24V electrical system and CAN-bus in trucks
How the truck's control systems work together
A modern truck is full of electronics. Engine control transmission brakes instrument panel and superstructure equipment are each controlled by a small computer — a control unit (ECU) — and they must all be able to exchange information with each other. As a truck mechanic you therefore encounter an electrical system that is far more interconnected than in an ordinary car and where troubleshooting requires understanding both power and data.
§Why 24 volt?
Heavy vehicles run on a 24V electrical system, while most cars use 12V. The higher voltage makes it possible to draw the same power with lower current and thus thinner cables — an advantage in a vehicle with long cable runs to trailer, superstructure and extra equipment. The battery pack typically consists of two 12V batteries connected in series, and the starter motor must be able to crank a large diesel engine, often in cold, which places high demands on both battery capacity and charging system.
§CAN-bus — the network under the cabin
Instead of running a separate cable for each function, the control units are connected on a shared data bus: CAN bus (Controller Area Network). Here, all units send and receive short data messages on the same pair of wires, and each unit listens only to the messages it needs. This drastically reduces the amount of cabling and makes it possible for, for example, the brake system to know the engine's rpm or for the instrument panel to show the gearbox's state — without a direct connection between them.
§J1939 – the language on top of the bus
The CAN bus itself is only the transport layer. On top of it runs a standardised protocol developed by SAE's (Society of Automotive Engineers) committee for truck and bus communication: SAE J1939. It defines how messages about eg engine load brake pressure or tank level are named and structured so components from different manufacturers can understand each other. J1939 is today the common standard in heavy vehicles construction machinery and buses and underlies the diagnostic equipment you use in the workshop.
- 01Engine control (ECU) — controls injection, turbocharging and exhaust treatment.
- 02Transmission control (TCU) — controls clutch and gear shifts in automatic transmission.
- 03Brake control (EBS/ABS) — monitors wheel speed and distributes brake force.
- 04Instrument panel – collects information from the other units and displays status and faults to the driver.
- 05Body control — controls lights, lifts, compressors and other auxiliary equipment.
§Troubleshooting: power or data?
When something acts up, the first question is always whether the error is electrical (poor connection, corrosion, faulty sensor) or communication (a control unit not sending or receiving correctly on the bus). Loose plugs, moisture in connectors, and damaged cables are still the most common causes of electrical faults on heavy vehicles precisely because cables are exposed to vibrations, salt water, and temperature swings. Diagnostic equipment can show whether a control unit doesn't answer on the bus at all, which often points to power supply or bus connection rather than the unit itself.
| Symptom | Probable cause |
|---|---|
| One function fails | Local sensor, actuator or cable |
| More functions fail at the same time | Common power supply, ground connection or bus fault |
| Control unit does not respond at all | Power supply to the unit or interrupted bus connection |
| Sporadic errors with vibration/cold | PLACEHOLDER_12 |
The better you understand how the 24V system and CAN bus work together, the faster you can narrow down a fault before you start replacing parts. This saves time in the workshop and results in less downtime for the driver.