Mine is a SWB but we all have what i am going on to describe, so first of all in a shorty then the fuel pump assy is behind the seats.....

 

 

I know in a LWB there is just one assembly, which is like the one we SWB owners have in the l/h chamber, strangely though we have another and it's just a gauge read float thing so no pump and no low fuel sender on this r/h one but all this is present on the l/h one........go figure i know.

 

Pic below shows the two SWB assemblies. Clearly you can see there are two of each type because i acquired a 'set' from a Jspec series 1 1989 car and apart from a slight brown colour difference, they are no different to my series 4 car a late 1994.

 

 

In 7 years of ownership my low fuel light has never worked, i have used at least 3 different instrument clusters in this time and so i assumed the problem was not there. I have also had the fuel pump out before but couldn't just see anything wrong, a broken solder joint for instance and i also know the previous owner has written about his cluster fuel gauge and i could see evidence of 'fiddling' in the fuel pump area, wiring, etc.

 

Having fit the 89 assy and simulated low fuel my cluster gas light for the first time in a long time illuminated, thus telling me the problem was my original 94 fuel pump assembly.

 

Now lets look at the pic below and i'll point your attention to the cylinder chambers that are in the center of this shot, this and only this and what is inside triggers your dash low gas light.

 

 

Inside this chamber is this thing - a thermister and i've read up and it's a type of resistor, what happens is when it's immersed in fuel it's at a certain temperature, kept cool if you like and it's when it is NOT immersed its resistance changes and it then sends a signal to the bulb in the cluster to illuminate.

 

You can buy thermisters and realizing this one on ebay looked identical i bought one, £20 though so not cheap but since i have found tremendous value for money.

 

 

Below you can see i have popped the thermistor in its cylinder and soldered on the two wires, simples and all secured, this is my broken 94 fuel pump assy being mended btw. Red wire to the top solder point and its black one at the bottom of the pic, original thermistor wires removed of course.

 

 

I tested/simulated and my 94 assy is working perfectly and with this fuel pump known to be 100% with a new thermistor that is not 20 plus years old, this is what i have re fit, the 89 one breaker acquired will be kept on the shelf.

Edited May 9, 20196 yr by phutumsch

Great fix Andy. What was the replacement thermistor part number?

 

Essentially the circuit works by supplying +12v (whilst the ignition is on) to the bulb which then grounds through the thermistor.

Current passes through the circuit and is limited by the resistance of the thermistor which varies with temperature.

 

When the thermistor is exposed it is no longer cooled by the fuel and heats up dropping it's resistance to almost zero ohms,

inreasing the current in the circuit and the lamp comes on. When submerged the thermistor is cooled by the fuel and it's

resistance increases limiting the current flow to the point that the bulb turns off.

Great write up there Andy nice one, but what a case of over engineering by Nissan as is the case on many parts of the 300zx, why did they not just add an extra section to the track on the fuel gauge slider ?? would have worked just as well without the need for an extra power circuit and thermister, but that`s Nissan for you.

 

Note: The low resistive value of led bulbs actually causes an issue when changing them on the dash cluster as there is enough pass through the thermister even when cooled by the petrol to light up, seen many low fuel lights on for this reason even with full tanks.

 

Jeff

Edited January 21, 20178 yr by JeffTT

What was the replacement thermistor part number?

 

I have read that many low lights do not come on, you do get some forum idiots saying "my light never comes on because i don't let it run that low" !!!

 

Anyway here's the link and what you state Andrew is my take on how it all works as well and a great addition to this thread :yes:.

 

http://www.ebay.co.uk/itm/252492475091?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

 

 

Great write up there Andy nice one, but what a case of over engineering by Nissan as is the case on many parts of the 300zx, why did they not just add an extra section to the track on the fuel gauge slider ??

 

That's how i thought it worked when about a year ago i had a look at the assembly, it was only recently did i start to trace back wires did i soon realize what exactly was going on, with me having two identical assemblies at my disposal and trying to work out why one would work and one wouldn't.

 

Note: The low resistive value of led bulbs actually causes an issue when changing them on the dash cluster as there is enough pass through the thermister even when cooled by the petrol to light up, seen many low fuel lights on for this reason even with full tanks.

