Alternator wires

[hanleyclifford]

It sounds like the instruments are grounded TO the alternator, which is not really the best place for that.
Both the 35 and 50 amp alternators that came stock on A4s ground through the case to the engine. They would not have a ground wire added to them nor need one.

Has anyone ever tested the resistance thru the engine block? Might the value vary depending on where the connection to batt (-) is made?

[joe_db]

That might possibly be a problem, especially if there is a lot of corrosion on the bolts.
What I would do:
Connect the ground wire to the engine close to the starter, not on the far side of the engine. Run a ground wire from the alternator to the same bolt that the engine ground wire connects to.
Keep in mind a very large percentage of boats and cars ground through the block, so this is not a huge problem. Not that it can never happen, but probably 95% or more of A4s don't have a separate alternator ground with no issues. If you want to check, put the alternator under load and use a voltmeter between the alternator ground and the ground wire connection to the engine. That will tell you the voltage drop through the engine block.

* there once was an issue with PT-6 turboprop engines where on some airplanes the ground wire was connected to a point that made the starter/generator current go through a bearing and corrode it causing engine failure. I have never heard of this happening to a boat, but I guess there is a first time for everything.

[hanleyclifford]

That might possibly be a problem, especially if there is a lot of corrosion on the bolts.
What I would do:
Connect the ground wire to the engine close to the starter, not on the far side of the engine. Run a ground wire from the alternator to the same bolt that the engine ground wire connects to.
Keep in mind a very large percentage of boats and cars ground through the block, so this is not a huge problem. Not that it can never happen, but probably 95% or more of A4s don't have a separate alternator ground with no issues. If you want to check, put the alternator under load and use a voltmeter between the alternator ground and the ground wire connection to the engine. That will tell you the voltage drop through the engine block.

* there once was an issue with PT-6 turboprop engines where on some airplanes the ground wire was connected to a point that made the starter/generator current go through a bearing and corrode it causing engine failure. I have never heard of this happening to a boat, but I guess there is a first time for everything.

That is exactly how I make the connection.

[ndutton]

The MMI information is for the basic 35 amp alternator . . . .

Looks to me like that's what Ben has.