How To Direct battery power to fuel pump

Mad

Well-Known Member
To increase voltage too and re earth the fuel pump.

Basically it involves running a power feed direct to the fuel pump via a 12v switching relay to increase the voltage available to the pump and at the same time improve the fuel pumps earthing points.......

It is best done if you have upgraded your fuel pump (Walbro or equivalent) and having fitted your battery in the boot to make life a little easier.

Solder all cable joints and insulate the joints well.

Run a length of 30 amp cable from the battery to a 12v switching relay (with pins 85,86 ,87,30) which can be located next to the fuel pump cover. It would also pay to fit a fuse in this power line no more than 12 inches from the battery.
Connect this power cable to pin 87 of the relay. This is the new power feed for the pump, not the relay switching feed.
Cut the stock live feed at the fuel pump plug leaving an inch or so of cable, strip this cable back and solder one end of a another length of 30 amp cable to the cable at the plug end solder the other end of this to pin 30.
Solder the other end of the cable you've just cut (the OE pump power feed) to relay pin 86 (this is the switch power feed for the relay).
Solder a short length of the 30 amp cable to pin 85 and earth this as close as possible. I think I cut the original earth feed from the pump plug and soldered it here too, then soldered a 30 amp earth cable to the fuel pump plug and earthed that to the same place as the relay.

You need to keep the original fuel pump relay in as, you're still using that feed, to switch the new relay on and off.
It should be noted that you won't get any voltage on the pump side of the relay, unless the fuel pump is priming (the 2 or 3 seconds after you turn the ignition on) or the engine is running. That may sound strange but, the fuel pump only needs to operate at these times so, that's how it works.

So, if you're trying to measure a voltage on the fuel pump side of the relay, without the ecu priming the pump or the engine running, you won't get a reading.

Steve

 
M

midnightpurple

Guest
The operating voltage of the relay is normal +- 10% anyway, its only energising the coil so nothing to worry about within the world of operatiing tolerances. If buying a relay check out the cycles/operations it can handle and the contact amp rating. At the end of the day, you pays your money, you get cheap chinese rubbish :) As mentioned before, bosch relays are the way forward. Electronics outlets have all the info you require on their tech sheets. Nothing worse than fitting it all to have the contacts burn out after a few months.....
 

williams

New Member
i bought a 20v 20a relay as i thought it would have needed to take a higher voltage, should i get a 12v 30a relay instead?
 
M

midnightpurple

Guest
Coil wont energise properly if at all so 12v it is I'm afraid
 

Fusion Ed

Active Member
Fuse by the battery is essential. This modification should be done for all cars running high power. It can make a MASSIVE difference to fuel pressure when running 400 or so bhp, and as a result cars map much nicer with a good supply to a fuel pump. (this is one mod I recommend to a lot of people)
 

Mad

Well-Known Member
Fuse by the battery is essential. This modification should be done for all cars running high power. It can make a MASSIVE difference to fuel pressure when running 400 or so bhp, and as a result cars map much nicer with a good supply to a fuel pump. (this is one mod I recommend to a lot of people)
I've said "how to" but for the record could you explain technically why it works and makes so much difference over just re earthing the pump.......please :)

Steve
 
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midnightpurple

Guest
My 2pence worth...... Skyline guys drop the earth return to the ecu completely to stop the fuel pump being stepped. At the end of the day earth is always going to take the path of least resistance so if it sees the ecu earth load and a dead short, it will take the dead short to ground but the original earth is in series with the motor and new earth still... Its been a while since my apprenticeship so feel free to put me right, and no it wasnt for automotive :) High resistance earth gives a volt drop across it, same as when you indicate and your brake lights flash as well is due to high resistance earth so does leaving the orignal earth return in matter.... more than likely... How applicable it is in this case, well I was just bored and got my oars out :) Me personally I would only have one earth as earth resistance imbalances can cause noise.... hey its all just ideas LOL Have I asked more questions than given answers....
 
M

midnightpurple

Guest
Maybe I should look at the wiring diagram and answer with more conviction..... but where is the fun in that.....
 
The existing cables were meant for the standard pump. By fitting an uprated fuel pump it obviously needs more power. As the voltage can't increase then it draws more current to get the power it needs (P=VI). If you have a shitty earth it adds more resistance to the circuit which ultimately limits the current the pump can draw. The heat is probably and issue aswell as when the cables/pump gets hot, the resistance increases and you get more of a voltage drop. Putting in your own cables means you can use a good gauge cable (one that will happily carry the maximum current the pump will draw; taking in to account the de-rating factors such as operating temps and location of cables for example).
Everyone on here laughed at me ages ago when i said my fuel pump was wired to a switch on dashboard (via relay). They all said it was dangerous as if you had an accident it would just keep pumping (as standard the ecu can cut the fuel i believe)
 
M

midnightpurple

Guest
Forgot to mention, noise imbalance is due to it, the component be it fuel pump or head unit, is still seeing the high load on the earth line. Am guessing that the ecu could also do this on the positive side if it wanted. Also going straight from the battery is going to see alternator voltage with no drops unlike going through its normal distribution route....maybe its not in the earth but purely where the supply is tapped from in this case as wiring diagram shows pump earth to deck and not ecu return so sounds like a lazy 12v in original setup...
 
M

midnightpurple

Guest
Out of morbid curiousity now, how much does the pump really draw, amps that is. And over the length of the cable in the car the resistance is going to be..... not alot and as with all jap cars the standard earth is going to be..... rubbish. I have fitted quite a few pumps over the years and changing the cable was not one of the things I did. for example, good 20awg cable will give you 10amps all day long.....
 
M

midnightpurple

Guest
do we really need to know the U value of the conductors :) I doubt it in this instance.... LOL
 
Forgot to mention, noise imbalance is due to it, the component be it fuel pump or head unit, is still seeing the high load on the earth line.
:? ay?
its not a 'noise' issue. The pump itself is an inductive load so it will generate a bit of noise if thats what you mean?. With regards to brake lights coming on when indicators do.... thats just because the current goes back through the brake light to find earth as it's own earth has higher resistance.

Also going straight from the battery is going to see alternator voltage with no drops unlike going through its normal distribution route
I totally agree with that though :thumbsup:
 
Out of morbid curiousity now, how much does the pump really draw, amps that is. And over the length of the cable in the car the resistance is going to be..... not alot and as with all jap cars the standard earth is going to be..... rubbish. I have fitted quite a few pumps over the years and changing the cable was not one of the things I did. for example, good 20awg cable will give you 10amps all day long.....
I concur lol.... i was just informing people of the things that need to be taken into account in general.
20awg cable in a tight wire loom at high temperatures wouldn't do 10 amps all day long though would it :lol:.... i see your point tho, in a short run its negligable.... but i was just saying lol.
 
M

midnightpurple

Guest
was just throwing ideas out there inbetween brews in general terms of earth impacts on circuits and not really directed to the pump itself :) should be fixing my r but waiting for bits to arrive :) I guess I didnt go too in depth as to why the brake lights flash as well but thought there was enough logic in it for people to figure it out..... or not LOL I will never be a teacher :) havent got any spare dumbells for a start.....
 
Think we were both trying to educate people here when in reality no1 probably wants to know lol. Electronics usually makes most people fall asleep lol.
 
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