I believe two functions going on: I think this starter uses magnet to engage the drive pinion vs. just helix. If the ground not solid for the coil but is for the motor you'll get a spin no engagement.

I could be wrong on the starter engagement, and if it is just helical gear engagement, if the voltage is not high enough you could get your symptom.

Check your B+ jump terminal. If that gets loose it can cause you symptoms.

BMW uses a solenoid to pull the starting gear into the flywheel. The fact that you hear the stater spinning but the engine is not turning is because the the starter gear is not engaging the flywheel, or that what you think is the starter is not the starter but something else spinning.

Did you test the starter (old starter) before replacing it or are you one of those guy that like to fire off the parts cannon?

As mentioned earlier, the old shade tree mechanic troubleshooting method that still applies is to tap the solenoid and starter a few times and then try to start it. May not work so it doesn't eliminate the possibility that the solenoid or starter is bad but it always worth a try or two.

This past weekend after testing relays, fuses, etc. for a few hours on my daughter's 335i I went back to the beginning sequence of troubleshooting I rechecked the battery and then tapped the starter and solenoid and it started first try. I didn't do that the first time. It reminded of what I learned many decades ago that a lot of the time you don't need special equipment to find the problem, keep the trouble shooting as simple as possible.

Did you purchase both the starter and the starter solenoid? They are separate parts. When you replaced the starter you had to disconnect the wires to the solenoid so you should be able to rule out a loose connection but you could still have a broken for frawed wire.

Do you have a code reader?

Seems puzzling, but here's what I know, and a suggestion.

I have a 2001 3.0i, rebuilt the starter when I did the engine work a couple of years ago. I'm sure I posted some photos and info on that on this forum somewhere.

Here are my own notes from that:

"Bench tested before reinstall - O'Reilly's was not able to do that successfully (told me it was failed) since they were not able to ground the solenoid, even though I pointed that out during the test."

"Part numbers: BMW 12-41-7-501-738. Bosch PN: SR-0474-N. On the starter shell itself: Bosch 0 001 108 190. Decoding the Bosch #: all Bosch starters start with 0 001; next 3 digits, 108 are the series of the starter; final 3 digits relate to exactly what type of car it is for."

So there are the Bosch and BMW part numbers for this. I'm sure I posted a photo up here of the actual starter with the actual Bosch PN shown (since I don't trust anything either). Those numbers (and stock photo below) confirm that the solenoid and starter are indeed one assembly here, not separate. They share a common casting, and the fork lever that actuates the gear makes it a little tricky if a new solenoid were to be acquired and installed on an existing starter. Possible, but not as easy as say installing a new VR in an old alternator.

That note I had up there about how O'Reilly's was unable to get a successful test due to not grounding the solenoid may be an important clue. As I recall, the effect of them not doing that was that the starter would spin but the solenoid would not plunge.

And confirming the above answers - yes, on these, gear engagement is driven by the solenoid's solenoid. :D

I'd try to find and figure out the exact wiring of this starter / solenoid, not assuming anything. So you know exactly what those two little control wires are supposed to be doing, and that the +12V and grounds are all correct. The black with yellow striped control wire with the 8mm nut is the one that gets switched on when the key is turned to start.

If you have the old starter, I'd bench test that (I ended up bench testing mine myself after O'Reilly's could not) to see if your old starter appears fine. If it does, that would point to a problem with the wiring rather than the new starter.

Or perhaps the corrosion in there is so bad that you have a ground problem - agree, can test for that with a jumper cable. Very unlikely that the old and new starters both have this exact problem, so I'd be looking outside the starter. Also it seems odd that limited current would spin the starter motor but not engage the gear, so I'm preferring the control wire ground theory vs. corrosion / bad overall ground (via the engine, not via the thin black wire).

BTW, that exposed braided copper cable connecting the solenoid to the starter is +12V, bridging from the solenoid to the starter motor. It is not ground.

Attachment 78772

If I had to guess and go for the answer now, I'd say that the thin black wire is not actually grounded (failed wire or component on the path to ground, etc.). I'd ground it with an alligator clip of some sort, and see what happens. Grounding a ground is a pretty safe thing to test as a guess.

I believe the black+yellow striped wire secured with an 8mm nut is the control wire (that goes high during starting, via the ignition switch, starter relay, etc.), and the thicker, black wire secured with a 10mm nut would be the one to ground for testing. It may go somewhere other than directly to ground when installed in the car, and a failure in that path may actually be the root cause of these problems.

And BTW, if it gives you any faith in these Bosch starters, my original is still in there, going strong as I approach 200k miles. I replaced the brushes assembly, and the originals probably had about 70% left. I basically just took it all apart, cleaned things, and replaced the grease.

Ground is via the aluminum nose of the starter that is bolted to the bellhousing. The positive cable goes to the terminal on the solenoid that does not have the cable going to the starter and the signal wire goes to the term on the solenoid

If you have the 2 wires 30h and 50 (smaller start wire on wrong small terminal) mixed up you will get the starter spinning and not engaging but that wouldn't happen while the car was just sitting unless somebody is playing a prank on you.

I wonder how this turned out?

I'm sure he finally figure it out. Or else he would be back here asking more questions.

Just wanted to update this thread, sorry it took awhile guys, I haven't had a need for this car but now my son is back from school and is bugging me to get it running so here we are.

I bought a power probe and used to determine that the starter was good. I dug a little deeper and found the smaller of the two wires was indeed loose. Tightened that connection and I was able to start it at the key.

I am happy that it was an easy fix and I don't have to pull out that starter again.
That makes THREE advanced auto starters I have bought and all have worked fine so IDK why everybody hates on them so much. I also get Delphi coils for my n54 for $50 and they work great every time.

Its cool having this resource to pull knowledge from you guys, I just wanted to make sure there wasn't some weird BMW trickery that i was missing LOL