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please help!! what sends signal to fuel pump relay to engage fuel pump????

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please help!! what sends signal to fuel pump relay to engage fuel pump????

  #1  
Old 08-22-2009, 06:42 PM
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Default please help!! what sends signal to fuel pump relay to engage fuel pump????

i know this is b5 a4, but maybe u guys can help me with me b5 s4. out of nowhere it wont start. crank and cranks, no start tho. the fuel pump relay and fuel pump are good, but what sends the switched ground signal to fuel pump relay to turn it on??? there is constant power going to all the prongs on relay, but the actual one that is switched to engage relay which is a ground, is not getting its signal????? and no power to fuel pump fuse. i jumped relay and it engaged fuel pump, but wont do it without me jumping that relay. does the ignition or ecm or what send the switched signal to that relay?? please help so i dont have to take it to the stealership. and by the way i did check for CEL lights and isnt showing any codes. thanx in advance!!!
 
  #2  
Old 08-23-2009, 02:09 AM
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the ecu is what sends the signal to the fuel pump relay, which then sends power to the fuse (#28 for my car) then to the pump itself. i hope you were testing it with either the fuse jumped or the car on, because when you turn the key on but without the engine started the fuel pump primes for about 2 seconds and shuts off. there will not be constant power to the fuel pump unless the engine is running. i dont know how different the S4s are so this may not even be applicable, but this is true for the A4s.
 
  #3  
Old 08-23-2009, 03:34 AM
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I think its true for ALL cars..

The signal comes from the ECU. You wont get power to the fuse until this relay closes. Do you hear the pump kick in when you first turn the key? You can jumper the relay to check the pump. If that works, you need to see why you not getting the signal to the relay. As said, if the engine is not turning, the fuel pump relay will not be active. If you crank for awhile w/o starting, the relay will disengage. If you have no crank or cam signal, the relay will disengage so check those. If you dont hear the pump when you jumper it, you have a dead pump.
 

Last edited by Midniteoyl; 08-23-2009 at 03:38 AM.
  #4  
Old 08-23-2009, 10:31 AM
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the pump and relay are both new and GOOD. but there is no ground signal going to that relay to tell it to engage pump. the pump isnt even priming when i turn key before starting it. the only way i can even get pump to come on is by jumping the relay. but i know relay is good because i just switched it with my b5 a4 relay and the a4 started fine with s4.s fuel pump relay. So the problem is from what sends the signal to that relay to tell it to prime pump, etc. do i need to get a vag cable and try getting codes??? i used a autozone ob2 scanner and it pulled one code, it was for the tps, but i erased that code and it never came back, and tps seemed to be working fine. is there a sensor that sends signal to relay to tell it to pump fuel?? thanx alot for help guys
 
  #5  
Old 08-23-2009, 11:41 AM
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is there a sensor that sends signal to relay to tell it to pump fuel??
no, but as he said there are sensors that tell the ecu to not pump fuel. make sure those are working. the fuel systems on these cars are pretty much mechanical only, there are no pressure or flow sensors or anything in the fuel lines that would go bad. there would be no codes directly for the fuel pump or anything- i dont even think you get a code when the pump dies, but there may be other codes for other things that somehow relate back to it (like the crank sensor or something, the tps code shouldnt be related though).
 
  #6  
Old 08-23-2009, 12:55 PM
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so should i just start these sensors??? is so which ones?? crank, cam, ?? or is there a way to test the sensors or the ecu? thanx again for the help, really trying to get this car running and not have to take it to the STEALERship to get repaired
 
  #7  
Old 08-23-2009, 01:04 PM
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You need the wiring diagram and a vag-com. Without it, you'll chase your tail all day.

Are you sure your getting power from the ignition too the relay? Do this: put a small wire on the ground side of the relay coil prong, insert it, turn the ignition on and ground the wire. The relay should click and the pump run. If not, find out why it doesnt - like ignition switch. This is where you need the wire diagram for your car/engine/year.
 
  #8  
Old 08-24-2009, 04:18 PM
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that relay has power to it and everything, just has no signal coming from the one prong that seems like the ground signal coming from the ecu. that relay kicks the fuel pump on if i have key turned and then jump the relay. still wont start tho even when im manually jumpn relay and i hear fuel pump pumping gas??? does this mean my ecu is bad since it isnt sending a signal to engage fuel pump, or is there a sensor that tells the ecu to send that signal that is not allowing it to pump fuel??? car was running fine and then just stopped!! and now wont start at all!!
 
  #9  
Old 08-24-2009, 10:37 PM
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or is there a sensor that tells the ecu to send that signal that is not allowing it to pump fuel?
as was said earlier, if the ecu isnt getting signal from the cam or crank sensors, its not going to kick the fuel pump on. you could try removing the fuel pump relay and just hotwiring it so the fuel pump is always on and see if the car starts. that would atleast tell you if this relay is your problem or if something else died and is preventing the ecu from giving you the signal you need. also check the resistance in the wire between that relay and the ecu just to make sure there isnt a break in the wire for some reason.

this may be useful to you...
http://www.mediafire.com/?ywlguyyiwky
 
  #10  
Old 08-25-2009, 04:24 PM
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i tried starting it with that relay bypassed. that made the fuel pump kick on and everything and it still wouldnt start. just cranks and cranks. ive bn told by alot of people that the crank sensor is my problem. that it is not sending signal to ecu to prime to fuel pump and whatever else it tell ecu to engage. just hope its a sensor and not ECU!!! if it is ecu, can i transfer my mtm chip to new ecu, or do i gotta get new chip also??
 

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