Starter motor problem (325 m20)

[louca21]

Hi all, my e21 wont turn over, at first all i was getting was a click, which after checking the web narrowed it down to immobiliser or starter motor. I managed to get a hold of a second hand starter, but after replacing, i was getting a "whirring" sound, which then narrowed it down to the solenoid not working, and the starter not engaging the fly wheel.

I then bit the bullet and bought a new starter.. Hoping this would resolve it all, but again this starter wouldnt connect with the flywheel..

Then realised the battery was dead and before replacing thought id try with a booster, could the reason the solenoid not be working because off no power in the battery, even when connected to a booster?
If not what else could it be??

I checked the voltage to the starter and off ignition it showed 11v, but once i turned the key its dropped to 1.6-1.8v.
Also are there any relays that could be affecting start up??

Any input is appreciated,
Thank you
Chris

[trash-berd]

There's only a starting relay on automatic transmissions if I remember correctly.

[Jeroen]

Check all wiring as most likely you don't get enough power. Connections all good? Especially ground straps like the one to the chassis, that is likely to be corroded not allowing enough current. As the starter draws a lot of current, make sure to have a strong starter booster or a fully charged good condition battery.