The situation when a truck with a powerful diesel engine does not start due to discharged batteries is familiar to many long-distance drivers. Unlike passenger cars, commercial vehicles are often equipped with an on-board voltage 24 volts, which requires a special approach and specialized equipment. Connection errors in this case can be very costly, including failure of expensive electronics or battery explosion.
The process of starting an engine from an external source, popularly called “lighting up,” requires strict adherence to the sequence of actions. Volvo, Scania, MAN and other European truck manufacturers have their own nuances in terminal locations and starting current requirements. Ignoring these features may result in a short circuit in the engine control system.
In this article, we will go over all the technical aspects of starting a heavy truck safely. You'll learn how to choose the right jumper leads, how to connect them to a two- or four-battery system, and what precautions you need to take to avoid arcing and damage to the vehicle's electrical system.
Features of the on-board network of a truck
The main difference between freight vehicles and passenger vehicles is the voltage of the on-board network. While a standard passenger car runs on 12 volts, most heavy trucks use a 24 volts. This is done in order to reduce the current in the circuit when starting a powerful starter, which allows the use of smaller wires and reduces energy losses.
A truck battery often consists of two 12-volt units connected in series, or one large battery with the appropriate voltage. In some cases, especially older models or specialized equipment, a parallel circuit may be found to increase the capacity at 12 volts. Before starting any work, the driver must know exactly the parameters of his system.
⚠️ Warning: Attempting to connect a 12-volt power source to the truck's 24-volt power supply will result in the starter not being able to crank the engine. Connecting 24 volts back to a 12-volt network is guaranteed to damage lamps, electronics and fuses.
Diesel engines require significantly higher starting current, especially in winter. Thick oil and high compression create enormous drag. Therefore starting current (CCA) for cargo batteries can reach 1000-1500 Amps and higher. Ordinary passenger cigarette lighters with thin wires will simply burn out or melt in the first seconds of a startup attempt.
Modern trucks are full of electronics: engine control units ECU, telematics, navigation and comfort systems are sensitive to power surges. If there is a sudden surge in current or a polarity reversal, not only the fuse can burn out, but also the control unit itself, the replacement of which will cost a tidy sum.
Selecting the Right Starting Equipment
For successful and safe starting of a truck, standard wires for passenger cars are absolutely not suitable. Their cross-section (usually 16-25 mm²) is not enough to transmit the hundreds of amperes required by a diesel engine. The use of such wires will lead to their heating, a drop in voltage at the ends and the inability to crank the starter at the required speed.
The optimal solution is professional starting wires with a copper core cross-section of at least 50-70 mm². Such cables can withstand currents of up to 2000 Amps in short-term mode. The crocodile clips must be massive, with a powerful spring and copper contacts that ensure reliable contact with the battery terminals.
- 🔌 Wire size: For 24V trucks, the minimum cross-section should be 50 mm², preferably 70 mm² or more.
- 🌡️ Insulation material: Rubber must remain elastic at low temperatures (down to -40°C) and not crack in the cold.
- 🔩 Clamp design: The teeth of the “crocodiles” must be sharp to penetrate oxides on the terminals, and the handles must be dielectric.
An alternative to donor wires are portable jump starters, often called boosters. Trucks require specialized 24-volt, high-current models. Lithium iron phosphate (LiFePO4) the batteries in such devices withstand frost better than conventional lithium-ion batteries.
Can a welding machine be used?
In theory, some drivers use inverter welders in "start" mode for charging or starting. However, this is extremely risky for modern truck electronics due to unstable voltage and ripple current. Without special filtering devices and experience, it is better not to risk expensive equipment.
Preparing for the startup procedure
Before connecting the wires, careful preparation is necessary. Both cars - the faulty one and the donor - must be turned off. The ignition keys should be removed from the locks or switched to the “off” position to prevent accidental starting of the starter or switching on of consumers during switching.
Inspect the batteries for damage to the housing. If cracks are visible, electrolyte has leaked or the battery is swollen, use it for starting or as a donor prohibited. During charging or discharging, a damaged battery may release gas that, if sparked, will cause an explosion.
Clean the battery terminals from oxides and dirt. To do this, you can use a metal brush or a special contact cleaner spray. Clean contact is the key to successful current transfer. If the terminals are coated with oxide, the resistance will increase and the starter will not receive the necessary energy.
