Before I replace the starter motor...

300tdi and when I turn the key it some times doesn't start..

- I get one loud click,
- the battery monitor shows the battery voltage drop as it would if the engine was cranking.
- When the car does start (after a few tries, or rocking it seems to help (Sometimes)) there is a split second where i can hear the starter motor still engaged with the engine.

I have another starter motor from another 300 tdi which i can swap in, but was wondering if there might be something else to consider.

could be poor connections of main battery lead but most likely a worn starter

As long as you’re measuring battery voltage drop at the actual posts, not the clamps or cables, you can rule out poor connections and crimps, and blame the starter.

Keep the old starter, and it may just need a clean-up and grease of the Bendix gear and solenoid and perhaps brushes, and you’ll have a spare.

jamesmartin said:

could be poor connections of main battery lead but most likely a worn starter

Click to expand...

I was ruling out lead connections due to the voltage drop when the key goes to III but nothing happens.

Often a new solonoid will sort it.As you have another swap it out. You could then try a new solonoid [ not expensive ] on the original motor and have a spare.

NiallRussell said:

I was ruling out lead connections due to the voltage drop when the key goes to III but nothing happens.

Click to expand...

I meant if you measure, say 9.5V at the battery clamps during cranking, but get no cranking, it could still be voltage drop at the clamp-post connections. The battery post voltage would remain close to normal because the drop would be after them.

Not so common in cars but in older tractors where the clamps get a green fur on them. Many a starter has been changed for no good reason in these cases.

thanks everyone - sorting it tomorrow will let you know.

Cleaned everything up - and so far no problems. So it seems it might just have been the connections. Time will tell.