. Like (87->30 + mystery connections) = (start and run).

Last edited by d1hartman; 07-05-2013 at 03:31 PM. Reason: CLARIFY
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Old 07-06-2013, 12:14 AM   #6 (permalink)
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I use a mini metro fuel pump relay 3.95p it works as good as merc one
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Old 07-06-2013, 02:28 AM   #7 (permalink)
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FPR on my SEC finally gave up the ghost last week,Ive re soldered it a couple of times
I just bought a cheap generic relay spliced it into the wires underneath the box 87/30 and a switchable power sorce and earthed it on the body
Works mint
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Old 07-06-2013, 05:12 AM   #8 (permalink)
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You dont always have to use merc bits you all need to Do is Step back and think about things I've got a 4.2 sel engine and gear box in a trike frame Volvo diff no wiring to talk about no CPU or idle valv unit and it runs lovely just the etz powering the coil
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Old 07-06-2013, 11:11 AM   #9 (permalink)
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OK so I got the pump running. But it won't actually start up. It sputters a bit at first, then nothing. It just cranks and cranks. I Could this be a fuel injection problem? Like it's getting a start up squirt, but no running squirts?
Dave
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Old 07-07-2013, 08:55 AM   #10 (permalink)
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Dave,

FWIW, I also have an '84 280SE, and when i've had M110 specific questions, I went to the W123 forum since those cars came with the M110 in the US. Bigger knowledge pool. No offense to anyone here...

Good luck with the fuel issue.
David
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