Early 'i' car to later 'i' harness swap FAQ

[Lunner]

I posted this in the electrical section, but on second thought this is probably the appropriate place. Hopefully the first one will get deleted/forgotten.

This started out as a cry for help, but now I'll just let it be a DIY/Technical thread/FAQ/etc., to help people who attempt this in the future. My car still doesn't run yet (probably for reasons other than the wiring harness at this point though), but I think this information will be vital to those who potentially will be in my situation, as I couldn't find this information in one place anywhere.

If you have an early model 'i' car, and for whatever reason (say you're retarded like me and crushed your first one) you need another wiring harness, the first thing I'd recommend is that you get a second early 'i' harness (up to 9/87 according to realoem).

If however, for varying reasons you wind up with a 10/87+ wiring harness, which is probably the more common one, you can make it work, but there are definitely some things to consider:

1. You will need BOTH the fuel rail wiring harness and the fuel rail, the inlet/outlet nipples are both on the front of the rail in the later cars, so the fuel injector wiring harness can fit.

2. You will need an O2 sensor from the late model car, or you will need to splice/solder/etc. the appropriate connector onto either the wiring harness or your original O2 sensor. Be comfortable with wiring / soldering, or definitely give up at this point and source the early harness.

3. You will need to do the same thing for the oil check sender, as the connections are different there too.

4. The tachometer/fuel impulse signals on early model 'i's are picked up with C104 (3 plug) at the glovebox, near the ECU. You can ignore the green wire, but you'll need to get those signals to the other two wires some how. It looks like the later 'i' harnesses sent that information directly to C101, so I'm guessing the best approach is to tee into the wiring harness at the glovebox, but I haven't done so yet (nor am I confident which wires to use), any help here would be great, when I find the answer I'll edit this.

5. The starter wires on later model cars has a third smaller wire (I think it's a ground) that goes to the top post on the starter. I'm told you can ignore this, but my starter had a post there (that was never used, it was a refurbished one), so I just attached it there.

And I think that's it for now, hopefully this'll be helpful for the next person in my predicament. I'm off to figure out why the car won't start (I might have lost a transfer pump...)

-Brett

[JGood]

Another thing to add: If you put a post-87 harness on a pre-87 car, you will fry it. There is a power wire in the engine harness that conveniently uses the same pin as a ground wire on the body harness. I asked on this forum if I could put a 91 harness on my 3/87 325is, and everybody said yep, it will go right together. Well after my engine harness caught on fire and my body harness ground melted, I learned that you cannot trust anyone else, so whenever you do this type of work, look at the schematics for yourself.

It's pin 20. On the engine side of the 88+, it's switched power from the main relay to power the ABS unit on the body side. On the car side of 87 and older models, it's just a spare ground.

[Lunner]

Another thing to add: If you put a post-87 harness on a pre-87 car, you will fry it. There is a power wire in the engine harness that conveniently uses the same pin as a ground wire on the body harness. I asked on this forum if I could put a 91 harness on my 3/87 325is, and everybody said yep, it will go right together. Well after my engine harness caught on fire and my body harness ground melted, I learned that you cannot trust anyone else, so whenever you do this type of work, look at the schematics for yourself.

It's pin 20. On the engine side of the 88+, it's switched power from the main relay to power the ABS unit on the body side. On the car side of 87 and older models, it's just a spare ground.

Well maybe that's why it won't start. I got it to run for 5+ minutes, and then when I came back to move it it wouldn't start again. After trying my damndest (replacing parts and such, my voltemeter is broken), I was able to get it to just start 'combusting' (I guess?) and it immediately died.

I'll have to look into this, thanks...

And pin 20 at which connection? C101?

[JGood]

Power + ground = instant failure

I highly doubt that's your problem. Within 10 seconds of cranking, my whole harness was melted into one big mess of copper and rubber.

[j0nny5]

not to argue the point of short to ground=insta-smoke but i bought a e30 with the early-late i swap n my pin 20 took a cple weeks to melt...maybe poor ground? idk but i was lucky and it didnt melt anything else important...

anyways,heres the c104 info(cut&pasted)...i just cut the pin 8 n 9 wires a few inches from the ecu connector n splice them into the c104 or u could splice in the connector from your old harness...and i left the green wire out when i did mine

Your c104 plug has 3 wires
Pin #1 Black Wire-Engine speed
Pin #2 green wire-OBC Fuel cut function
Pin #3 Yellow/White wire-Fuel rate
On the 89 harness you'll need to splice or connect the following wires to the ones on the c104.
Pin # 3 from c104 to Pin 8 on the c101 of your 89(Yellow/White or White/Black wire)
Pin #2 from the c104, run a wire from directly to the + terminal on the coil(Green wire)
Pin #1 from the c104 to pin 9 on the c101(black wire)

[Trptguy]

so right now my car is having a very hard time starting. It cranks a ton before it finally starts. I have a '89 engine that I swapped in there, the car is an early '87.

I was thinking that the fuel pump was shot but could it be the c104? The car does eventually start and then runs seemingly fine. It has been about 3 months since the swap and the harness doesn't seem to be melted.

Also the tach and econometer don't work.

[Lunner]

For my last contribution to this thread (and I'm glad to find out I'm not the only one to have this problem), I wanna add that I went through the whole rigamorole of troubleshooting why my car started once then wouldn't start again (but *almost start* twice in my fiddling). I eventually got frustrated enough to go to pick n pull where I luckily found (at the last car too) an 87 i harness.

Plugged that in and the car started first try. So my conclusion is a) early cars should use early harnesses, and b) late harnesses might work, but it looks like way too much of a hassle.

[imwhyteyo]

any issues when swapping in the harnesses to EtA variety?... i have n 86 eta.. and an early 87is motor/harness with the harness cut at the round connector towards the fuse box. im wondering if i can chop mine off the eta, and splice it to work just the same for the "I" harness??

[imwhyteyo]

either that or i DO have another later style "I" harness.. but i cannot figure out if chopping the connector from that will work the same way either if i splice it in. anybody know if the pin-out on that round plug is the same? need to finish up this swap.