6BTA Isolated Ground vs Negative Ground ?

[Alex]

Hi,

Looking thru the diagrams found the “Wiring Diagram – Standard (B/C Series)”.

It shows that when the ground is connected to the starter is an Isolated Ground and…

ground to the engine block is a Negative Ground system.

My 6BTAs have grounds attached to the bellhousing and not to the starter negative stud which makes them a Negative Ground system, I think.

From a marine electrical requirements/system standpoint, what is correct?

Just thinking that the block and everything (gear, sensors, solenoids, alternator etc) attached to it is grounded, including the starter when grounds are attached to the block.

And in an Isolated Ground system only attached to the starter stud, only the starter internally is grounded but not the block?

Looking for some clarity in this area.

Thanks!

Topic

[Alex]

Excellent Tony, Thank you all for the advise.

Is there anything else “electronic” wise that I need/Must have in my 6BTAs?

Oil pressure sender, Coolant Temp sender, RPM Sender on bell housing, ignition On for fuel solenoid hold and ignition signal for starting and fuel solenoid pull, alternator?

[Tony Athens]

Gene is basically right.. Most starters today have a ground stud but it IS NOT Isolated.. The reason it’s there is mostly for electronic engines, as it’s best to not pass any current thru any part of the block but for that current to take the shortest possible path — Would the neg work if fastened to the bell housing–Of course it would………….Lot’s of people think they have an isolated starter because of the visual ground stud, but in realty they do not..

Most alternators are also “case ground”, but then again on electronic engines it’s best to use the case gnd stud on the alternator housing and put a #4, #2, or #1 wire from there to the main neg going to the motor to eliminate current going thru the block. Current will always takes the easies path.. Even Cummins knows that.

[Alex]

Thank you for all the great advise Gene, Mark and Clark.

Mostly for convenience and to clean/organize things better:

I will be changing the negative connection from the block to the starter.
Add extra negative strap from starter to block (optional).
Keeping the negative cable from engine to engine.
Keeping the bonding cable from each engine to the bonding busbar.

Something else I have to sort is engine harness and keep it to a minimum. (Oil pressure, Coolant Temp, , RPM Sender, ignition On for fuel solenoid hold and ignition signal for starting and fuel solenoid pull).

Eliminating intake air heater module/solenoids etc and the temp sensor in the aftercooler.

Anything else that I MUST have?

[Clark Leighs]

There are isolated ground starters and alternators. My own engine has an isolated ground starter.
Usually the isolated ground starters/alternators are needed and used in metal boats to ensure the large loads , actually all loads, do not get a chance for the current to use the hull metal as a path back to the batteries, chargers, etc.

Our fiberglass boats don’t generally need this. However, they often come that way which is the case with my engine.

My starter neg. goes back directly to the batteries..
My alternator is not an isolated ground unit but the neg. is attached to the heavy bracketry of the mount. It also serves as the engine ground for all the small loads for guages and such.

[Mark]

Yes you can connect the negative direct to the starter which is technically better than connecting to block but i doubt if you’d notice a difference. But yes its better.

And yes you can add the extra negative strap from starter stud to block that would do no harm, I believe the alternator relies on the block for its negative return. all other devices use a black negative already in the harness.

Yes a heavy negative cable between engines is a good idea just in case you have to crank the stb engine of the port battery or visa versa assuming you have the battery selector switches set up that way.

Really dont know why that diagram refers to the starter stud as an isolated ground. Probably something to with a grounded system versus a floating system.

[Gene Fuller]

That looks like a convenient stud to attach the negative battery feed, but it does not look “isolated”. If it were isolated there would be an insulator on that terminal just like the positive terminal. An isolated connection would not feed back to the engine block through the starter mounting flange. (That’s what isolated means.)

The difference between using that terminal and using a bolt on the engine should be negligible if everything is clean and tight.

Gene

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Tony Athens

[Alex]

Thank you Gene, I have the two bolt starter which has the negative stud also in addition to the positive stud.

Not sure if the three bolt has this negative stud, but given that I have the two options I could connect to either block or starter.

The only reason for me changing to starter stud negatives is to freeup the block connections which are multiple and very low in the bellhousing and against the stringer. Trying to clean things up and route some cables in a more organized manner.

I would still connect to the block the bonding cable (green) to the bonding strip but that’s a different story/system.

Why would one starter have the option of negative stud connection and another starter not? Perhaps just designs and applications over the years have steered those differences. But both work either way you wire them? No idea

[Gene Fuller]

You may be overthinking this a bit. Is there some problem you are trying to solve?

Most starters don’t even have a separate negative terminal. If you look at the SBM page for 6B starters,

B Series 3-Bolt Cummins Marine Starter

you can see that connecting the battery negative to the engine is acceptable.

The diagram you posted clearly says “OR” in the wording about grounding. No doubt that diagram is somewhat generic and covers installations of various types.

Gene

[Alex]

Thank you Mark,

I did hear or read somewhere that battery negative should be connected to the starter neg terminal stud. I had envisioned to do this (change from block to starter), but wanted to consult first.

If I connect the battery negative to the neg starter stud as you said it would help in cranking amps.

Should I also add a jumper from the starter negative stud to the block itself or it makes no difference based on what you said?
“the starter frame and block are electrically identical. block or starter negative connected to same thing.”

Also, to tie negative corrections from one engine to the other: Should I add an engine to engine jumper cable from the blocks or from the starter studs?

[Mark]

I feel nervous about replying this but here goes.

Quote;
it shows that when the ground is connected to the starter is an Isolated Ground and…

ground to the engine block is a Negative Ground system.

Im a 40 year exp elec professional, the above that you posted makes zero sense to me. the starter frame and block are electrically identical. block or starter negative connected to same thing.

the only thing ide advise is to take the negative from the battery direct to starter so the cranking high amps dont have to pass thru the cast iron block to get to the starter. copper conducts better than iron.

[Alex]

Adding reference file – diagram.

[Author]

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