There are several relays involved, note the diagram. A relay has two sides, a load side, the load side is that line with arrowhead, the control side is the other side, looks like a coil. The control side of relay needs voltage and ground to energize.
With fuel pump relay on the control side, the computer controls the ground, bottom wire, but, on the control side top wire, there should be no voltage with everything off. You can use test light to check voltage and ground on those circuits.
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Re: Fuel Pump Runs Continuously - Not Relay
It could be some water in a wiring harness plug causing the relay to stay on constantly, or a short to earth in the switching side of the fuel pump circuit, but really it needs a auto electrician to have a look over it to find out why the relay is staying powered on all the time. When you plug in the relay can you feel it click on straight away? Or does the pump run regardless of whether the relay is plugged in or not? If the latter is true, check for damage to the wiring loom from the relay to the pump. Other than that, its probably auto sparky time.
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the operation of most fuel pumps is controlled by the fuel pressure regulator
when the fuel pump gets to set pressure , the pump is switched of by the ECM at the relay level
maybe nothing wrong with your system just the system working as designed
if you have pressure in the fuel manifold when you turn the ignition on then the pump will not run but will if there is a fuel leak somewhere and the pressure drops off
the best way is to have someone turn car to on to activate the fuel pump. get under car and place hand on fuel tank. if pump is working you will feel a vibration in tank. but you may have to recycle key a few times, for a fuel pump only works for a few seconds to get fuel pressure to engine. then when it starts, pump runs continuously. have a helper help you on this. if the pump does not work. check the fuse and relay for pump first. have a good day !!
3. have one person listen in the gas tank at the gas cap opening.
4. have the other person turn the ignition switch to the ON position ONLY so as the dash lights come on. Do NOT attempt to start the engine.
5. As soon as the ignition key is turned to the ON position, The person at the gas tank should hear the fuel pump run for TWO SECONDS and then it should STOP.
6. Does it ?.
If you do NOT hear it run, in most cases the fault is the fuel pump relay or the fuel pump..
As soon as you turn ON the ignition switch to start the engine a 2 second ONLY battery voltage signal is sent from the computer to the fuel pump relay and from the relay back to the fuel pump in the fuel tank. This runs the fuel pump for ONLY 2 seconds of which pressurizes the fuel system in preparation to start the engine.
Once the engine starts, The computer BYPASSES the fuel pump relay and sends constant contionous battery voltage from the computer to the oil pressure switch of which is now CLOSED because the oil pressure is UP because the engine is running. Therefore the battery voltage flows thru the oil pressure switch back to the fuel pump to run the fuel pump constantly until you turn OFF the engine.
most of fuel pumps were in tank, wire corrodes is harness under right front seat floor pan to tank
if fuel pump buzz is heard and car runs and when it doesnt run you cant hear the fuel pump try
turn ignition off wait 10 seconds
turn ignition to on position to (prime or hear pump buzz)
no buzz fuel pump not working check fuel pump relay(swap with another) and try again?
you could use test light to pump wiring and make sure your getting power to pump on this prime test no power relay or wiring problem.
power at pump, but no buzz fuel pump faulty
I would check the ground for the fuel pump relay that operates the switch side of the relay.
You should have 2 12v inputs to the relay. When the relay gets 12v input, it closes the relay switch on the heavy current side that sends 12v to the fuel pump.
A relay is needed because you don't want heavy current going through the switch.
Also, most cars only run the fuel pump for 2 seconds when the key is turned on because you don't want the fuel pump running without the car running. If you listen closely at the fuel tank, you should hear the pump run for 2 seconds when you turn the key to "ON"
When the key is turned to "START", the fuel pump should run continuously.
The engine computer tells the fuel pump to shut off if the engine rpms are zero.
So, I would check the switch side of the relay. Power amd ground.
Fuses mean short somewhere.Fuse is always live it feeds relay but in tank pump [ fuse#4or#6 i forget anyway visa versa ] is not live until this.Relay clicks once when key tuned to on and once more when key turned to crank engine it is under dash by glove box and loud enough for someone with ear at this spot to hear.In tank pump should not be live until engine starts to crank.