M20 to M30 Swap - Sanity Check Needed

[brownsw44]

Chassis is a 1986 528e. Came as an auto. I swapped it to a manual using the usual guidance documents found on this site. Everything worked fine.

I've had a couple of M30s sitting around and figured now was the time to do the swap. The M30 that went in was from an '86 535. I used the associated wiring harness and ECU. Hopes were high the '86 model year commonality would make this a simple swap.

The problem I'm experiencing is the starter (none of the 3 I've tried) engage when the ignition key is turned start. The only thing that comes on is the auxiliary fan at the front of the car! The starters don't even engage the flywheel when I manually feed 12v to terminal 50 of the starter. However, all 3 of the starters bench test fully functional.

Looking for some suggestion/experiences/knowledge about what to investigate. Are there significant differences in the wiring harnesses between an M20 and M30? Is the fuse box wired differently? I have the ETM and nothing is standing out to me as a difference.

Only thing I can think of is the starter relay, but the car started perfect after the manual swap with the M20 so I feel like all of that wiring is isolated correctly (maybe even removed. It's been a few years.) I'll have to check.

Thanks in advance for any suggestions/information.

[Mike W.]

I took a quick look at the ETMs and didn't see anything different. Since jumping to 50 doesn't do anything that suggests battery power, either positive or negative, isn't getting to the starter. I forget what now, but on my first BMW, a Bavaria, with carbs, I could only sort of get it to crank. Eventually figured out I was trying to get a ground to the starter thru the electric choke for the carbs. That obviously isn't your exact problem, but make sure you have a good ground between the body and the engine and a good connection between the battery and the starter. IIRC there is supposed to be a ground strap bypassing the motor mount on the pass side and one by the firewall going to the valve cover.

The more I think about it the more I'm thinking ground.

[brownsw44]

Mike W. FTW! It was a ground, the one off the firewall to the back corner of the engine on the passenger side. Happy to have it be a single 10mm bolt fix!

Thanks again.

[Federico]

The strap between the cylinder head and firewall is for the ignition noise suppression. Make sure you have the thicker one for the motor mount, or add copper wire between the block and frame/battery.

Obviously, you want as much ground as the thick '+' wire for the starter.