* High Current Draw: The starter motor draws a very high current to crank the engine. If there's a problem with the starter itself (worn brushes, damaged windings), or with the connections to the battery or solenoid, the resistance increases. This increased resistance causes more heat generation (I²R losses, where I is current and R is resistance). Excessive heat can melt insulation, potentially igniting nearby materials.
* Faulty Solenoid: The solenoid is the electromagnetic switch that engages the starter motor. A sticking or internally shorted solenoid can continuously draw high current even when the key is released. This prolonged high current draw is a major fire hazard. A failed solenoid can also cause arcing, which produces sparks and intense heat.
* Corroded Connections: Corrosion at the battery terminals, the solenoid connections, or the starter motor connections dramatically increases resistance, leading to excessive heat buildup and potentially causing a fire. This is especially common in older vehicles.
* Wiring Issues: Damaged or frayed wiring anywhere in the starter circuit can create high resistance and sparking, leading to a fire. This can be caused by rodent damage, chafing against metal parts, or simply age and wear.
* Poor Ground Connection: A poor ground connection between the engine block and the battery's negative terminal can force current to seek alternate paths, leading to overheating and sparking.
* Battery Problems: While less likely to directly cause a fire, a weak or failing battery can place extra strain on the starter and solenoid, exacerbating the problems mentioned above. A low battery voltage forces the starter motor to work harder.
In short, it wasn't just *one* thing, but likely a combination of these issues that led to the near-fire situation in your truck. The high current draw, compounded by poor connections and potentially a faulty component (starter or solenoid), generated enough heat to create a dangerous situation. You were extremely lucky it didn't fully ignite. It's crucial to have a qualified mechanic inspect the entire starting system to prevent a future incident. Don't attempt repairs yourself unless you're experienced with automotive electrical systems—working with high amperage can be dangerous.
