As most of you already know, e36 coupes, sedans and convertibles have a trunk mounted battery while the 318ti houses the battery on the passenger side of the engine compartment. '96 and newer 318ti's are OBDII which means that they are equipped with an M44 engine that uses a secondary air pump as a component for the emissions system. The secondary air pump is normally mounted on the driver's side strut tower in the engine compartment. On 6cyl e36 models, the secondary air pump is mounted where the battery would normally be located on the 318ti.
One of the major benefits of relocating the battery is the vehicle's weight distribution. If you're installing a 6cyl powerplant into your 318ti(M50, M52, S50, S52), you should know that it weighs about 80lbs more than the M42/M44 engine that you're removing. In order to restore the 50/50ish weight balance of the car, you'll need to shift approximately 40lbs to the rear of the vehicle.
Another benefit is the ease of wiring. The factory 318ti engine wiring harness has a positive cable with a clamp to attach to the battery. The 6cyl wiring harnesses do not. Instead, they connect to a power distribution block which is mounted to the passenger's side inner fender just aft of the strut tower.
And finally, the other reason to relocate the battery is the secondary air pump. The 318ti has the same plate pre-installed to mount the 6cyl secondary air pump. It's used as a battery tray on the 318ti, but once you remove the 318ti's plastic battery box you'll notice that the plate below it has threaded holes in exactly the same configuration as the other e36 models for mounting the secondary air pump.
Since all of the parts needed to relocate the battery are sold individually, I recommend sourcing them used from a donor vehicle. It will probably cost a fortune if you decide to order everything new. However, if you're having a dificult time locating a part or if you're rich, feel free to look up the part numbers @
www.realoem.com or have the dealership find them for you
I'd like to add that my install and wire routing is exactly the same as the factory installation on my M3 that I removed the components from. You're welcome to route your wire however you you want on your car, but I'd recommend sticking to the factory specs for safety reasons. Take some time to read through this before you begin this project. Always inspect the components thoroughly for damaged/exposed wire and safety hazards prior to and after installing.
Enough talking, lets get to work!
Step 1) REMOVE BATTERY
If you can't figure out how to remove your battery, this project is not for you.
Step 2)REMOVE BATTERY BOX
This will require you to remove the negative cable. Save this cable, you'll need it later when you install the battery in the cargo area. The plastic battery box may be used to contain the battery once it's relocated.
Step 3)REMOVE PASSENGER SEAT
Only after the battery is disconnected and removed, should you attempt to remove the seat. The reason for this is because if you disconnect the electrical connectors under the seat while the battery is still hooked up, it will trigger an SRS light that requires a special tool to reset.
once mounting hardware is removed(two 16mm nuts & two 16mm bolts) tilt the seat back to expose the underside:
Disconnect underseat connectors: