Bad earth?

[the old grind]

I would make sure the battery ground is clean and secure at the frame or engine where it attaches. I'd also consider having the battery lead-tested if it requires a jump to start up. It's not always a bad connection.

Also check battery voltage at rest and with the engine running. Check for leakage/drain between startups, and voltage again after sitting overnight. I've had batteries on chargers that would hold up for a day or two but not four or five. Polished terminals can only pass what the battery is capable of, and neither lasts forever.

 

[Sbns]

On subject of bad grounds----
I've had occasional ground related problems.
I once discovered that the corrosion in a ground cable had travelled as much as 2-3 inches back from the connector.
Stiffness in the adjacent (more than elsewhere) inches is your indicator.
Only cure is, cut and shorten OR replace.

Thanks!
Not a big cable ~12 inch, so probably worth replacing anyway.

 

[Sbns]

I would make sure the battery ground is clean and secure at the frame or engine where it attaches. I'd also consider having the battery lead-tested if it requires a jump to start up. It's not always a bad connection.

Also check battery voltage at rest and with the engine running. Check for leakage/drain between startups, and voltage again after sitting overnight. I've had batteries on chargers that would hold up for a day or two but not four or five. Polished terminals can only pass what the battery is capable of, and neither lasts forever.

Yeh I took the battery to get tested and it is fine (not that old).

 

[Frankenkubota]

I had an old old proff that used the term earth vs ground. He was one of the few that could actually tell you how something worked.

A good teacher is worth their weight in gold.....or bit coins.

 

[pmsmechanic]

It could just be that after the current goes through all the old safety/neutral switches etc. that there just isn't enough current left to activate the starter solenoid. Measure the voltage to the starter solenoid while holding the key in the start position. It should be within a volt of battery voltage. If not use the wire going to the starter solenoid to activate a relay to feed adequate volts to the starter solenoid.

 

[docone]

You have already proven a bad ground by attaching your jumper cable to the frame.
Bite the bullet and buy a new ground cable.
JMHO