I have a 2002 Fabia 1.9 TDI and it won't start. When I turn the key there is a clunking sound and then nothing, at this point the battery voltage dives and the lights dim. Battery voltage appear OK, it has also been on charge but this didn't help. Tried jump starting it from my Landrover but still no joy. I have just swapped the starter motor for a new reconditioned one and this didn't change anything. Looks like I have just wasted my money on the starter motor, anyone any ideas as to what might be wrong as the wife is starting to complain!

Do you have a live at the starter motor? Do you have a cranking live at the solenoid? Is the engine earth ok?

Connect your jump lead from the negative battery post to a metal part of the engine and see if it then starts.

Failing that

 

Quick electric theory up date to OP.

( from ohms LAW) VOLTS = AMPS X RESISTANCE.

And hence ,it follows that if the current flows through a resistance ( e.g. bad connection), then voltage will be developed, and if in a feed to something like a starter motor, then this voltage will reduce the battery voltage.

principle of operation- key to start position- battery voltage sent to solenoid ( high current relay) ,which energises and connects battery to starter motor through high current handling switch. Starter turns etc.

The current on starting is very high, so any small bad connection will drop the battery volts to nil at the starter when trying to start.

Could be battery ,as when they age, bad connections form inside the battery and more current is drawn ,then output volts drop. Suggestion 1- try the 1.9 battery in the land rover . No go =dud battery. Starts LR, then( obviously replace battery in 1.9) try a bridging wire from solenoid operate to the battery +  on 1.9. No go, then you've got a bad connection from either the battery -ve to engine or from battery +ve to starter. Hint- often a high current bad connection can be found by looking for a connection that gets HOT.