 

Spot on Jeff, the same seller does sell a module to allow a led bulb to be run i notice, not specifically for the 300ZX i might add, but what's the point of led in this location.

 

I have fit the oe orange filament bulb and is perfect......

 

 

Still visible when i light up my red cluster.....

 

Edited May 12, 20196 yr by phutumsch

Both my cars must suffer from this Andy ive run both of them well below the last bar on the guage and no light.

Mind you not as low as the needle on yr guage. Be interested to see how much fuel it takes to put the light out and the coresponding needle position. Think i might put a jerry can of fuel in the car and run it as low as yours and see if the light comes on

Edited January 22, 20178 yr by Iangreenfield

The reason why LEDs will stay on when simply substituted for a filament lamp in a "low fuel circuit" is as follows.

 

Most LEDs have a forward voltage drop of around 1.2V and a current flow of around 10mA to light up. When supplying 12V

to them you need to include and additional resistor to drop the 12V down to 1.2V and limit the current to 10mA. Using ohms

law the value of that additional resistor can be calculated as R = V/I where R is the resistor, V is voltage and I is current.

So to get 10.8V drop at 10mA we need a resistor of 1080 ohms. This is almost the same value as the thermistors used in

the "low fuel circuits" when submerged in fuel - 1 to 1.5K ohms - so the LED will light up.

Both my cars must suffer from this Andy ive run both of them well below the last bar on the guage and no light.

Mind you not as low as the needle on yr guage. Be interested to see how much fuel it takes to put the light out and the coresponding needle position. Think i might put a jerry can of fuel in the car and run it as low as yours and see if the light comes on

 

Don't go off the needle position in the pic Ian as the point where the symbol comes on, i have near half a tank of fuel so to make my light come on i was having to lift the assembly out of the fuel and then cup the thermistor chamber in my hand to make the light come on, hence speeding up the simulation process.

Needle reaction and low fuel symbol illumination is not immediate or instant.

To make the needle drop quicker the electrics were unplugged, done for purposes for pics for this thread.

I don't know until i road test but i will want the symbol to come on when it's on or slightly above the last notch, i think you'll run out of gas if no one as ever removed the needle before somewhere a few mm below the last notch or totally clearing it, my pictures needle is in it's unplugged state at the very bottom.

I know when i put both float senders to the 'full' position the cluster gauge was bang on the top notch, so i expect i have got the 'empty' area not far off either, it's finding that happy medium, i bet some on here when they fill to the brim the needle is above the full notch and the knock on of this is that they'll run empty above the empty notch, the needle on the spindle is on incorrectly therefore.

This however will have no bearing on the low fuel light, it'll still illuminate and therefore cannot lie but what will happen the needle position will appear higher up on the cluster's gauge.

Hope i've got that across ok.

 

Most LEDs have a forward voltage drop of around 1.2V and a current flow of around 10mA to light up. When supplying 12V

to them you need to include and additional resistor to drop the 12V down to 1.2V and limit the current to 10mA. Using ohms

law the value of that additional resistor can be calculated as R = V/I where R is the resistor, V is voltage and I is current.

So to get 10.8V drop at 10mA we need a resistor of 1080 ohms. This is almost the same value as the thermistors used in

the "low fuel circuits" when submerged in fuel - 1 to 1.5K ohms - so the LED will light up.

 

Is that in laymans terms Andrew, haha, again nice addition.

Edited January 23, 20178 yr by phutumsch

Is that in laymans terms Andrew, haha, again nice addition.

 

You're welcome!

Lets update...………

 

All the work above was done with tricking the car to thinking that I was low on fuel but now having started the car pretty much every 2 months for 2 years whilst in storage, the fuel finally ran low enough for the the fuel symbol to come on naturally :thumbup1:.

 

I knew last time the fuel level was getting low, so yesterday on firing up and driving out of the garage I pleasantly noticed the symbol was on, so I went inside to get my camera, on my return though the light was off !!!

 

Anyway, a bit more rolling about and gentle revving and back it came on, I have the needle perfectly positioned as well, looking straight on it's sitting a shade above the minimum mark :cool3:.

 

So for the first time since my ownership began in 2009 I have a working low fuel light :yes:.

 

Edited May 9, 20196 yr by phutumsch