Fried Engine Harness

I have a 1995 Defender 110 (RHD) with 300tdi and I swapped the original 65A alternator for a 100A disco alternator about two months ago. There were no issues until one morning while warming the truck up for work, I noticed the B+ wire had come loose on the alternator while it was running. I was in a hurry and, instead of turning the vehicle off and disconnecting the battery, I just reconnected the wire without thinking. The alternator sparked and the vehicle went dead and there was no longer a click or anything when I turned the key in the ignition. I disconnected the battery and got a ride to work.
After work, I checked connections of all the wires to the starter and alternator. Before connecting the battery cables, I checked their condition with no issues and confirmed the negative ground to the transfer case was clean as well. Everything seemed fine so I reconnected the battery cables, and without even turning the ignition, the engine bay started billowing thick white smoke. After spraying it down, I saw the following damage.


It's probably something really obvious but I hope someone can help me figure out the problem.

Battery + has gone direct to earth somehow.

I presume those starter solenoid cables were removed after the meltdown?

Did you connect the earth cable that normally goes to the block to the solenoid?

The positive cable was on the solenoid and I removed it after the meltdown. The black cable shown at the bottom of the starter picture goes to the engine bay fuse box and it was connected to the solenoid as well. The battery negative cable is grounded to the transfer case.

You may be able to tell by carefully stripping back the insulation and see which cables are welded together but that looks too far gone for forensics.
I don't know if a dead short in the alternator would be a thing?

OneZero said:

I noticed the B+ wire had come loose

Click to expand...

If by the above you mean it become disconnected, then ...

OneZero said:

I just reconnected the wire without thinking.

Click to expand...

Reconnecting it with the engine running might have let the smoke out of the rectifier within the alt ..

If you meant just literally come loose, which is probably more likely, then I'd say you've been very unlucky to fry the alt .... a DMM should give you resistance readings - it's quite a simple circuit ...

Does your LR not have a maxi fuse protecting the B+ circuit ?

Either way - Ouch !! Good luck with it

The positive cable from the engine fuse box (with maxi fuses) was connected to the starter solenoid. Is that correct or should the fuse box cable connect directly to the battery?

Does this cable from the engine fuse box connect to the battery or the starter solenoid?

Quite common for that wire to be on starter solenoid but no idea if your vehicle was.

Everything is connected. Is this correct? Positive battery lead, engine fuse box cable, and cable to alternator are all connected to the starter solenoid.


I also added another ground strap from the starter bolt to the chassis.


The negative battery lead is also new and is grounded to the transfer case. I already fried one engine harness so I'm nervous to connect the battery cables!

OneZero said:

I already fried one engine harness so I'm nervous to connect the battery cables!

Click to expand...

If you have serious doubts, then I'd connect the +ve lead via some small fuses AND a DC clamp meter, whilst progressively increasing the load - e.g. a 10A fuse should get your auxiliary circuits up and running, a 20A might get your "ignition" on ( but not the glowplugs - disconnect them, for now ) .... etc.

Hopefully, that might build enough confidence to connect the battery directly to allow you to try and start her up ... but there are lots of things you could check beforehand - like voltage drop to the alternator - which might add some more confidence that you have it all connected up correctly....

Good call on the extra earth strap IMO ...