Where did my Alternator ground go???

[Simi 60]

On a genuine Delco 33si

Pic in instructions shows ground screw location
Reality shows it not there or at least the hole not drilled.

Suggestions as to what to do?
I'm guessing drill and tap?
Or will one of the small bolts on back of case suffice?

[Simi 60]

This is the last thing holding me up before we start the engine
Confirmation please

[Uncle Bob]

Originally Posted by Simi 60

This is the last thing holding me up before we start the engine
Confirmation please

Not exactly my forte, but that point appears to be the metal case which is in direct contact, at multiple points, with the engine so perhaps it is already grounded to the engine itself.

[Simi 60]

Originally Posted by Uncle Bob

Not exactly my forte, but that point appears to be the metal case which is in direct contact, at multiple points, with the engine so perhaps it is already grounded to the engine itself.

Yes, it is grounded to the engine
But if that was sufficient why do they mention a grounding post and why is there a wire?

I am assuming that the wire, like the + wire, goes to starter motor then onto battery.

Further into the manual I found this

A threaded 1/4" hole (with screw and lockwasher assembly)
in the rectifier end housing is provided to connect a ground
lead if used; otherwise, the ground path is through the
mounting hardware and brackets to the engine. On
replacement units, a paper tag is present identifying the
“ground screw”; remove and discard the tag. The screw
and lockwasher assembly should be installed in the housing
regardless of whether a ground lead is connected, to
prevent entry of dirt and water.

Of course mine has no 1/4 inch hole, screw, or paper tag

[wholybee]

It depends on the specific engine and alternator. Some engines are required to have an isolated ground. That is, there are no ground connections to the engine block. The starter and alternator cases are not connected to the engine block, the oil pressure and water temp sending units are not grounded to the block, etc. That is why some alternators will require a direct ground wire connection to the case itself.

So, it would depend on what engine you have, and how it is electrically wired, whether or not the alternator needs a ground wire to it.

[Simi 60]

Originally Posted by wholybee

It depends on the specific engine and alternator. Some engines are required to have an isolated ground. That is, there are no ground connections to the engine block. The starter and alternator cases are not connected to the engine block, the oil pressure and water temp sending units are not grounded to the block, etc. That is why some alternators will require a direct ground wire connection to the case itself.

So, it would depend on what engine you have, and how it is electrically wired, whether or not the alternator needs a ground wire to it.

It's an nta855m Cummins
We ran if for a couple of years with no alternator at all if that helps.

[Wotname]

Ignore the grounding issue; the case is grounded by the mechanical connection to the engine ground.

All the fluff about screws and paper tags etc is just Delco's way of covering every possibility no matter how or where the alternator is installed. An auto electrician will make the necessary judgement depending on the installation.

In your instance, nothing more needs doing.

[Wotname]

However I don't see any wire on the 'I' terminal. I don't have the Delco installation document but I would have thought this wire is essential - might be wrong of course!

Typically the I terminal provides the initial field voltage (via the indicator lamp or resistor) until the alternator rpm becomes self exciting.

[cal40john]

From: https://www.delcoremy.com/documents/...35si-36si.aspx

Ground Screw – Ground lead ensures alternator is grounded. A Ground lead is strongly recommended for optimum performance.

But it appears that the alternator is available with an isolated ground, which would require a negative lead, but it wouldn't be directly on the case.
From: https://www.delcoremy.com/alternator...el-family/33si

Negative Ground (Standard)
Isolated Ground (Available)

Did you buy an isolated ground version?

[Putt-Putt]

Simi 60

You have what Delco calls the Negative Ground version of this alternator that has the Negative running through the engine block to the batteries by way of the alternator's case, the mounting brackets and likely the starter's Negative wire.. If it is possible, I would route the existing Negative wire that was connected to your old alternator to the pivot bolt on the mounting bracket. Thats the best you can do as the rear cover bolts are only a #10 machine screw.

Only the Isolated Ground version (which is somewhat rare) has the Negative terminal on the top of the case.

I doubt if this change in the ground path will have any effect on your engine's operation.


Your picture shows an interesting color (yellow) for the Positive wire to the new alternator. Then again perhaps it is actually a white wire?

[Simi 60]

Originally Posted by Wotname

However I don't see any wire on the 'I' terminal. I don't have the Delco installation document but I would have thought this wire is essential - might be wrong of course!

Typically the I terminal provides the initial field voltage (via the indicator lamp or resistor) until the alternator rpm becomes self exciting.

From the manual
Under features, one wire alternator.

[Simi 60]

Originally Posted by cal40john

Did you buy an isolated ground version?

Yes, no, maybe.

I bought 2 alts for very small money (33si 100amp and 36si 105amp) that had been taken off of new engine installs in commercial trawlers.

Originally Posted by cal40john

From: https://www.delcoremy.com/documents/...35si-36si.aspx

Ground Screw – Ground lead ensures alternator is grounded. A Ground lead is strongly recommended for optimum performance.

And as posted above, from the 33si service manual it also says

A threaded 1/4" hole (with screw and lockwasher assembly)
in the rectifier end housing is provided to connect a ground
lead if used; otherwise, the ground path is through the
mounting hardware and brackets to the engine. On
replacement units, a paper tag is present identifying the
“ground screw”; remove and discard the tag. The screw
and lockwasher assembly should be installed in the housing
regardless of whether a ground lead is connected, to
prevent entry of dirt and water.

As I have no 1/4 hole with screw and lock washer assembly....

[Simi 60]

Originally Posted by Putt-Putt

Simi 60

You have what Delco calls the Negative Ground version of this alternator that has the Negative running through the engine block to the batteries by way of the alternator's case, the mounting brackets and likely the starter's Negative wire.. If it is possible, I would route the existing Negative wire that was connected to your old alternator to the pivot bolt on the mounting bracket. Thats the best you can do as the rear cover bolts are only a #10 machine screw.

Sounds like a plan - thanks


The two alts I had prior that I smoked trying to charge big AGM banks never had a neg stud as such
But did have a tapped 1/4 hole

[Simi 60]

Just checked the 36si
No ground bolt on that one either

[Wotname]

Originally Posted by Simi 60

From the manual
Under features, one wire alternator.

Thanks for the info - all makes sense now!