Hello. 2005 R53 with an automatic transmission. No crank at start, brand new battery. Passenger side ground strap is secure and looks clean.

I was going to test the starter solenoid with a multi meter. One probe on the engine ground wire, the other on the solenoid post to make sure it was getting juice at start.
Multi meter probe connected to ground, person turned the key to start position, I touched the other probe to the solenoid power terminal AND THE CAR FIRED RIGHT UP! I was able to start the car several times after that. It then failed to start so I repeated the process and it started up.
So, no crank start but if I put a ground to the solenoid power terminal it starts. What is this telling me? Replace the starter because the solenoid is bad? Some kind of ground issue?
I can't figure out why putting ground to power would make the solenoid work.
Thanks everyone.

 

I suck with electrical so keep that in mind

i think you just proved it’s a ground issue

 

I did have continuity between the chassis and the ground strap that connects the solenoid to the starter. Perhaps the connection between that strap and the solenoid itself is compromised? I'm installing a new starter this weekend, I'll report back with results. Hopefully that's all it is.

 

Did you have the meter in DC volts mode? If so it has very high impedance & would not pass current to engage the solenoid. Besides, providing a direct short from a 12V circuit to ground is not a good thing. Something else is wrong. The electrical schematic is your best bet to figuring it out.

 

Yes, the meter was set to DC. I was just trying to test the voltage going to the solenoid at start up. Next thing I know, starter engaged. If it had a bad ground I could see jumping from ground to ground working. If it wasn't getting power I could see jumping power to positive working. I can't figure out why putting ground to positive works.... clearly, I'm not too smart when it comes to automotive electrical. Googling and figuring stuff out as i go.

 

So, got the old starter off now. That one has the large post for power, in the middle, and the smaller post for ignition wire above it. The new starter has both of those plus one more tiny post, bellow the power. What's that for? Can I just ignore it?

 

Update: I got the old starter off, bench tested it, and it worked. That was a bit discouraging. Maybe it wasn't the starter after all. I did install the new starter and ignored that extra tiny connector that I mentioned before. The Cooper started without issue. The car has been used regularly since the repair with no problems at all.
Recap: car wouldn't start. Starter would work if you jumped a ground to the power terminal. Bench tested starter worked every time. Installed a new starter and haven't had a problem since.
Conclusion: automotive electronics are a mysterious pile of crap when you don't know what you're doing.

 

amen

 

Believe your first instinct was correct: it was always the starter. Just because it spins with battery voltage applied doesn't mean it's strong enough to turn the engine over. A bench test machine - which puts the starter under load - at a place like CRAPA or Pep Girls likely would have shown the original was in fact worn out. The replacement route was the correct one.