Upgrades 29. Stack Fuel Gauge
Onto the fuel gauge. Probably the most frustrating one of the lot, but the most rewarding. My fuel gauge was originally replaced under warranty as it originally didn't work. Great, I thought, but then comes the classic problem of the fuel gauge not really being calibrated for the sender that is using. I found that the fuel sat on full mark for about 50 miles, then dropped to half over the next 50 miles, and then suddenly to empty for the next 25 miles. This meant that after 125 miles my tank was reading empty. I then filled the tank up to find that I could get in about 22 litres of fuel. Due to the shape of the fuel tank, the sender doesn't report linear results for the guage to use, and the gauge is clearly setup for linear readings.
I have the SV model and have been told that the tank has a capacity of 41 litres when full, and that 5-6 litres of these are unusable due to the shape of the tank. This means then that I should be able to get at least 34-35 litres into the tank when it is reading empty, and not the mere 22 litres that I was getting. So with this in mind, I opted for the stack programmable gauge.
I thought it would be as simple as wiring it up, plugging it in and calibrating, but this was not the case. The initial wiring is quite straight forward. Firstly make note that because the fuel sender wire is simply a wire under resistance, we cannot wire up the original and new one together and compare the readings on both gauges, as these will introduce unwanted resistance in the circuit and cause the readings to be inaccurate. I decided that whilst getting it all to work, to allow mine to be interchangeable by allowing a wago connector to switch between the two (removing each gauge from the circuit). Once complete I removed this.
As for the water/oil temperature, the wiring is connected up the same as before. Red/white is for the lighting circuit, green is for the ignition live, two blacks are for earth (use any for each) and purple is for the fuel sender wire. I also chose to add spade connectors to the original wiring to make switching it back easy if needed.
Once wired I set about calibrating the gauge as per the instructions, firstly let's be clear of the error I made from the start:
On installation I tested the resistance of the sender with a full tank of fuel and got a reading of 480 Ohms, the gauge will error and show empty when the sender passes anything over 280 Ohms to it, this means it will not work until you get that resistance down to anything. The stack gauge manual is also clear that the resistance MUST be under 280 Ohms.
With that in mind, I returned to my physics days at school and remembered that I had to add another resistor into the circuit in parallel which will bring this resistance down. Shown below is me testing the resistance (with slightly less than a full tank), to show that it is over 280 Ohms without this trick. Also shown is the little wire I made with a resistor in it which I connected in parallel around the gauge circuit. I'll explain more below.
The trick is to add another path which "bypasses" the gauge to allow some current to also bypass the gauge. This is basically adding a resistor in parallel around the circuit. We want to drop the resistance such that when the tank is full the reading at the gauge is just below 280 Ohms (maybe even more to make the gauge readings more linear), but to start with I chose 280 Ohms as my target.
Ohms law for n resistors in parallel basically states that "Resistance = 1 / ( (1/R1) + (1/R2) + ... + (1/Rn))". So in our case we are looking for "280 = 1 / (1/480 + 1/R)", which equates to "(1/280-1/480) = 1/R", Which basically ends up with R = 672 Ohms.
I chose to use a 650 Ohm resistor in parallel which will safely bring that resistance at the gauge to just under 280 Ohms when full. This will have the effect of making the readings from the sender tend slightly more to the linear side of things, which should hopefully make the gauge a bit better to read.
The instructions are clear on how to configure the gauge so I won't go into detail but I did you a trick to avoid tyhe hassle of configuring the gauge at the pump. I simply removed the sender wire and the parallel bypass, and in it's place used a potentiometer to fake the readings. I knew that my full tank was going to be about 280 Ohms, but didn't really know what my empty reading was going to be. I decided to take a guess at about 50 Ohms to start with. When configuring the potentiometer I made sure the resistance was accurate using a multimeter.
Knowing that my readings were just an estimate at the moment, I decided that I would keep track of the actual sender readings before going to fill up the tank. My first trip to the fuel pump was after about 160 miles and I managed to get 29 litres in the tank, the resistance without the parallel bypass measured 142 Ohms (with it, 119 Ohms), my stack gauge was reading just over 1/4 at this point, my Caterham gauge when swapped in again, was on empty.
For the next tank of petrol, I let it drop to 1/8 tank, which was about 30-40 miles more than I did before, in theory this should now be actually empty. I did this with a can of petrol in the boot just in case. I took some readings and the resistance was measuring 79 Ohms without the bypass (and 70 Ohms with it). I decided that I was happy with this being the zero reading and so headed to the fuel pumps and I calibrated the gauge whilst there with this as the "Empty" value. I managed to get 33.8l in the tank, which means there was about 2 litres of usable fuel left in the tank before it did run out.
I appreciate that this is quite a quick summary, but hopefully it does show how it can be achieved, and a proper working gauge is a real treat from what I had before.
The reading shown above is much more accurate as the Caterham gauge at this point was indicating that I had a full tank still.
