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How do i install a fuel pump relay?


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You guys say i need a fuel pump relay. I buy a new relay and understand all the wiring to it. Crawl under the truck and find 2 wires going to my fuel pump. ! red and 1 black. OK find the ground. Use ohm meter, both go to ground.  Fire the truck up and red reads 13.9 and black reads .3.  Check black with ohm meter and it does not show ground. ?Probable goes to ground through computer??  Do not want to screw anything up!  HELP?

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Since you understand how relays work and how they are wired, I would simply say the hot from the ECM should be on one side of the relay, and the other wire should be on the ground. You already know how the other 2 wires are, so you should be good. 

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33 minutes ago, KATOOM said:

This is a diagram I got from Mike years ago.  You should be able to follow the schematics but if not just ask. :thumbup2:

lift-pump-dleay.jpg

 

   In the above diagram, the fuel pump will only work when the starter motor is being engaged and pin # 85 is energized.  What else is energizing pin #85 when the starter motor is not being used?  Also the ECM is still powering the fuel pump.

   The diagram below takes the load off the ECM/ECU and puts it on the battery/alternator.

 

FuelPumpWiring[1].png

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:think: I'm not sure what you're seeing about the fuel pump running only when the starter is engaged.....  The relay in the diagram I posted is an ON / switched OFF relay.  So when pin 85 is energized with the key in START position, the relay switches OFF and stops the signal to the fuel pump.

 

And in regards to the ECM, yes you're right.....but also wrong.  Dont forget that most aftermarket electric fuel pumps also include within their wiring loom kit a new relay which removes the voltage load from the ECM, although still retaining all ECM pump operations.  However this relay diagram I posted is merely acting as a secondary switch overriding the ECM's "control" of the fuel pump when the key is in the START position, but again retaining all the factory fuel pump functions. :thumbup2:

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I guess I'm not getting it either Katoom. In the original diagram the ecm is supplying power through a relay straight to a fuel pump. Then when you hit the key the relay opens and kills power to the fuel pump, but isn't that exactly what the ECM does anyways? 

 

I guess to make this work and take load off the ECM you'd have to run 87 from relay #1 to 86 of another relay that's powered from the battery. But at that point wouldn't it be easier to just wire like @IBMobile suggested and just use one relay? 

 

Maybe if the "power off while cranking" function of the ECM wasn't working you could wire it like the first diagram but at that point your just using the ECM as a hot in run power source. 

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It would be good to get the wire harness. It has a plug that plugs in to the ecm plug that normally is pluged into the lift pump and powers the lift pump, but that's bad thing putting a load on the ECM.

 

So the new better way the ecm only controls a small switch, hardly any load, and that switch turns on the power to the lift pump which is now powered by the battery and not thru any sensitive electronics.

 

Eric at Vulcan Performance can send you a harness with the directions. It's a cool set up.

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4 hours ago, KATOOM said:

So when pin 85 is energized with the key in START position, the relay switches OFF and stops the signal to the fuel pump.

I see what you're saying now but why do you need to delay the fuel pump when staring? 

 

4 hours ago, KATOOM said:

Dont forget that most aftermarket electric fuel pumps also include within their wiring loom kit a new relay which removes the voltage load from the ECM, although still retaining all ECM pump operations.  

I think oldbeek was just putting in a regular fuel pump relay

My Fuel Boss didn't come with a relay for the backup pump so I wired it like this.

 

transfer pump.jpg

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Also remember that relays come as constant duty cycle and intermittent or short duty cycle as well. I have been bit in the back side numerous time chasing ghostly electrical issues and found this to be the case where folks buy a relay off the shelf for constant duty use  and it winds up being a light duty relay and they have issues where an accessory works for a while and then just quits for no reason.

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Sorry to leave anyone hanging.....  Glad you see what I mean about the relay ON/OFF switch.

 

And yes, you're all correct.  This relay is ONLY for those who have an aftermarket "electric" fuel pump which pulls more amps than the OEM lift pump and already came with a relay to delegate load away from the ECM.

 

The Fuel Boss kit wouldnt use any other relay either because the assumption would be that the ECM is already doing the job just fine.  No need for another relay unless there's something going on with the ECM not properly powering the lift pump.

 

The relay I posted was simply how to stop those aftermarket fuel pumps from over pressurizing the VP during engine cranking.  If someone doesnt have one of those aftermarket relays in place then you can use IBMobile relay drawing.  But I just wouldn't remove any relay provided by FASS or Airdog. :thumbup2:

Edited by KATOOM
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   After I installed the Fuel Boss and an Airtex pump I disconnected the Airtex.  This set a p code in the ECM and when it was time for the 'smog' inspection it would have failed.  The problem was solved by having the relay (load) energized by the ECM and the main power from the PDC.  I still run with just the Fuel Boss.  The Airtex is a just incase I need it pump.  

   Not all relays are made the same.  All ways get a heavy duty high amp than needed relay.  If you have a short the fuse will go and not the relay. 

   Some of the relays I work with get very hot in their normal applications due to the amp load and after several years in service the solder will 'flow' off a terminal and open circuit.  The same thing can happen when a light duty fuel pump starts to go bad it will increase the amp load in the ECM, increase the heat and degradation of a solder joint.  This is another reason to install a fuel pump relay.     

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50 minutes ago, IBMobile said:

   After I installed the Fuel Boss and an Airtex pump I disconnected the Airtex.  This set a p code in the ECM and when it was time for the 'smog' inspection it would have failed.  The problem was solved by having the relay (load) energized by the ECM and the main power from the PDC.  I still run with just the Fuel Boss.  The Airtex is a just incase I need it pump.  

   Not all relays are made the same.  All ways get a heavy duty high amp than needed relay.  If you have a short the fuse will go and not the relay. 

   Some of the relays I work with get very hot in their normal applications due to the amp load and after several years in service the solder will 'flow' off a terminal and open circuit.  The same thing can happen when a light duty fuel pump starts to go bad it will increase the amp load in the ECM, increase the heat and degradation of a solder joint.  This is another reason to install a fuel pump relay.     

 

Exactly.  And probably why some ECM's suddenly stop powering the lift pump. :thumbup2:

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