Hilux Undervoltage Issue

Undervoltage can stem from several causes. Before delving into the voltage regulator, have you had the battery's health assessed? A heavily discharged or defective battery can manifest in the ways you describe. The starting system demands considerable current, and a weak battery will struggle to meet that demand, causing voltage drops that affect other systems. It's worth having a professional load test performed on the battery. Also, check the earth connections.

Thanks. I hadn't considered the battery itself, given its relatively recent replacement – about two years ago. But your point regarding a load test is well-taken. Could you elaborate on the earth connections you mentioned? Where are they located in the engine bay, and what should I inspect for?

Two years isn't necessarily an indicator of perfect battery health; environmental factors play a large role. Regarding earth connections, focus on the main battery ground strap that connects the negative terminal to the vehicle's chassis. Also, inspect the engine-to-chassis ground strap, typically found near the engine mounts. Corrosion can impede electron flow, resulting in voltage drops. Given the symptoms and the potential complexity, I would advise a qualified technician to inspect the entire charging system. It can be frustrating, but it is the best course of action.

I appreciate your guidance. I took it to a workshop, and it turned out the battery was defective, as you suspected. A new battery set me back 290€, but the Hilux is now running smoothly. Thanks again for your assistance!