I2C LCD Module with ruggeduino

Hi !

I'd like to use an I2C Lcd module with a ruggeduino. I'm using LiquidCrystal_I2C.h
I understand that SDA and SCL are on analog pins 4 and 5. What else do I need to do to make it work?
So far I've wired the module like this: SDA--A4, SCL--A5 (and of course 5V, and GND).
The backlight turns on when I power the arduino, then it turns off.
The Lcd module works as expected with a teensy++ (SCL--PD0, SDA--PD1), and a DFRobot Romeo with three I2C headers.

Thank you for reading, bye!

Welcome Antoino,

What else do I need to do to make it work?

Of course a piece of code,
The library may contain calls to set the backlight on/off,

Please post your code and a link to the I2C LCD library if you want us to dive into your problem,

The library is here http://hmario.home.xs4all.nl/arduino/LiquidCrystal_I2C/

I tried the Helloworld file

#include <Wire.h> 
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27,16,2);  // set the LCD address to 0x27 for a 16 chars and 2 line display

void setup()
  lcd.init();                      // initialize the lcd 
  // Print a message to the LCD.
  lcd.print("Hello, world!");

void loop()

By backlight turning off, I meant the LCD module completely turns off as if it was not powered.
I don’t think that the calls to the methods of lcd object have something to do with my problem.
Do I have to setup something before using A4 and A5 as SDA and SCL ?
This sample code is working fine on the two others platforms I talked about in the first message.