battery problems
 

I have a 2002 4WD 2.0 TD and when we are having problems starting the engine. I give the battery a jump start, and everything is fine. I've been told it could be a bad earth, but how would I know if this solves it ? Could the battery be on it's way out ?

Bad earth or bad ground or bad negative terminal connection
It doesn't necessarily mean a bad battery.

It is like having the positive post of the battery connected to the circuit but the negative is not.
Take a test light and connect the other side to negative
Touch the test light to battery positive to verify it is working (lights up)
Touch the test light to engine block and turn key to crank position
If the test light lite up then there is a grounding problem.
If no light at all then you may have a bad battery.

Check the voltage at the battery with a DVM or Mutli-meter
The voltage across the terminal posts should be around 12.5 volts
Start the engine. The alternator should be charging the battery
And the reading should now be 14 volts to 14.5 volts

Good luck