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  #1  
Old 03-25-2003, 03:16 PM
carguy1953
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fuel pump relay

If you hear the fuel pump(s) running when you turn on the ignition does that indicate that the fuel pump relay is working properly? If you replace the fuel pump(s) do you need to replace the check valve? Are the check valves inside the fuel pumps?
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Old 03-25-2003, 04:49 PM
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It would help to know what year and model you're talking about. Some fuel pumps come with the check valves installed. You might check with whoever you're buying from.
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1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino
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  #3  
Old 03-25-2003, 06:47 PM
carguy1953
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the car is a 1992 190e 2.6
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Old 03-25-2003, 06:54 PM
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OK, hearing the fuel pump run is no guarantee that the fuel pump relay is fine. What symptoms are you seeing that would make you question it?
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1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino
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  #5  
Old 03-25-2003, 07:35 PM
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Hi carguy1953,

On a W126 and I assume on all others the fuel pump relay lets the pumps start in position II, just one notch before the start turn. Should you then not turn the key and start the car, the pump should stop after about a second, as there is no need to pump anymore. I guess it is part of the safety concept that lets the pump stop if you have an accident and don't want the pump to spill gasoline in a dangerous situation. There is a sensor that feeds the turning of the flywheel or crankshaft to the ignition system. No turning of engine, no fuel pumping.
You can override this relay in case of malfunction by connecting I believe pin 30 and pin 87 (all on a W126, yours might differ). Pump then gets electricity from battery bypassing the relay and will pump.

Again on a 126 ('87) the check valve is on the exit side of the pump to prevent backflow of fuel and the pump I bought came with new checkvalve installed. I didn't know either and have now 2 spare check valves.

Hope this answers your questions

Reinhard Kreutzer
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  #6  
Old 03-26-2003, 11:53 AM
carguy1953
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The car isn't getting gas to start. I changed the fuel filter first but that did not help. I have read quite a few posts on this forum that said these relays are bad about causing problems, so I am trying to narrow down the possibilities of the problem before I buy new fuel pumps without really knowing the exact cause of the problem. I would take it somewhere to have it checked out but the nearest place is a 100 miles away and I hated to pay somebody to tow it that far away.
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Old 03-26-2003, 05:27 PM
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I do not understand why 'hearing the pumps run' does not indicate that the FP relay is working properly.
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Old 03-26-2003, 06:54 PM
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Hi carguy1953,

Not starting can be a host of issues. I would do a search on that.
If you hear the pumps running, thats good. But do you get enough fuel to the injectors? You can loosen the fuel line there and see if fuel is coming out when someone else tries to start the car. Do you have spark? How did it develop? Did the car run before you changed the fuel filter and now it won't start? Are you sure there are no kinks in any lines?

Sorry I can only respond in general terms, as I don't know your car.

Good luck

Reinhard Kreutzer
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  #9  
Old 03-26-2003, 09:13 PM
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Hearing the pumps run is certainly a good thing, but that doesn't mean the fuel pump relay is good. One symptom of a bad fuel pump relay is stalling at anything but idle in a KE-Jetronic system. Obviously, the car needs to have been started for this to happen.

If you suspect a fuel pressure/delivery problem, you need to hookup a fuel pressure gauge. Anything else is just guessing.

Personally, if the car is not starting, I'd check the ignition system first. I'm assuming that you've already done that.
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1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino
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