Classic mini fuel gauge problem

[Frodgey]

My son is trying to use a fancy Drift fuel gauge for his classic mini.

It effectively is a volt meter - 100% at about 2 volts and 0% at zero.

The mini has a fuel sender that is a variable resistor - 400 ohms at full and 0 when empty - which means that the gauge is reading 100% when empty and about 40% when full.

I have never done any electronics (I taught physics as a missionary for 3 years in Africa, but that never got near anything electronic).

Do you have any suggestions that are relatively simple - and only use a +12V power supply.

He has lost the details of the gauge and we have had no reponse trying to get new ones.

Thanks in advance

 

[chuckey]

I Googled for "Drift fuel gauge", got loads of hits but no installation details. One site did mention about a convertor for resistance fuel tank units.
Without such scant details I have had a think and what I think is worth trying out is :- 1. earth the -ve terminal of the gauge. 2. connect the tank unit to +12V via a 8K resistor. 3. Wire this junction to the fuel gauges + terminal. What is now happening (in theory!) is the tank unit now has +2V on it the gauge measures this voltage as 100%. when the tank unit goes to empty, it has 0V on it and the gauge reads this as 0% full.
In the bad old days, the fuel gauge used to be fed from a dash board regulator which pulsed the battery voltage on and off but giving an average of 5V, this was to stop the fuel level on the gauge changing with the state of the battery.
Frank

 

[Frodgey]

Frank,

Thanks for the reply, there are lots of places where modern car wiring expects one thing and the mini does something else (modern cars expect to ground a signal to activate the horn, the mini supplies power).

I have decided to try to use 2 LM117 voltage regulators:



One with R2 as the fuel sender (which has the advantage of needing to be earthed) on one. The other creates a reference voltage that indicates that the tank is full. With R1 on both being pots I can adjust both so that empty is actually shown as 0% and full as 100%.

Any comments - such as the current drawn would be appreciated. R2 will range (on both) from 0 to 400 ohm and R1 will have to be around 50ohm to give the correct voltage for "empty".

 

[chuckey]

The LM 317 regulates by trying to keep 1.25 V between its Vo term and its Vadj terminal. There is also a small current (< 100 micro amps flowing from the Vadj terminal). So if your tank unit is connected between Vadj and earth, when the tank is empty, the resistance is zero, so Vo = 1.25V. So if the other LM317 is used with zero ohms its Vo should also be 1.25 V, so the meter would have 0V on it . When the tank unit is at max, its resistance is 400 ohms and we know that it must produce 1.25 (off set from other LM317) +2V (for the meter) = 3.25 V, so doing the maths, I reckon 250 ohms should be OK for R1.
Frank

 

[chuckey]

I have had a thought,the LM317 used for the 1.25V offset might not be able to sink the current flowing from the meter, a 100 ohm resistor or so as a load for it should sort it out?
Frank