MoTeC Global Forum

Hey Guys, I know this is a really simple question but I got a car with an SDL3 in it so I was kind of thrown into the Fire.

What im trying to do is use an Existing AV input and adapt it for a fuel level sender that is resistance based. What use to be in that input was a Damper pointometer.

So my question is how would I wire and Program this 0-90 Ohms sensor to work on this channel? What pull up resistor should I have? Currently it is a 3 wire Plug with 0v, signal and 5v.

thanks guys!

With fuel sensors like this one that have very low resistance you have to balance the resolution you will get from the pin with the maximum current you can draw from the 5V rail. A very low resistance pull up resistor will give you better resolution but will draw more current. The 5V rail on the SDL3 is rated at 0.2A so you probably don't want to go over around 0.02A (20mA) to allow overhead for other sensors etc.

I would use something like a 270 or 330 ohm resistor as the pull up, this is about the best compromise. The 270 ohm will only draw 18mA so you can use a 1/4 Watt resistor.

To calibrate the sensor, change the method to Ratiometric and then add a set amount of fuel and read the voltage. Continue doing this from empty to full and you are good to go.

adrian wrote:With fuel sensors like this one that have very low resistance you have to balance the resolution you will get from the pin with the maximum current you can draw from the 5V rail. A very low resistance pull up resistor will give you better resolution but will draw more current. The 5V rail on the SDL3 is rated at 0.2A so you probably don't want to go over around 0.02A (20mA) to allow overhead for other sensors etc.

I would use something like a 270 or 330 ohm resistor as the pull up, this is about the best compromise. The 270 ohm will only draw 18mA so you can use a 1/4 Watt resistor.

To calibrate the sensor, change the method to Ratiometric and then add a set amount of fuel and read the voltage. Continue doing this from empty to full and you are good to go.

Which way would I wire it? One to Signal and One to 0v or 0v and 5v

Assuming its a 2 wire sensor, connect one wire to the 0V and the other to signal. Then put the resistor between the signal wire and the 5V.

Just jumping in to kickstart here as I have the exact same setup (0 - 90 sender and SDL level dash).

Would wiring to Ground instead of 0V make any difference in this scenario (270ohm 1/4W between 5V and AV)?

Car has good grounds and the senders aren't exactly NASA-accurate. Not going to effect the reading greatly?

Technically you should always wire sensor grounds to a 0V but in this case it probably won't make that much difference.

As long as you have a good earth the movement of fuel will be the main cause of inaccurate readings.

Cheers! Going to set it up soon.

adrian wrote:Technically you should always wire sensor grounds to a 0V but in this case it probably won't make that much difference.

As long as you have a good earth the movement of fuel will be the main cause of inaccurate readings.

Just to triple check this, as I am not intimately familiar with resistors:

I have obtained a 270R 1/2W (no 1/4's available) and I am wondering if I can install this INTO a Deutsch 2 way connector instead of hard-wiring it?

If you can imagine, Signal wire splice into a V with the resistor wire heading into one side of the connector. Resistor sitting inside the connector, with the other resistor wire crimped inside the pin, and then the 5V and Signal coming out the other side of the connector.

Any issues in doing this? Probably need a few dabs of solder on the resistor wire crimp...

That setup sounds fine, I can't see any issue with doing it that way.

A 1/2W resistor will work fine as well, it is just physically bigger than the 1/4W. I would however advise against using solder in the terminal. There are two ways I would wire the connector:

1. Crimp the DTM pins onto some wire and then crimp the resistor to the wires
2. Add wire to the DTM pin to get a better crimp.


I would always do it using method 1, it takes a bit more effort but it is the most robust way (I have found) of wiring in a resistor.

Thanks Adrian! Particularly for the tips.

I'd originally envisaged something along these lines.