FAQ - Fuel Pump Circuit for full power high pressure

What sort of problems have you guys heard of people having with the jwt pump mod?

JWT fuel pump mod

I have run two systems now of the JWT modified ECU. I don't know where you guys get your information about the ECU, but they are incorrect.
The Z31 ECU provides a GROUND to the RELAY coil windings. So to hook that part up correctly, you find the wire from the ECU (pin16) this gets connected to the relay coil. The second relay coil wire gets hooked to 12 VDC or the battery. This will energize the relay when the key is turned on, and denergize the relay with the ignition off or ACC positions.
The second thiny you need to do, is first of all get a new Bosch or equilivant relay that is rated for at least 30 Amps.
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NOTE: I have tested the Nissan 20 Amp relays, and the Bosch 30 Amp relays. I did not like the results, the Nissan relay would not sustain a 20 AMP load for extended periods and not becone to hot , and I was seeing a 1 VDC drop across the relay contacts, not good. So the pump would be getting only 11 volts if the battery was at 12 volts. This is not a good option for the fuel pump. The Bosch Relay could sustain a 20 AMP load, it still did get a little hot, but manageable for extended periods, provided the spade lugs were tight.
:wink:
So you get a good #10 wire, I dont recommend #12 it may be ok to use for short runs. connect the battery up to the common lug of the relay. connect the N/O normally open lug to the fuel pump positive.
Now to overcome the modulated speed of the pump and have it run at a continious rate: remove the ground on the pump, and install a ground wire to chassis ground and the pump negative. This will overcome the modulation, and I would highly recommend this. You will need a steady flow of fuel to make tuning easier, and to ensure you have good drivability on the street. Also, fuel flowing through the rail and back to the tank is then cooled, you can think of the fuel somewhat as a good heat sink to keep the injectors cooler and the rail cooler. The ONLY reason Nissan installed a modulated pump configuration, was to decrease the noise of the pump @ idle.
That should clear that up.
The MSD box, YES it does work. I went through hell trying to get my car to flow enough fuel when I went to 4 BAR static fuel pressure with the 420cc injectors. The pump would fall on its face after 78psi and the flow would slow down to the point the pump was not able to supply enough fuel at WOT.
I opted for the Nissan twin turbo pump, which of all my testing, calling, and inquiries, turns out to be the very best pump you can get, provided you have an in tank fuel pump, or if like me had an external pump and retrofitted my pump in the tank, not fun at all, total PITA, but it works now after three revisions LOL. I was able to get 3300cc's a minute @ 90psi with that pump, and I tested it in line through the whole system.
So back to MSD, My friend is running the Bosch 944 twin turbo external pump in his car. He also had the same problems with the fuel pressure. He installed the voltage stepper box from MSD, and that solved his problems 100% so YES it works.
You know I know Clark and Jim a little better then most, and I don't consider their work hack work at all. They do a good job, of modifing, and retrofitting upgrades to the Nissan cars. I am currently working with Clark on my car to work out the bugs with the injection for a 600Hp L28 turbo application.
anyway, have a good day.

like I said, if you also connect the pump priming relay ground to the ecu's pump ground - a simple cross splice - then you'll have more grounding and full ground voltage at all times

so if your ECU throws a fuel pump circut code u can by bass the circut by doin this mod.

Got a question for you guys about this
I have it to an arming switch which I can flip on and off when ever I want
I have question tho about the fuel pump
How long before it will just crap itself getting a full 12V's? Or will it not?
Sorry I'm a newb about this and all
But rather be safe than sorry.
Because I just dont want to be driving and opps there goes my fuel :lol:
Oh and with my bypass, I did it with (2) 10gauage wire, and 4 females, and looped it over the switch and back into itself, instead of using the relay any problems would come of this does anyone know?

hey great info. one problem. i dont think words. could you make a diagram wire color and whatnot and a parts list. also wouldnt you only really want the pump at 100% at or starting near WOT? sort of ike the way the clutch safty switch is with the starter or the brake lite switch on the pedel? if so could I do a switch on the pedel for a closed circult @ WOT or make it ajustable so that when i get to the trottl point where its enough to boost switch from computer variable to full on? it would effectively be how the system should have been.

Having the pump run at 14 volts all the time is just fine. Hopefully you are seeing more than 12 volts with your car running if you have it wired on a switch. The pump running 14 volts all the time is not going to hurt anything so long as the pump always has fuel surrounding it to keep it cooler. This can be a problem if yo are running 1/4 tank of fuel or less.

my sender is f%^*ed

actual-----gague
F-----------F
------------1/2
1/2----------E
--------------
E-------------

so yes cuz the light still works! so do you thing my idea would work? i could also use the z32 pump witch would solve everything but the f^%! gave me an n/a one.

[quote]Z-day wrote: Sure you could make it work. It is just a lot of unneeded crap. You have a fuel pressure regulator for a reason. You can run the pump maxed out. Any additional fuel that is not needed will just get returned to the tank. As far as I know some pumps cannot run at less than the specified voltage. Problems arise when you do so.

thanks for you help on both posts satan

I did something similar when a drive transistor in my stand alone ecu went bad. However I used a switch as the ecu was running the pump full time. While the switch is useless if I am rubber side up and taking a nap, it does give me a nifty theft deterrent. Unless you know where the switch is and what to look for you won't start my car. well you might start it, but it will die in 2 seconds and not start again.

Some one doing your mod could add a switch and get the best of both worlds.

This has alot to do with the other question asked about running @ 22v.

My father used to repair fuel pumps and small motors throughout college to learn more about his field (ac/dc motor technician/engineer/teacher)… he's seen quite a couple of weird things. Back when wiring on some older cars or first-model vehicles were using newer fuel pump setups and new wiring techniques as science experiments from various car manufacturers, he noticed a few things…

He's told me that if your voltage drops anywhere below 50% of rated output, you're driving on the thin line between having a low voltage that will create enough heat to destroy the windings and not pump enough fluid to dissipate the heat. Whereas running a pump at over 175% rated voltage would the voltage you have to worry about on a mid-sized car with about 5 to 10 litres left (highly based on tank sump design/pump location… internal/external…yada yada).

If anyone knows about AC/DC motors, he's the guy.
Reading books about them while waiting for a quad-bypass (twice)
Dedication, I'd say.

That's neat, I was thinking about putting an inertia switch in the fuel pump circuit when I get around to putting the standalone in. One reason being for safety and also to bump it open when not running so that nobody can steal the entire car unless they are actually smart and look for the switch.

Is this mod only helpful if you have the JWT ecu, or will it help if you put in an aftermarket pump like the walbro?

Awesome thread… I shoulda came to this board to begin with! lol

Question.. I have a 24 hours of Lemons Z31 Turbo.. and I've ruined 2 sets of heads.. the last set running lean enough to eat two exhaust valves. Motor is stock. Cooling system is in excellent shape. All sensors are new except the O2.

Is this mod worthwhile in a stock application? Its lookin like I'm starvin for fuel. Fuel pressure gauge on the motor starts at 28psi (idle) and goes to 37 (turbo should hit 44 under boost) as soon as you touch the throttle, but I cant see whats happening under load (gauge at regulator).

I'm thinking a JWT regulator, and this mod would do the trick. Any thoughts?

Also looking into the CHTS mod, and simply leaving the O2 unplugged. More thoughts? lol

This car is to be raced only, never smogged.