Right I have had this problem for about 18 months but only now am I going to try and resolve it as it driving me mad.

My battery keeps going flat after a few days, even if I leave it unlocked with the alarm off. Done all the draw checks On the aftermarket alarm and stereo and all seems ok but the one thing I have noticed is that when ever I disconnect my battery and re connect it, the starter clicks into engagement but does not spin.

So the starter is constantly holding the sprocket onto the flywheel although not turning. The car still starts fine when the battery is ok but I have had to change the motor before due to it dying which I think is related.

The ign switch live is working as it should ( the little one on the starter ) only gives 12v when the car is de immobilised and in the cranking position on the ign.
The engine earth is fine and also so it the main power cable that runs from battery to starter to alt.

It only seems to do it when the ign live is connected as I took that off earlier and re connected the battery and it didn't click.
Only when the ign live and the main live are connected to the starter as it should be and I re connect the battery.

Weirdly if I leave the battery connected and disconnect both starter wires, then re connect them, it does not happen.

Obviously the starter will only engage and crank with a signal from the ign live so why would it engage but not crank when ever the battery is re connected?

Could this be an alarm fault or something to do with the ecu as the ecu is from a ph1 and mine is a ph3?

Oh and also my car has no factory fitted alarm from standard, import apparently.

Any opinions welcome.

________________________________________

***Breaking*** Ph3 Silver GTI 6 High Boost

Right its not an ECU issue. There is only 2 supplies to the starter, a constant 12v from the battery and a switch live direct from the barrel.
I would suggest doing some voltage drop tests on your engine earths because it could well be you're getting some sort of back feed through the switch live on the starter. Only a few Ma so enough to engage the teeth but not enough to properly engage the solenoid to crank.