Alternator wires

[BenCT]

Hi, per my Motorola alternator & regulator in the picture attached. Do I have my labels correct in this photo? I believe the 4 wires I’ve labeled are the exciter, the output, the regulator ground and the alternator ground. I’m double checking that my thinking is sound. Thanks for feedback.

[hanleyclifford]

Are the output and alt ground cables the same size?

[BenCT]

Yes, same size if I’m correct about that being the ground on the upper left.

[hanleyclifford]

Yes, same size if I’m correct about that being the ground on the upper left.

Where does that black cable terminate at the other end?

[BenCT]

From my old photos (I took out the gauges as I’m changing some things) the large black wire looks like it went to the ground of the ammeter. that’s why I’m assuming it’s the ground. Also ion the alternator is on the same terminal as what I’m pretty sure is the ground for the regulator. So that would make sense too.

[hanleyclifford]

From my old photos (I took out the gauges as I’m changing some things) the large black wire looks like it went to the ground of the ammeter. that’s why I’m assuming it’s the ground. Also ion the alternator is on the same terminal as what I’m pretty sure is the ground for the regulator. So that would make sense too.

No. The alternator should have it's dedicated path to ground in the same size as the alternator output. You can ground to the engine and thence to the batt (-) IF you are confident there is little or no resistance in the engine mass. Of course the engine must also be grounded using at least the size capable of carrying your maximum load (the starter). Do not count on the alternator mounting arrangement to be an effective ground. I like to run the ground on the flywheel housing (close to the starter) to the negative post, thence to the shunt, thence to the breaker, thence to the battery gang post.

[joe_db]

From my old photos (I took out the gauges as I’m changing some things) the large black wire looks like it went to the ground of the ammeter. that’s why I’m assuming it’s the ground. Also ion the alternator is on the same terminal as what I’m pretty sure is the ground for the regulator. So that would make sense too.

Ammeters do not have a ground connection
That alternator is not isolated, so most of the time they ground through the mounting bolts to the engine block. Can you trace that black wire? I sort-of-suspect someone may be doing the reverse of what you expect, using that wire to ground something else to the alternator, not the reverse.
The correct way to ground the alternator would be to run that wire to the ship's ground bus or if you don't have one, to run it to the same spot on the engine were the ground wire runs from the engine to battery negative.

[ndutton]

Ben, I've never had an alternator ground wire on any of my boats dating back 50 years or any of the boats I built (100's of them, probably half with Atomic 4s straight from Universal) dating back longer and cars even longer than that No problems that required a redundant alternator ground wire to solve.

[joe_db]

Ben, I've never had an alternator ground wire on any of my boats dating back 50 years or any of the boats I built (100's of them, probably half with Atomic 4s straight from Universal) dating back longer and cars even longer than that No problems that required a redundant alternator ground wire to solve.

I have one, but only because my alternator is isolated ground. Other than that, a ground wire is very rarely used.

[ndutton]

I have one, but only because my alternator is isolated ground. Other than that, a ground wire is very rarely used.

That's a specialized condition. As I was watching this thread I got the feeling Ben was being encouraged to chase a very rare zephyr. There's no downside to adding a redundant ground wire, just that no builder or engine manufacturer I've ever run across ever provided one. Notice also that it does not exist on the MMI wiring diagram, their engine rewiring kit, their API alternator or their exchange engines. They do however mention an OPTIONAL redundant ground on their high output alternator instructions.

[BenCT]

I came up with the ground idea as if you see my ammeter photo before I took it apart I’m pretty sure that black wire with yellow heat shrink is one of the wires going to the g terminal on the ammeter or possibly the fuel meter. And in my search for wire diagrams of the model of my Motorola mr12n451d Alternator I came across references to other alternators that looked similar, not the same, but were compatible with westerbekea and they and other older alternators all show a ground in the diagram. So…seemed logical. I may be able to find an old diagram somewhere for this alternator in my old original boat documentation that has been passed down to me from all the other owners of my boat. I will double check in morning.

[hanleyclifford]

I came up with the ground idea as if you see my ammeter photo before I took it apart I’m pretty sure that black wire with yellow heat shrink is one of the wires going to the g terminal on the ammeter or possibly the fuel meter. And in my search for wire diagrams of the model of my Motorola mr12n451d Alternator I came across references to other alternators that looked similar, not the same, but were compatible with westerbekea and they and other older alternators all show a ground in the diagram. So…seemed logical. I may be able to find an old diagram somewhere for this alternator in my old original boat documentation that has been passed down to me from all the other owners of my boat. I will double check in morning.

Could it be that black wire is simply on the mounting bracket serving as a ground for the instrument light?

[hanleyclifford]

That's a specialized condition. As I was watching this thread I got the feeling Ben was being encouraged to chase a very rare zephyr. There's no downside to adding a redundant ground wire, just that no builder or engine manufacturer I've ever run across ever provided one. Notice also that it does not exist on the MMI wiring diagram, their engine rewiring kit, their API alternator or their exchange engines. They do however mention an OPTIONAL redundant ground on their high output alternator instructions.

I'm no expert on this but as it was explained to me by the "expert", using the same size cable (presumably with the same resistance value) helps with the diode balance, which is above my pay grade, but I have to give the man his due as he is in the business of manufacturing high output alternators.

[hanleyclifford]

That's a specialized condition. As I was watching this thread I got the feeling Ben was being encouraged to chase a very rare zephyr. There's no downside to adding a redundant ground wire, just that no builder or engine manufacturer I've ever run across ever provided one. Notice also that it does not exist on the MMI wiring diagram, their engine rewiring kit, their API alternator or their exchange engines. They do however mention an OPTIONAL redundant ground on their high output alternator instructions.

Neil - The MMI information is for the basic 35 amp alternator and the system with few accessories and requirements, from which we have gone far afield (if you'll pardon the pun).

[joe_db]

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.