Силовой кабель питания окислен

Corrosion inside a high current cable or at its terminations — the battery leads, main earths, and the heavy feeds to glow plug and intake heater circuits. What makes it distinct from an ordinary poor connection is the current involved, and current is what turns a small resistance into a large voltage drop.

It is invisible to the test most people reach for. A meter's continuity check passes a few milliamps and reads near zero ohms across a joint that will drop several volts under a hundred. Worse, corrosion often travels inside the insulation from a chafed point or a terminal that has wicked salt water, so the cable looks perfect along its whole visible length while its usable section has been eaten away.

Measure voltage drop with the circuit loaded, which is the only test that finds it. Probe across each joint and cable while the load is commanded on. Heat is the other clue — a joint dissipating enough power to drop volts gets warm, so a terminal noticeably warmer than its neighbours after the circuit has run is the finding. It explains why intake heater and glow plug faults cluster in winter: those circuits only draw their full current when it is cold, which is exactly when a marginal joint is asked to prove itself.