One of the issues with any engine conversion is dealing with potential differences in fuel tank level and the fitted fuel level gauge. In my case, the classic Mini EFI fuel tank has a level sender with a resistance reading of 30 Ohms when the tank is full and 290 Ohms when it's empty but online reading revealed that most Toyotas use a range of something around 0-110 Ohms range
I first needed to confirm what the actual Toyota fuel level gauge needed so with the instrument cluster and wiring looms installed and a battery connected, I put a 200 Ohm potentiometer between the fuel level signal and ground wires at the back of the instrument cluster, simulating the fuel level sender resistance. I confirmed that the fuel level gauge worked, and confirmed the required range required by the gauge was 4 Ohms to approx 114 Ohms. I also noted that the gauge took a long time to respond to changes which means that the signal to the gauge needle is heavily dampened and any sloshing around of fuel in the tank would not have an immediate impact on the fuel level gauge.
In regards to the low fuel warning light in Toyota Starlet instrument cluster, the fuel sender in the Mini tank does not have a low fuel warning so unfortunately I won't be able to use that function.
There are kits available like the Gauge Wizard from https://www.spiyda.com/ which electronically alters the outgoing signal from the level sender but a friend of mine told me about a solution he had successfully used in his Mini engine conversion years ago - replacing the rheostat in the fuel level sender with a custom circuit board and I considered this idea for my Mini.
Making the custom fuel sender circuit board was a relatively easy process. I had to pull part the factory Mini sender assembly to expose the rheostat (wire wrapped around a plastic board) and take some measurements.
I spent some time learning a free PCB maker program (Circuitmaker) and copied the layout of the factory rheostat. The small custom board (37mm x 33mm) was designed to have twenty four 4.7 Ohm SMD resistors wired in series to get a ~5-113Ohm resistance range. It was a great way to solve the problem of mismatching ranges between the Mini and Toyota Starlet. I had a bare PCB milled and the board does not have a typical green or blue solder mask covering the board. Soldermask generally protects the copper tracks from oxidation which shouldn't be a concern within a petrol tank.
Soldering the tiny SMD resistors (2.0mm x 1.3mm each) to the circuit board was fiddly, but relatively simple using solder paste. The paste is placed on the copper pads and with each SMD resistor held in place with some tweezers, a soldering iron melts the paste to solder the component on.
Once the SMDs were soldered on, I tested the resistance values at each track to confirm the correct resistance range and make sure there weren't any short circuits across the resistors.
The PCB install was easy - I drilled a couple of holes in the backplate of the original sender assembly and mounted it with some M3 bolts through the two holes in the PCB board.
I first needed to confirm what the actual Toyota fuel level gauge needed so with the instrument cluster and wiring looms installed and a battery connected, I put a 200 Ohm potentiometer between the fuel level signal and ground wires at the back of the instrument cluster, simulating the fuel level sender resistance. I confirmed that the fuel level gauge worked, and confirmed the required range required by the gauge was 4 Ohms to approx 114 Ohms. I also noted that the gauge took a long time to respond to changes which means that the signal to the gauge needle is heavily dampened and any sloshing around of fuel in the tank would not have an immediate impact on the fuel level gauge.
In regards to the low fuel warning light in Toyota Starlet instrument cluster, the fuel sender in the Mini tank does not have a low fuel warning so unfortunately I won't be able to use that function.
There are kits available like the Gauge Wizard from https://www.spiyda.com/ which electronically alters the outgoing signal from the level sender but a friend of mine told me about a solution he had successfully used in his Mini engine conversion years ago - replacing the rheostat in the fuel level sender with a custom circuit board and I considered this idea for my Mini.
Making the custom fuel sender circuit board was a relatively easy process. I had to pull part the factory Mini sender assembly to expose the rheostat (wire wrapped around a plastic board) and take some measurements.
I spent some time learning a free PCB maker program (Circuitmaker) and copied the layout of the factory rheostat. The small custom board (37mm x 33mm) was designed to have twenty four 4.7 Ohm SMD resistors wired in series to get a ~5-113Ohm resistance range. It was a great way to solve the problem of mismatching ranges between the Mini and Toyota Starlet. I had a bare PCB milled and the board does not have a typical green or blue solder mask covering the board. Soldermask generally protects the copper tracks from oxidation which shouldn't be a concern within a petrol tank.
Soldering the tiny SMD resistors (2.0mm x 1.3mm each) to the circuit board was fiddly, but relatively simple using solder paste. The paste is placed on the copper pads and with each SMD resistor held in place with some tweezers, a soldering iron melts the paste to solder the component on.
Once the SMDs were soldered on, I tested the resistance values at each track to confirm the correct resistance range and make sure there weren't any short circuits across the resistors.
The original sender wire was also soldered directly to the circuit board (the copper pad next to the "Full" label) and reassembled with the float. I then connected the multimeter to again confirm the resistance range. The alligator clamps I have 1.9 Ohms of resistance so adjusted/corrected range pretty much spot on to match the Toyota Starlet gauge requirements of 4-114 Ohms.
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