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Old 11-21-2003, 12:05 AM
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HELP! Where is the fuel pump relay located???

My 96' S420 is not getting fuel. I checked the voltage by turning the motor over at the fuel pump and getting 10.5 volts. Should be getting at least 12.0, right? I have to assume the relay is going bad, HUH??? Any input will be greatly appriciated as it will have to be towed.
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Old 11-21-2003, 09:02 AM
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In the trunk just in front of the rear fuse box, behind the trunk liner. Should be all by itself.
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Old 11-21-2003, 11:21 AM
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Turn ignition switch OFF.
Remove RH side trunk liner.
Remove fuel pump relay (K27).
Install in reverse order.
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Old 11-21-2003, 01:26 PM
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What you need to remember is that the voltage at the pump should only be a few tenths of a volt less than BATTERY voltage and this will not be 12v while cranking. Battery voltage at cranking is acceptable to be as low as 9.6v, 10.5v is very acceptable.
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Old 11-21-2003, 02:29 PM
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Fuel Pump Problem

So if the voltage of 10.5 at the pump cranking the engine over is OK, would you think the pump is dead?
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Old 11-21-2003, 02:44 PM
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The fuel pump is located under the car, in the rear, on the passenger side -under the vehicle in front of rear suspension.
If you will turn the ignition key to the on position, (not starting the car) you will hear a humming nose coming from the fuel pump - if the pump is operating. If you dont' hear a humming noise, I would suspect the pump has quit. Sometimes you can "knock" on the pump to get it stated again.
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Old 11-21-2003, 02:58 PM
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The pump only hums for a couple of seconds to prime as you turn the key on, and hums fully when the engine cranks when it is working properly.
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Old 11-21-2003, 03:00 PM
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When was the last time you changed the Fuel filter?
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Old 11-21-2003, 03:16 PM
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Approx 5000 miles ago.
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Old 11-21-2003, 03:21 PM
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Is fuel pressure your problem?
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Old 11-21-2003, 03:23 PM
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You mentioned that you suspect your problem is fuel related. Can you give more detail on what symptoms the car is experiencing? Do you have random stalling? Does the problem only occur when the engine is hot or cold? Is the car only idling rough? Does the problem get better when you press the gas peddle?
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Old 11-21-2003, 03:24 PM
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Yes, no fuel pressure.
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Old 11-21-2003, 03:27 PM
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Driving down the road a few days ago, come to a stop sign & she stalls. Cranks up go and every few miles sputters & lacks power. Drove home, next day cranked up then dies and nothing since. Checked the fuel rail cranking the motor over and no fuel.
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Old 11-21-2003, 03:47 PM
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If you have fuel, you might by pass the pump relay and watch how the pump functions; best with a gauge on the fuel rail.

If its not the pump it would then have to be a restriction at the outlet screen of the tank. You can verify flow by removing the line at the pump inlet. Fuel should gush out from gravity feed - BE CAREFULL!
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Old 11-21-2003, 03:51 PM
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You can check fuel delivery by unhooking the return line place it in a container, and run the pump for 30 -40 seconds by removing the fuel pump relay and jumping pins 7 & 8. You should get about 1 liter of fuel.
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