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Yes, you are doubling the alternator output, and the old wires were sized for half that current flow. If you have a large current use by the battery, the alternator will force too much current through the old wire = fire.
I would add a new second wire to the alternator output lug and run to the battery. Also add a large ground wire to the alternator case to a good ground.
Running a larger wire to the battery (should really just go to the solenoid) screws up the ammeter readings. There really is no good solution for this except changing to a voltmeter.
The ground wire at the alternator is to ground all the circuits on the front of the car (horn, lights etc) and not to provide the charging path. The charging path is through the brackets and engine block to the large negative starting cable which is more than adequate for the charging current.
From: Las Vegas - Just stop perpetuating myths please.
Yes sorry for the bad news but u can look ampacity charts for yourself and from what i am surprised to see u need a #2AWG in copper wire for copper at 140*F (95 amp rating with a little safety margin). Good old #10AWG is only 30amps. And for full charge capacity u need to run #2AWG not only from the new alt to the dist block on fender but from the fender dist block to the starter terminal. Like said this will ruin your stock amp gauge reading as it uses that dist block to starter wire for a shunt and it will no longer have the same resistance.
On the other hand the only time u would draw 85@ is with a dead batt or a short to ground. U can always install an inline fuse or use a fuse-able link wire to protect the output/charging wire from the alt - but that kinda undoes the higher capacity alt. Just a thought, its your car and $$$ or excuse me euros. Please let us know what u find for a solution as it helps us all.
Running a larger wire to the battery (should really just go to the solenoid) screws up the ammeter readings. There really is no good solution for this except changing to a voltmeter.
The ground wire at the alternator is to ground all the circuits on the front of the car (horn, lights etc) and not to provide the charging path. The charging path is through the brackets and engine block to the large negative starting cable which is more than adequate for the charging current.
I was replying to OP's question. It was not how I would/have upgraded my own wiring. The 40 amp ammeter is not going to read correctly - no matter what he does with a 140 amp alternator. And of course I changed out my ammeter years ago. For the mere fact that he is asking this question, he is more likely to have a poor connection if he takes the wire to the solenoid. That is why I said what I did. I stand by my suggestion of grounding the alternator. The bracket cannot always be relied on for proper grounding - and you need the same capacity for both + & -.
As a point of reference, GM uses a 13mm^2 wire which is just shy of 6 gauge for their 140A alternators for the main run from the alternator to the starter solenoid. There is a fusible link that is 5mm^2 or just under 10 gauge at the solenoid end. I wouldn't hesitate to use that wire size for your alternator.
Here is a quick schematic showing a suggested way to update the wiring.
A = alternator
S = starter
H -horn relay
The circle where wires are connecting is a junction block. Use it for any new high current loads. Put it in a handy location.
The junction block feeds the old horn relay and existing power to the fuse block.
The S terminal on the alternator is the sensing connection back to the connector plug.
New generator, new cable?
And for full charge capacity u need to run #2AWG not only from the new alt to the dist block on fender but from the fender dist block to the starter terminal. Like said this will ruin your stock amp gauge reading as it uses that dist block to starter wire for a shunt and it will no longer have the same resistance.
