Starter motor not working when hot
The Toyota Landcruiser LJ70 TD2.4 had occasional starting problems due to a bad starter relay as seen in this video.
With that starting problem clearly diagnosed another no-start situation emerged some time later.
What happened was the starter motor being completely silent even with connecting a lead directly from +12 Volt battery to the starter lead.
This ruled out a faulty starter relay including all wiring in this circuirty.
What's left was the starter motor and the battery - including the thick starter leads in between.
Remarkably, both the starter motor and the 12V battery were just a few months old.
Even more interresting was that the starting problem only occured (occasionally) when the engine was hot!
This with no sound at all from the starter motor when hot and a guaranteed start when the engine was cold.
A visual inspection of the earth lead from the 12V battery didn't indicate excessive corrosion - the lead was in good shape.
Corrosion causes resistance in the wire and results in a voltage drop and (exessive) heat on the bad spots.
Because the engine - always - started in the morning it was a good test to see if the earth strap heats up when repeatedly starting the engine.
One morning this test was done, though the earth lead itslef wasn't really heating up from the starting.
What did heat up was the earth plate connected to the engine!
Unbolting the plate showed excessive corrosion under the bolt and at the attachment surface to the engine.
Even the bolt thread itself was corroded with rust.
The resistance of the earth plate connection was higher than normal causing it to heat up!
With the engine heating it up even more, the resistance caused by corrosion increases even more resulting in a large voltage drop.
This is also known as - HEAT SOAK.
The corrosive earth plate gets really hot from the running engine building up even a higher resistance.
Leaving the engine standing for 15 minutes reaches the highest temperature, specially in warm / hot summer weather.
Trying to start it then results in the best conditions to cause such a large voltage drop, the starter motor solenoid can not move with so little power.
Cleaning the earth plate and connection surface at the engine was an easy job.
The result was noticeable in the morning when the engine was cold - a subtle more powerful and quick engine start!
In comparison, the starter motor sounded like it's dragging a bit in the morning.