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  #16  
Old 02-11-2014, 10:36 AM
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Originally Posted by jimandsuzy View Post
THANKS - that's excellent. I'd already traced most FPR signals on schematic - later found that 87k went to kickdown (and see it on the link's schematic).

I put a dummy load (2 ohms) on FP terminals and cleaned all FP socket sleeves/pins (7 and 8 had much crud from arcing [now I know why it's good to use a fused jumper]), but the contacts still don't close for 1-2 seconds when key is first switched on.

I'm beginning to think this is a function of a timer on the FPR, independent of inputs - if so, the relay is bad. I remember hearing the FP run briefly when I turned the key on in the past. I wouldn't think this will stop the car from starting/running, but if it does, I'll find the cause and maybe it'll fix this.

Once again, with a meter on the FP terminals and relay contacts closed, I wasn't getting voltage back there. I thought cleaning the contacts fixed this, so did it again - it worked a few times, then didn't again... pulling on the relay/wires a bit while cranking "fixed" it. I've read about loose connections, bad harnesses, and something about a main power relay causing no voltage at FP so I'll dig into this later.

Voltage at the FP (now 9.3) is close to battery voltage (10.6) while cranking, but not equal. May have to look into this later (could be related to intermittent power at FP).

The tach signal at the FP showed 11v DC with ignition on, and 7v while cranking (scope showed ~70% duty cycle square wave), so tach signal is working.

Will post back after I drain tank, install pump, clear lines, and try to start it. May be a few days.

I've learned a lot - thanks again for the input.

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  #17  
Old 02-11-2014, 03:21 PM
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A spare FPR in your glove box is a must at this point of an SLs life.

If it fails you can replace it or if it fails you can have a part ON HAND for someone else to fix it for you.
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  #18  
Old 02-12-2014, 02:11 AM
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FP should run on demand only. Most run for 1 or 2 secs on power up and quit as the system is pressurized. You might want to pop your fuel rail and see if the pump stays on from there
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  #19  
Old 02-12-2014, 09:56 AM
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Originally Posted by TnBob View Post
FP should run on demand only. Most run for 1 or 2 secs on power up and quit as the system is pressurized. You might want to pop your fuel rail and see if the pump stays on from there
Pump runs a few seconds until the timer stops and will not run again until the tach signal says the engine is turning over. It's not the pressure that makes the pump stop.
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  #20  
Old 02-12-2014, 04:03 PM
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Originally Posted by jimandsuzy View Post
Pump runs a few seconds until the timer stops and will not run again until the tach signal says the engine is turning over. It's not the pressure that makes the pump stop.
Seems the timer would be a circuit on the FPR. From schematics, there are "tach" and "start/crank" (fm START terminal on ignition switch) inputs to the FP - either could turn on FP while cranking, but there's no other input that would do this when ignition is first turned on (like "FP on" from a control unit).

Couldn't find a schematic of this FPR, but ran across detailed info for a D-jetronic (Porsche 914) system.
The ECU

" Fuel Pump Control (FPC) - Sheet 2
... When the key is turned to the "on" position, +12V is supplied to the ECU, and the FPR is activated for 1.5 seconds..... The pulse charges C551, turning the FPR on, then C551 discharges (taking about 1.5 sec), turning the FPR off. Turning the key to the "start" position sends the Start signal to the base of T552, activating the FPR. If the car starts, and the engine speed is greater than 100 rpm, pulses from the ES pump up capacitor C551, turning T551 and T552 on, activating the FPR . If the engine speed drops below 100 rpm, there is insufficient pumping to keep the voltage on C551 high enough to keep T551 on, shutting off the FPR. "


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  #21  
Old 02-12-2014, 04:12 PM
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Originally Posted by Chris8 View Post
Seems the timer would be a circuit on the FPR. From schematics, there are "tach" and "start/crank" (fm START terminal on ignition switch) inputs to the FP - either could turn on FP while cranking, but there's no other input that would do this when ignition is first turned on (like "FP on" from a control unit).

Couldn't find a schematic of this FPR, but ran across detailed info for a D-jetronic (Porsche 914) system.

[/FONT]
Timer is inside FPR. Great write up on what happens inside Porsche unit for anyone to repair FPR. Must be the same or similar for MB. Never seen such info for MBs.
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  #22  
Old 02-12-2014, 05:08 PM
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This may help

Fuel Injection


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  #23  
Old 02-12-2014, 08:45 PM
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Thanks much - especially for the fuel injection links.

Got the fuel pump in, and as expected, it doesn't run when ignition is first turned on. The car started, ran briefly, and died. I'll dig around a bit (through those links).

I started another thread about a fuel leak I caused on the FD while trying to check pressure.
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  #24  
Old 02-19-2014, 10:56 AM
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Found the +12v at relay pin 30 (socket pin 8, directly from battery +) drops when the fuel pump stops.

I can stop/cause this by holding/moving the relay/wiring while the car is running, though can't get the voltage to stay/drop doing same while monitoring the voltage with a DVM. I've cleaned pin/socket and both look good - may be a resistive open circuit that needs a load to show up (but couldn't cause it with a 2 ohm load resistor).

Several posts say that relay solder joints go bad, and relays can be fixed by resoldering, but it will soon happen again. One said the fuel filter gets restrictive, pump works harder, draws more current/burns relay contacts, and heats joints (and recommends replacing the filter along with relay).

My pump was stuck/shorted - repeated attempts to start would have pulled a lot of current through wires/pins 7 and 8.

I want to inspect the connection at pin 8. Does anyone know how the relay socket housing comes apart? Looks like there are tabs, but I'd like to know (at least) if socket side or back cover comes off.

BTW, this won't cause the "doesn't run 1.5 seconds when key first turned" issue - that's a function of the relay. Below links a similar circuit from a 924. The 7-pin 380SL FP relay has two 555 timers, two dual flip-flops, and a smaller 2-contact relay (I started making a schematic).
924Board.org :: View topic - Fuel Pump Relay circuit diagram

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