where is my Fuel Cut-Off box/switch?

[NSX123]

I have previously asked this question, still no progress. ((NEW UPDATES Below)) Can someone please help me find a mysterious small box in my 91 NSX. (Fuel Cut-Off box/switch).

I am almost certain Hondas have them, in fact even my Range Rover & Mercedes has one. As well as all Ford’s & Chevy's. This cut-off switch looks almost the same in each of the mentioned cars...its just a mystery to figure out where its at?

Trust me, from experience it is a Good Idea to always know where this box is @ when you travel.

Can someone please help me here???

 

[DanO]

Since you seem to know about these switches, can you tell us if the switch is in-line with the fuel supply? Is it electric or manual? I’ve been over almost every inch of the NSX and I’ve never seen such a switch—but I’m just a hack mechanic. Located on the home page of this site, there is a link to the Service Manual where one can find pages that detail the fuel system. You might want to take a look at those pages to see if the mystery switch is reveled. Please don’t be upset that this forum is not too eager to help you locate a switch that does not exist.



DanO

 

[Larry Bastanza]

The fuel pump is controlled by the ECU when the car is running, and by the ignition switch upon starting. There is a relay called the "main relay" that activates the fuel pump on startup, then gives control over to the ECU once the car is running. The main relay is located by the ECU (behind the passenger seat, just towards the center of the car. The service manual on line (as DanO mentions) has it all covered.

Page 23-13 shows the physical location
Page 11-8 shows the relay schematically
Page 11-103 to 11-105 is the troubleshooting procedure

Obviously I have had a few issues with this relay.

HTH,
LarryB

 

[NSX123]

In response to DanO / Then Larry

In response to DanO / Then Larry

Yes & No...the switch works in conjunction with the fuel supply, however it is not always directly in-line with the fuel supply. (From my experience) 'Manual', but works exactly like an on/off light switch, thus its also 'Electric'. For example in my Rover this
little (black box w/ a red button) is located under the Driver seat. In the Mercedes, its in the Trunk passenger side. Honda’s near rear Taillight. The function of this small box is to cut-off the engines fuel supply in the event of a serious accident. Case in point...my own mother hit a massive pot hole and this switch did its job! Thus the local police department wanted to know where her cut-off box was to restart her car. This cut-off devise is a
DOT Government mandated requirement for most vehicles in the USA, just as it is in all
Non-DOT race cars. If the NSX does not have one i suppose that’s nothing to fuss about...

However,
i am strongly leaning towards Mr. Bastanza and his advise. As we all know the Japanese auto group does things differently than the Europeans. Thus, I am beginning to wonder if they turned a simple black box with a small red button, into an ingenious
electrical nightmare. Thank you Larry for the prompt understanding & detailed advise...I trust you have been down a similar road at some point.

Off to the Online Service Manual...Thanks Prime!

 

[DanO]

Re: In response to DanO / Then Larry

This is turning out to be an interesting discussion. If I understand correctly, we are looking for a box that might have some sort of accelerometer which opens a circuit (maybe to the fuel pump?) when a certain acceleration threshold has been achieved; after the circuit has opened, it requires a manual “push” to close the circuit. As far as I know, the early model NSX does not have such a box.

This cut-off devise is a DOT Government mandated requirement for most vehicles in the USA, just as it is in all Non-DOT race cars.

There is a NHTSA regulation that applies to ALL passenger cars (No. 571.301) which deals with fuel system integrity during a crash: http://www.fmcsa.dot.gov/rulesregs/fmcsr/regs/571.301.htm. The regulation stipulates minimum fuel leakage during and after a crash (S5.5, S5.6). There is no mention of mandatory fuel cutoff devices like you describe. For race cars, there are often class rules that require a BATTERY cutoff switch to be accessible to corner workers and fire crews from outside the car.

In the NSX, I have never experienced a situation when the fuel was cut due to excessive g forces—even after some pretty ridiculous spins on and off the track. Like I said in a previous post, there are two fuel-cut devices mounted on the top of the tank. These devices help with emissions and will close to prevent a spill if they are rotated enough, but these do not interfere with the main fuel supply in any way. The fuel pump has a check valve which helps maintain pressure in the lines when the pump is off and MAY prevent leakage in the event of an accident. As far as I know, there is no electrical or mechanical component that directly cuts engine fuel supply as a safety device in the early model NSX.

Maybe this is a device which is found on the later model NSXes? An EU requirement?

.02

DanO

 

[j14nsx]

Don't know of any such thing on UK NSXs. Almost grateful too, that I have the ability to drive back onto the track after spinning off. Unlike Boxsters which cut out. They have a timed cut out, which is just long enough to have to be towed back to pits after a spin, to add to the embarrassment. Then when back in the pits, the engine will start!

 

[Larry Bastanza]

DanO,

I agree, there is no such device in the NSX. The main relay, once switched to ECU control, MAY have some safety features, but I am not really sure how the ECU is programmed in this regard.

HTH,
LarryB

 

[DanO]

LarryB

It appears that the PGM-FI controls power to the fuel pump through the main relay but the PGM-FI module is dumb when it comes to safety inputs. From what I know, the 3 SRS system sensors (left, right and cowl) output only to the SRS system therefore the PGM-FI module has no way of knowing of these or other acceleration forces. Safety wise, the PGM-FI module can receive a reduce power request from the TCS module. The TCS uses the wheel sensors and steering wheel angle sensor to calculate yaw rate.

Seems to me that the only way to cut fuel in the event of an accident is to stall the engine by orienting the car in such a way that the pump can not pick up fuel from the tank or by stopping the car while in gear.

Interesting,

DanO

 

[Larry Bastanza]

My other thought was if the ECU stops seeing crank/cam sensor data it would know the engine stalled and probably stop fuel delivery. I just checked the service manual, page 11-24, and the ECU does shut the fuel pump off if the engine stalls

LArryB

 

[DanO]

So it looks like the engine has to stall BEFORE fuel delivery is cut, which was kind of the point in my post above. I wonder if the current NSX is designed in the same manner?



DannyO