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Fuel Pump Circuit

vifer

By vifer,
in General Maintenance

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vifer Contributor

vifer

Posted
Posted

could someone in geek speak, more or less explain to me how the fuel pump circuit works ? Is the relay meant to energize when the ignition is ON? see diag.

Im chasing a no start, no prime with a voltage seen on the relay. Relay is good, clicks when pulled to ground. Shorting pins 3 and 5 on the relay does nothing.

post-110427-0-62379600-1369746224_thumb.jpg

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vifer Contributor

vifer

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Posted (edited)

Pretty sure on a stock cicuit (i.e not hard wired or aftermarket pump) the Fuel Pump wont prime until the key is turned to "start".

I figured that much out. Whats does the initial priming the relay or the FPCM ? Cause the relay does not energize when i tun the key ON.

Check the FPCM (Fuel Pump Control Module)

That's next...

So is there power at those pins? If so, is the earth on the other end of the pump good?

Yep. All earths are good. At this stage Prob is either the pump or fpcm unless theres something else im missing.

Edited by vifer
vifer Contributor

vifer

Posted
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Posted

update. Changed the fuel pump and while it now starts. It still doenst prime. So im thinking if i ever leave the car sitting for a day and the fuel in the rail evaps, its only a matter of time before it wont start again. Is my thinking right?

vifer Contributor

vifer

Posted
  • Author
Posted

Why not hard wire the new pump (assuming walbro or bosch?) with a relay?

That way as soon as you get reds, you get power to the pump before starting.

The FPCM is there for a reason so I dont want to particularly bypass it unless its an absolute last resort. I believe the pump is meant to step down ( quiet down) when the car is idling not work its titz off.

I pity the next bloke who buys your cars finding these extra wires , relays, parallel circuits with no diagram to troubleshoot when things dont werk. lol

Kane_R32 Experienced

Kane_R32

Posted
Posted

I pity the next bloke who buys your cars finding these extra wires , relays, parallel circuits with no diagram to troubleshoot when things dont werk. lol

I think he would be quite impressed knowing that a simple fault (if there was one) can be traced through a couple of wires and a relay, rather then jumping on a forum and asking for peoples help ;)

Do a search on hard wiring a fuel pump and maybe you'll understand when you start to make more then stock power you'll appreciate constant 12v power.

vifer Contributor

vifer

Posted
  • Author
Posted

I think he would be quite impressed knowing that a simple fault (if there was one) can be traced through a couple of wires and a relay, rather then jumping on a forum and asking for peoples help ;)

As opposed to resounding a theory, (albeit proven within the right context) and not having a clue as to why they;re doing it? :rolleyes:

So now I have to search for hardwiring a fuel pump to appreciate the constant 12v power when i start making more than stock power. ic. :no:

note: I;ve solved the issue with the priming circuit and modified the FPCM to provide a good idle duty cycle without affecting power on transition. cheers for the responses.

scotty nm35 Grand Master

scotty nm35

Posted
Posted

Which pump did you install? Most aftermarket pumps will do similar things when supplied with the lower voltage the FPCM supplies at idle and cruise. They may also fail down the track (DC motors don't like being loaded up at low voltage as they burn out) which is why most of us have hardwired the power with a relay switched by the FPCM output.

If you want to run the FPCM for some reason, you will need to run either a stock pump or a Nismo/tomei.

Theory or not, modifying the FPCM sounds interesting, shame you gave us absolutely no information on what you modified... :rolleyes:

DVS JEZ Grand Master

DVS JEZ

Posted
Posted

When chasing a no start issue, if u suspect a lack of fuel supply, spray some aero start (or wd40) for 2 seconds into the intake throug a vacuum hose, then crank. If it starts it has spark etc. if it doesnt u have no spark.

Diagnosing from here, check All fuses and power at ecu, CAS etc

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