BMW Questions? Ask a BMW Repair Expert for Answers.
From your description it sounds like you have a poor ground connection from the engine to the body of the vehicle. There is 2 ways you can test this. One with a multimeter- check for voltage drop from the engine to body when someone is trying to start it. Anything above 0.3 volts is no good and the ground strap should be replaced. The other is to take a set of jumper cables and attach one side to the engine (oil filter housing would be fine) and the other to the chassis ground lug under the hood and try starting it.
Let me know how you make out.
Yes, check with a meter please. Respond back when the check is made.
So, you added power to the starter solenoid by what means? Jumper pack, power probe?
Sounds like the ground may be fine then. I would double check the voltage drop anyways to be safe. It sounds like you may have a voltage drop on the power side. Check the jump start terminal and connection point below the battery (at the body) for corrosion. These are both common places.
I'll need the VIN number of the car to pull up a wiring diagram,...