Fuel injection wiring strangeness

[Mistercorn]

When fitting an immobiliser I usually cut the power to the fuel pump relay coil, or to the main FI computer relay. On the NSX I am looking at doing the former. Power to the relay comes from the dash fuse box, fuse 5, and goes out on a yellow wire to connector C404. I got to the connector and buzzed the three yellow wires through the fuel pump relay to see which one provided the power. None of them buzzed through. I then decided to check that the relay was operating as expected so started the car without the relay attached. The car started and drove perfectly. So the wiring on my car has been altered at some point and a different relay location is being used. Anybody seen anything like this? I'll have to look at doing it on the main FI computer relay instead...

MC

[Kaz-kzukNA1]

Hi, Mistercorn.
What you experienced is quite normal and your wiring/relay were very likely not modified.

On our NSX, the fuel pump relay is not used for the on/off control of the fuel pump.
It is there to control the speed of the pump by enabling/bypassing the fuel pump resister.

NSX EFI software doesn’t use the fuel pressure dependent control and simply uses the mechanical pressure regulator to let the fuel pressure to always achieve 3.5kgf/cm2 higher pressure than that of intake manifold.

So, any excess fuel is simply returned to the fuel tank and since the OEM pump is designed to cope with all rpm/TH conditions including max rpm/WOT, most of the time under normal street driving conditions, there is no point in operating the fuel pump at full speed.

Hence, in order to keep long life of the fuel pump, the ECU will keep the fuel pump relay switched off and reduces the supply voltage to it with the resistor block (right side of engine bay, just above the fuel filter) and forces it into the low speed mode.

ECU will only switch on the pump relay to bypass the resistor block in order to put the pump into high speed mode under certain window such as engine start phase or above 3,800-4,000rpm with specific manifold pressure window.

So, if you had enough pressure left in the system, you will be able to start the engine even without the fuel pump relay and if you only drove around the street at moderate speed, then you probably never required the fuel pump to be in high speed mode or only for a short period that you didn’t notice the mixture imbalance when you were driving without the fuel pump relay.

For security reason, may I recommend not discussing about where and how to install immobiliser with specific instruction on the public website?

There are several ways to disable the fuel supply and since you are new to NSX, may I recommend studying the workshop manual first?
The system diagram, wiring, etc are all in there.

Kaz

[Mistercorn]

Kaz,
Thanks for that, I had looked at the wiring diagram but hadn't noticed that about the fuel pump. I intend to do the work, document it, then make it available to any other members who are interested. I won't post it on here, I'll only post what doesn't work. I have decided on what I think is probably the best way to do it, so I'll be doing that now I understand what is going on. That still doesn't explain the lack of continuity between the two ends of the yellow wire.

Thanks again for your expert knowledge and guidance.

MC

[Kaz-kzukNA1]

https://lh3.googleusercontent.com/-dwcAnJtbmAw/UdiajYIxctI/AAAAAAAAKeU/ATUmu2imh3o/s640/IMG_0078.JPG
The fuel pump relay.
The parts no. is different depending on the year model/country spec but it’s the same NO type.



https://lh3.googleusercontent.com/-h_gkCjeRWjk/UdiakLkGVCI/AAAAAAAAKec/nk31zIxMItM/s640/IMG_0080.JPG
This is from my NSX and the fuel pump relay has nothing to do with the immobiliser feature so
I'm happy to post this.

The area will look different depending on the spec/year model but it’s the one with the brown connector.
On the later model, the fuel pump relay is located at the similar position but facing sideway together with
another relay sandwiching the metal bracket but the connector and the wiring colour stayed the same.

Not sure what you meant by 'the continuity of the two ends of the yellow wire'.
I saw many aftermarket immobiliser/alarm system breaking into the wires at the fuel pump relay
because the installer misunderstood the actual feature of this relay.
You can’t stop the fuel pump by disabling this relay.

For immobiliser purpose, personally, I won’t just cut the fuel but also I will disable the starter.

Even without running the engine, you will be amazed how far you can move the car or climb steep slope by
putting the car in gear and using just the torque of the starter motor.
That’s how we were trained to move the car in emergency case if the engine stalled, the car got stuck at the rail crossing and
failed to restart the engine.


By disabling the starter circuit, it will also help in separating the main relay failure from the immobiliser issue.
If the main relay fails, the starter circuit is separated that you can still crank the engine.



https://lh4.googleusercontent.com/-yeQmUa3btUs/U4OM7cmv78I/AAAAAAAAQww/k7c86-wlU7c/s640/img_001.jpg
The wires at the fuel pump relay brown connector.

YEL, RED (or BLK), BLK/RED, BLK/YEL.

Coil side: YEL and RED (or BLK).
YEL at 12V when the IG Key is in P2/IG1/ON position. Lots of lights on the dash.
RED (or BLK) is open collector controlled by the ECU.

N.O. switched terminal: BLK/RED and BLK/YEL.
This side is connected to the high side of the fuel pump in parallel with the fuel pump resistor block.
When the relay is open, the power supply to the fuel pump is through the fuel pump resistor so it is in the low speed mode.
When it is closed (coil activated), the relay will bypass the resistor and full voltage will be applied to the pump and hence, at high speed mode with resistor block still connected in parallel.


Please note that every time when you turn the IG key into P2/IG1/On position, the fuel pump will be activated for only about 2sec.
During this 2sec period, the fuel pump relay is also activated in order to put the pump into the high speed mode.

After this 2sec period, the power source to the fuel pump is disabled and same goes to the control signal (RED wire) of the fuel pump relay.
It will stay in this state until you crank the engine.

Kaz

[Mistercorn]

All clean now Kaz. What I meant about the continuity is that the yellow wires comes from the back of the dash fuse box, but there was no continuity between the wire at the fuel pump and the connector on the fuse box.

I have sorted out how I am going to do it now. I won't be cutting the starter motor power because this system is designed for me to remotely cut the power to the car, this can quite possibly happen when the car is driving at slow speeds (<20kph). Leaving power to the starter motor would enable anybody to get themselves out of a dangerous situation as you mention above without them being able to drive off. The car has the standard immobiliser which is working fine, this is just an additional one for me to remotely track the car, listen in to the car, and turn the engine off if I want to. I should get it finished by the end of tomorrow. I will then make details available by email to anybody who is interested.

MC