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#1
02-09-2014, 08:10 PM
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What makes fuel pump run when key turned on? 380sl
Same story as many with fuel problems (car won't start but runs if gas poured into throttle body). Found fuel pump was stuck (ordered one) and then figured I'd confirm voltage at the pump - which there wasn't. The relay contacts were dirty (cleaned them), so now I get 10.5v at pump terminals when cranking the engine, BUT there is no voltage when the key is first turned on.
I pulled the pump, filter, and damper. I assume the pump should run initially to pressurize the lines - seems if there was no pressure that it would run continuously. Is there a signal/something that makes the pump run initially? I hope I didn't mess something up... when the pump didn't run with pins 7 and 8 on the relay socket shorted (which sparked/paper clip got warm quick), I turned the ignition on and did same - then I could hear something run faintly, which I first thought was the pump, but it was under the hood. 
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#2
02-09-2014, 11:11 PM
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IIRC pump only runs for a couple seconds with key on if engine not turning over. Gets a tach signal to keep pump on while engine is running. I know this is how it works on my sec. This is a safety thing to keep pump off in case of accident when engine stops and fuel line is busted open. Check EPC wiring diagram for pins to jumper to keep pump on with key on.
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85 300 SD ~ 115K 82 500 SEC Euro ??K 78 450SL 164K
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#3
02-10-2014, 12:06 AM
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Chris8:
With the air cleaner removed, a safety switch plug will be found just aft and to the right of the air flow sensor plate. When that plug is pulled off, and with the key turned to ON, there should be power at the pump. Pulling the plug off simulates a deflection of the air sensor plate, as though the engine were running.
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#4
02-10-2014, 12:59 AM
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Frank, your response is that of pure Scotty! I love it!!!
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#5
02-10-2014, 02:14 AM
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There's a fuel pump relay that takes a signal from the tach such that the pump only runs when the engine is turning over. It's behind the glove box. The kickdown switch is also routed through it.
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Current Stable
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#6
02-10-2014, 06:25 AM
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Thanks for all the input.
But now I'm a bit confused.... and head is spinning from reading a 10 pg tome (over almost one year) mostly between Frank and "rfitts46" which told me it's good I found a split/disconnected vacuum line while looking for the shutoff switch (which I never found) and that I should be able to duplicate disconnecting the switch by pressing on the air flow sensor plate... which I did and the pump still didn't run with key on. So maybe the tach also has to be moving to energize the FP relay with the key on??? (I didn't chk voltage - only listened for relay click while pushing on vane). If it makes a difference I have a 12 pin relay (only 7 pins used). From schematics: MB manual shows a 6 pins. I can't see a fuel safety switch connected to FPR [is it called something else?], but does show the tach input at pin TD. Haynes manual shows the safety switch, but doesn't show a tach signal (5 pins shown [it's not labeled for what car - book covers all MBs '74-'84]). I still don't know what energizes the relay for 1-2 seconds when the key is first turned on (to pressurize the fuel lines). Now wondering if the fuel pump has to be hooked up for it to do this??? (assuming the FPR board senses there is no load and doesn't energize the relay, and that the cranking circuit is separate). So… if the FP needs to be hooked up (or something else is wrong), the relay wouldn’t try to pressurize the system initially for 1-2 secs; and if the tach needs to be spinning, the FP wouldn’t get voltage even with the safety switch defeated when ignition is on. I'm just getting ready for the new pump. I'll have to drain tank/flush lines to get new gas into fuel distributor (hopefully it's not damaged like the pump). Gas is brown and smells bad.
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#7
02-10-2014, 06:58 AM
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Quote:
Does someone have a link/pinout diagram for the 12-pin socket/7-pin FP relay? (I've only seen diagrams for jumping pin 7 to 8)
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#10
02-10-2014, 07:21 AM
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Answer
Quote:
Unless you have a low / bad battery: Voltage at the fuel relay and pump should match battery. Initial power should be present for 3 seconds, if tach signal is not detected "activating relay RUN circuit", voltage to pump cuts off. The tach signal goes through OVP to fuel relay. .
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ASE Master Mechanic asemastermechanic@juno.com Prototype R&D/testing: Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician. Noise Vibration and Harshness (NVH). Dynamometer. Heat exchanger durability. HV-A/C Climate Control. Vehicle build. Fleet Durability Technical Quality Auditor. Automotive Technical Writer 1985 300SD 1983 300D 1984 190D 2003 Volvo V70 2002 Honda Civic https://www.boldegoist.com/
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#11
02-10-2014, 09:46 AM
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Quote:
VIN begin WDBBA45 or WDB107045? That is the correct relay for the '83 US model
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#12
02-10-2014, 02:02 PM
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Chris8:
I think that mach4 and rowdie have it right in this instance; there will be either a safety switch OR a timer in the FPR. Yours apparently has the timer, but no safety switch, yes? A careful cleaning of the relay pins, and the contact sleeves in the relay socket may restore the timer function.
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#13
02-10-2014, 02:20 PM
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Quote:
There's also a thin metal sticker signed by two guys in 1983 (interesting...), with a bit of blue paint residue remaining along the edges.
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#15
02-10-2014, 10:05 PM
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Quote:
Go here. http://www.startekinfo.com/StarTek/outside/11883/PROGRAM/ETM/83etm.pdf
__________________
85 300 SD ~ 115K 82 500 SEC Euro ??K 78 450SL 164K
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