My 93 500E has a problem.
Whenever it sits for an extended period, the car turns over slowly like the lights were left on for an hour. No really, really slow, but just not as happy as it should. Now that the cold has sat in, If I park the car outside in 38 degreee weather for 12 hours, it starts as though the lights were left on for maybe a 4 hours or so hours.(the lights on analogy will hopefully make sense to those reading. Its really slow , but still a smooth spin and not the Whaou, Whaou, Whaou, Whaou..tick, tick, tick...of the proverbial dead battery.
So, Its a new battery. Even after putting it on, it still never really spun free, and fast like my 400E does. ...Well maybe when I first put it in, but not after the first week of storage. I only drive it on the weekend.
So how can I differentiate if its the Solenoid, Starter, or the Alternator

__________________
Regards,

Albert
aldedmon@yahoo.com

Alternator - charges the battery. Has nothing to do with the cranking speed of the motor. You should see (I'd guess) 14.2 or so V at the battery with the car running, all accessories off and the battery fully charged.

Solenoid - engages the starter motor while simultaneously "kicking" the starter gear out to mesh with the gear on the engine. Far as I know, a failing solenoid will either result in a starter which just runs and never cranks the motor, or you just get a "click" when you turn the key.

Starter - if you've got a slow cranking motor and you know for sure that the battery is fully charged, then this is your likely culprit. HOWEVER - there are many factors that can result in a slow cranking motor. I'd make the following checks:
1) Good connection on positive cable at battery - no corrosion and no deteriorated cable end or cable wiring.
2) Good connection on negative cable at battery - same test as for #1.
3) Good connection from negative cable to car chassis. Probably no worries about corrosion here unless the cable is exposed to the weather and you live in salty road country.
4) Good connection on ground wire from engine block to chassis - same tests as #3.

If you've verified that 1 - 4 above are all OK and you know the battery is fully charged (should be about 13.5 volts when fully charged - maybe down around 12.9 after several days sitting), then the starter should be tested - most big-chain auto parts stores can provide this service for free.

If this behavior is worse after letting the car sit for days as you indicated that you do, maybe there's excessive draw at the battery with the car off and you've got a low battery condition at first start. You didn't indicate if this problem persists after you've driven the car for a bit and attempt to re-start.

Hope this helps.