Motronic upgrade, new loom smoking! NOT WIRE 20

[FEP]

'86 Tech 1 325i.

New 3 row ECU and loom fitted with front mounted CPS and Cylinder ID Sensor.

Connected up the battery with the ignition off and the whole engine loom started smoking! Disconnected the battery pronto - connected for maybe 4 seconds.

My old engine loom had no wire to pin 20 of the C101 plug, but a brown earth wire on the fuse box side which I cut short, sealed and moved well out of the way.

Few questions.

Why does the new loom have a new wire near the bulkhead grommet that the Engine Loom page on the Wiki (http://www.e30zone.net/e30zonewiki/inde ... ngine_Loom) labels "2" and says goes to battery +ve? What is this feeding that didn't need feeding before? DOES THIS WIRE DEFINATELY GO TO +VE?

How do you differentiate between the 3 pin plugs "8" and "9" on the Wiki diagram - ICV and TPS connectors. Surely having them connected the wrong way round wouldn't cause the whole loom to smoke.

How do you differentiate between the 3 pin plugs "15" and "16" on the Wiki diagram - CPS and CID? They have the same colour wires under the boot. Are they interchangeable?

I presume that the ECU doesn't need to be bolted in place to earth properly - I've still got it hanging down in the footwell.

Why is my loom trying to catch fire? Do I connect everything up one by one with the battery live to isolate where the problem is?

Many thanks experts.

FEP

[Brianmoooore]

The 'other' common cause of M20 loom fires is when the earth strap between the engine and the car chassis is faulty. The alternative current path is through the small brown/orange wire that goes to the oil level sensor and to earth through the sensor body. This 0.5mm wire objects to carrying starter motor current.

[FEP]

NO - It's not that - the earth strap is good and folowing the smoke (which came from the whole loom) that oil level sender wire seemed fine.

[Grrrmachine]

The small wire numbered 2 in the wiki diagram should run to the starter motor; there should be two wires on the large outer terminal of the starter, and that's one of them. From the starter, the second thick red wire goes to the alternator.

To differentiate the other plugs (CID, CPS, TPS, ICV) you'll need a multimeter and a paperclip. Use the ECU Pinouts page in the Wiki to stick the paperclip into one of the pins on the ECU plug, then use your multimeter to check for continuity to one of the unknown plugs on the loom. Slow, but effective.

As for your wire smoking problem; my first reaction was the same as Brian's. Can you see where exactly the loom has melted?

[FEP]

I think wires 10 and 7 go to the starter motor. To connect 2 to it I'm going to have to re-install the starter motor on the driver's side of the bellhousing!!!

I was never very happy connecting 2 to +ve as per the Wiki guide. Should it really be so connected?

No sign of melting of the loom at all. I could connect the battery for a bit longer then we'll know!

Does seem very bizarre that the TPS and ICV plugs and CID and TPS ones are not identified.

[Grrrmachine]

You've misunderstood. 2 connects to battery positive, but it RUNS to the starter, meaning its other end is one of the wires marked 10, for the starter. That's how the starter gets its power. Use a multimeter to confirm if you're not sure.

If you've no sign of wire damage anywhere, how do you know it was the loom that was smoking?

[FEP]

Surely wire 2 is not fat enough to carry voltage to the starter. Any way, there is a separate, non-engine loom wire that runs from the battery (well battery in boot so forward battery terminal) to the starter.

Pretty sure that it was the loom that was smoking - a kind of crescent of smoke rose up from servo to front pulley!

[Grrrmachine]

What wire colour and terminal type have you got for wire 2? And solid red means permanent live, and if it's a ring terminal then the only places for those in the engine bay are battery terminals (earth and live), starter, alternator or coil - all the high-power stuff.

As for the smoke, the safest thing to do is unplug the loom again and lay it out on the floor to inspect for damage. As to why it melted though...

[Brianmoooore]

A facelift E30 engine loom only only has three points of contact with the rest of the car, or four if it is a switchable auto or has aircon. (The small socket close to the large ECU plug, which will be unused on most cars.
The three connections are the 20 pin C101 plug and socket, near the fusebox, and two medium sized wires with ring terminals on them. Both of these wires have black outer sheaths, but under the sheath one of the wires is red and the other brown or brown orange.
The red wire goes to the battery positive, or the smaller post in the phantom battery +terminal in the case of cars with rear mounted batteries, and the brown or brown/orange goes to the earth terminal on the top of the RH suspension turret.
The battery + terminal also has a thick black cable which supplies power to the starter motor and from the alternator, a medium sized red wire that goes to the fusebox, and on some cars, another medium sized red wire that goes to the ABS pump. These all go the large post of the phantom battery + terminal on cars with rear mounted batteries.

If you have transposed the red and brown or brown/orange wires with ring terminals, then you will have burnt out the brown/orange to the oil level sensor, since you will have effectively connected the battery directly across the ends of it, and will possibly have fried the engine ECU as well.

[FEP]

Who would like a good laugh at my expense? Go on - it's on me.

At the top of the phantom battery terminal is a small post that is clearly not connected by anything metallic to the battery +ve terminal. This had a mediaum guage, black sheathed cable running into the inner wing along with the thick battery feed from battery to phantom terminal. My old loom had a cable attached to this small post and I assumed that it was an earth point. Curiously the old loom had no other apparent earth wires coming off it.

A combination of other vehicles using black as earth, my colour blindness making the browns and red wires look the same colour and there being no spare earth post on the suspension turret meant that I didn't pay much attention to these wires and happily assumed that the small post was earth! In reality of course the medium wire from the rear of the car was +ve.

Once re-connected and having found somewhere to attach the brown/orange earth wires, (and reversed the CPS and CID cables) it starts and runs with no problems. Even the oil level wire looks ok (I have not removed the sheath) so maybe I got away with it.

Enjoy you chuckle and thanks for the assistance.

FEP

[Brianmoooore]

Small phantom battery terminal is connected to the battery + terminal at the rear of the car by a second wire running along side the thick one to the large post. It's wired like this to isolate the engine ECU from the voltage pulses and variations that occur because of voltage drop in the thick cable from various electrical equipment, especially from the large drop that occurs when the starter motor is running.
If you didn't connect anything to this small post originally, the engine loom would have been dead apart from the ignition feed to the coil and the start signal, so I can't see where the power to cause this alleged smoke was coming from.

[FEP]

As I said, I did connect the loom earths to the small post, mistakenly assuming that it was a battery earth connector!