Fuel level and CDL3

[DJDINO]

I have a CDL3 in a Lancer Evo 6.5 and am using the original fuel level sender, I am connecting one side to an AT input on the dash the other side of the sense is connected to chassis ground, this is shared with the fuel pump also, is it OK to use this or do I have to use OV out of the dash ?

[adrian]

It is always recommended to use a 0V as the return path for a sensor signal. However as it is a fuel level sender which isn't very accurate in the first place using a chassis ground is probably not going to cause you too many issues as long as you have good ground connections.

Do you know what the resistance range of the sensor is? AT inputs have a 1k ohm pull up resistor to 5V and fuel level sensors are typically very low resistance. This means that if you wire the fuel level sensor into this input you will have a very low resolution reading.

The best way to do this is use an AV input and calculate the correct 5V pull up resistor depending on the resistance range of the sensor, also taking into account the 5V supply's maximum current rating of 0.2A.

[DJDINO]

Thanks for that Adrian, the stock sender specs are 1.8 ohms full and 65.2 ohms empty, would it be best to use a AV input ?

[adrian]

You can use either but an AV will be better. If you use an AT input you will have a voltage range of about 0.3V and the AT has a resolution of 3.66mV. So if for instance you had a 50 litre tank you would end up with a resolution of 0.6 litres.

If you use an AV input with a 330ohm pullup resistor to 5V, you get a voltage range of about 0.8V. The input resolution of the AV is 1.33mV so with a 50 litre tank you would have a resolution of 0.08 litres.

So you can see using the AV input is much more accurate but requires a bit more work to wire in the resistor. You might find that a resolution of 0.6 litres is good enough and not worth the extra effort.

[Scott@FP]

Does the CDL3 have an 8v supply?
What about using 8v pullup source (if not used for anything else) and a bit lower resistance pullup? Too much current thru the sensor? That would result in a lot more resolution from the 65 ohm to 3 ohm span.

I'd be a bit wary about getting close to the current limits of the 5v sensor supplies, the 8v has a much higher current limit and aren't used very often. Also putting a bunch of current on the 0v sensor can cause issues, going to chassis gnd won't cause a huge offset, but a bunch of current on the 0v can cause big problems with the other sensors on it.

[adrian]

The CDL3 has an 8V supply and it would be fine to use as long as the sensor can handle it. I would be a bit careful reducing the value of the pullup resistor too much. If drop down to the next E12 resistor value of 270ohms you draw about 30mA which is fine but the power dissipated by the resistor jumps up to about 0.25 Watts. This just means you would have to use a 0.5W resistor.

If you use a lower resistor value you would just have to make sure you are not drawing to much current (the 8V can supply 0.3A) and that the resistor can handle the current when the sensor is at its lowest resistance value.

[Scott@FP]

8v 270 ohm pullup would give it a near 1.5v range from 65 ohm to 3 ohm on the sensor. With the pulldown in the calc 65 ohms = 1.551v 3 ohms = .088v. Normally low resistances its not worth calculating equivalent resistance with the parallel pulldown in the input circuit but with such small steps probably worth it in this case.

Gain about 0.5v full span on the input going from a 5v pullup to an 8v pullup.

This is assuming the sensor itself can handle the higher current.

[Scott@FP]

Waitaminute, these cars have a saddle type fuel tank with two fuel level sensors wired in series and a siphon tube to keep the pump side of the saddle full. If they use the same ones as the EVO8 full will be ~110 ohms, empty ~4.0 ohms, resulting in ~1.90v span with the 8v 330 ohm pullup, which is pretty good resolution. To be able to calibrate you'll need to know total resistance when both sensors are in empty and full positions or get info from the service manual.

Also since the tank full resistance is so low, your voltage at the AV/AT will be very low (close to zero) and it will be difficult to be able to set up reliable fault conditions for the sensor, maybe add a 10 ohm resistor in series with the two fuel level sensors so that a low voltage fault condition can be set in the sensor calibration.

[DJDINO]

Thanks for that, yes the original tank is a saddle tank with 2 sender units, I have deleted this and am running an aluminium 1/2 tank with about 21L capacity using the main sender and fuel pump as lift pump so resistance will be 1.8ohms full and 65.2 ohms empty. I will try the AT input 1st and if that doesn't work I'll have to run another wire back and try the AV.

[Scott@FP]

If its an aftermarket tank just get a universal aftermarket sender that has a 'normal' 200-300 ohm range then everything works better. Or adapt another OEM unit to the pump hanger. That 63 ohm range kills the voltage span for the AT input.