Starter grounding issue

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johnbuoy1

Recruit
Joined
Apr 13, 2025
Messages
3
Hi

I was dealing with a bad starter - cranking slowly. I did a voltage drop test between the earthing mount on the motor and the negative battery terminal. Only losing 0.5v.
I then did a voltage drop test from the earthing mount on the motor and the starter casing - whopping 2.5v drop.

So I rerouted the negative battery terminal to be mounted on the starter bracket. No more slow cranking issues.

My question is, how am I losing 2.5v from the earth mount to the starter motor, when the whole engine block is basically an earth mount? And why did connecting directly to the starter fix this? Are there any repercussions to my fix?
 
M

Mc Tool

Petty Officer 1st Class
Joined
Aug 7, 2024
Messages
312
Metal bits that are bolted together get corrosion between them specially where there is electricity present . Start by removing starter and wire brush the surfaces where it bolts to the block ,clean up bolt hole and threads and spring washer if fitted . Then do same to the earth cable connections . It may well be an accumulation of a few joints.
Do your volt drop test in smaller bites and you may see the faults at each join . At the moment you know it's somewhere between the earthing mount and the starter casing .
 
J

johnbuoy1

Recruit
Joined
Apr 13, 2025
Messages
3
Mc Tool said:
Metal bits that are bolted together get corrosion between them specially where there is electricity present . Start by removing starter and wire brush the surfaces where it bolts to the block ,clean up bolt hole and threads and spring washer if fitted . Then do same to the earth cable connections . It may well be an accumulation of a few joints.
Do your volt drop test in smaller bites and you may see the faults at each join . At the moment you know it's somewhere between the earthing mount and the starter casing .
Click to expand...
I guess that's the cause of my confusion, there are no more connections between the starter motor and the earth mount, since the starter is bolted directly to the motor casing.

The mounts and contacts have been cleaned.
 
C

cyclops222

Lieutenant Commander
Joined
Mar 21, 2024
Messages
1,962
I have always ran the battery negative cable end DIRECTLY to the starter mounting bolt that is easiest to reach.
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,073
Gee, if in doubt, add a short battery cable from starter to frame.

Mercury always mounts their starters in rubber, and therefore always have to have a cable from starter to frame.
 
J

johnbuoy1

Recruit
Joined
Apr 13, 2025
Messages
3
cyclops222 said:
I have always ran the battery negative cable end DIRECTLY to the starter mounting bolt that is easiest to reach.
Click to expand...
And I will too from now on. I was just a bit taken aback that it isn't done from factory, considering the voltage drop lost from using the engine block/starter case bracket connection as the earth. Also that this quick, simple fix wasn't mentioned in any of the forum threads or videos I've searched trying to rectify the issue. Hope this helps a newb like me someday.
 
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