Sunday, 6 April 2008

Matching the gauge to sender.

Today I tried to connect the fuel gauge to the van. Being referenced 73 Ohms empty/8-12 Ohms full, I thought that it would a be straight forward job
But after connecting the gauge to the sender, the needle registered nearly empty while the tank was full.
I thought that it must be possible to fix, so decided to investigate.
This is what I have done....

From last week, I knew that when the petrol tank is empty, the resistance at the sender is 70 Ohms and when the tank is full the resistance is 14 Ohms.
Connecting a resistor between the "S" connector and the ground should give me a variation of the needle.I had to figure out what resistor corresponds to the needle on position empty, and what resistor corresponded on position full.
After a few attempts I found out that with a resistor of 45.7 Ohms the needle goes on empty and with a resistor of 10 Ohms the needle goes on full.
I needed to find the relation between the tank empty and the needle on "Empty" and then the tank full with the needle on position "Full"
If I put a resistor in parallel with the resistance of the tank I will reduce the overalL resistance and I might match the gauge to the sender.
The calculation gave me a resistor of 130 Ohms. I will then have 45.5 Ohms for the empty position and 12.6 Ohms for the full position.
We can notice that 45.5 Ohms is very close to 45.7 Ohms so the needle will be exactly on the mark. While 12.6 Ohms is 1.4 Ohms away from 14 Ohms so the needle will be a bit below the full mark.

Click to zoom

The schematic above shows a little diagram which will light an LED to show that the tank is nearly empty.

Simulation of half a tank of fuel with resistor.

Gauge on empty position with resistor.

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