98% of people overlook the grounds. Initial diagnostic testing always starts on the + side, for some reason??.
On mowers you need to check the engine mount to mower frame bolts are present and tight.
Some mowers will install the ground cable from the - batt post to the mower frame. Relying on rusty engine mounting bolts, paint and rusty frames to provide a poorer path back to the battery. They do this as it makes the ground cable shorter and long story short CHEAPER.
In reality, ground cable should go from batt - to the starter housing. Why? The starter is the highest amp draw device on a mower. Connected this way provides the best path for the starter to spin faster. Starter will perform better and last longer/run cooler with a better ground.
Looking at ground paths. A steel mower or car frame, with
CLEAN bare metal connections carries current at a 70% value. A clean over amp rated/sized full copper wire is valued at 100%. So a clean steel frame connection starts out inferior. Then you add paint and rust...... Copper is better than steel as we all know.
You need to research star grounding washers, dielectric grease, hydraulic crimp tools and full copper lug connectors.
If you want a little more info, watch the below video. Look at WHAT they are trying to do and not HOW they do it. Most of these videos are wrong for this purpose. It's called the big 3 or big 4 upgrade.