☑️ Ready for launch
Position the donor cars so that the starting wires are long enough to connect, but the car bodies do not touch. The donor truck should be idling (if already running) or ready to start. In winter, it is recommended to warm up the donor engine before connecting it so that the battery delivers maximum current.
Diagram of connecting wires to a 24V battery
The most important step is connecting the wires. In trucks with a 24-volt on-board network, the connection diagram differs from that of a passenger car. Typically, the system has two batteries connected in series: the “plus” of the first is connected to the “minus” of the second. Free terminals give a total voltage of 24 volts.
It is necessary to find the external conclusions of this bundle. Often, for ease of maintenance, special copper coins or terminals are located on the truck frame "+" and "-". It is to them that you need to connect the starting wires. If there are no such leads, you will have to connect directly to the battery terminals in the niche.
The connection procedure is strictly regulated to prevent sparking. First, connect the “positive” wire to the positive terminal of the discharged battery (or 24V terminal). Then the second end of the “plus” is connected to the positive of the charged donor battery. Only after this is the “minus” connected: first to the donor’s minus, and lastly to the minus of the discharged battery or the “ground” of the engine.
| Step | Action | Wire color | Risk of error |
|---|---|---|---|
| 1 | Plus to a discharged battery (24V) | Red | Short circuit |
| 2 | Plus to donor (24V) | Red | Sparking |
| 3 | Minus for the donor | Black | Low |
| 4 | Minus for a discharged battery/Ground | Black | Gas explosion |
⚠️ Attention: When connecting the last contact (minus), a spark often occurs. This is normal as current begins to flow through the circuit. However, if the spark is strong and long lasting, immediately disconnect the wire and check the correct pole connections.
If you are using two separate 12-volt donor batteries to start a 24-volt truck, you must first connect them in series (minus the first to the positive of the second), producing 24 volts, and only then connect this bundle to the truck. Connecting two 12-volt donors in parallel will only produce 12 volts, which is not enough to get started.
Engine starting and shutdown process
Once all the wires are securely fastened, you can start starting. If a donor vehicle is used, start its engine and let it run for 5-10 minutes at high speed (about 2000 rpm). This will allow you to slightly recharge dead batteries and warm up the electrolyte, which will increase its current output.
Try starting the truck. If the engine does not catch on the first try, do not turn the starter for more than 10-15 seconds. Let the batteries rest for a couple of minutes. Prolonged cranking of the starter can overheat its windings and permanently damage the donor. For a diesel engine, the shaft rotation speed is important: if the starter turns sluggishly, there will be no start.
If the truck does not start after 3-4 attempts, check the tension of the starter harness belts. Poor contact at the junction of the crocodile and the terminal can “eat up” up to 5 volts of voltage, making starting impossible.
Once the engine has been successfully started, it is important to properly disconnect the wires to avoid power surges. The disconnection procedure is the reverse of the connection order. First, remove the negative from the discharged battery (or ground), then the negative from the donor. After this, disconnect the “plus” from the donor and, lastly, the “plus” from the running truck.
Do not turn off the truck engine immediately after starting. Let it idle for at least 20-30 minutes so that the generator has time to restore charge to the batteries. If you plan to park for a short time, it is better to leave the engine running or use stationary charging.
The main rule for disconnecting: first we always break the negative circuit to prevent accidental shorting of the positive wire to ground when removing the second clamp.
Common mistakes and safety precautions
One of the most common mistakes is trying to “light” a truck from a car. Even if the voltages are the same (which is rare), the capacity and starting current of a passenger car battery are insufficient for a diesel engine. As a result, you will simply land the car and possibly overheat its wiring, but the truck will not start.
Another dangerous mistake is the clamps touching each other or metal parts of the body when the other end of the wire is connected. In a 24-volt circuit with hundreds of amperes of current, a spark can instantly melt metal and cause burns. Hold the clamp handles only by the insulated part.
- 🚫 You can't Smoking or using an open flame near batteries while charging, as oxyhydrogen gas is released.
- 🚫 You can't touch metal parts of both cars' engines during the process.
- 🚫 You can't leave the wires connected after starting the engine for a long time without the generator running.
In cold weather, batteries may freeze. If the electrolyte in the battery has turned to ice, try to charge or “light” such a battery strictly prohibited. This will lead to destruction of the plates and possible explosion of the housing. The battery must be brought into a warm room and defrosted.
⚠️ Attention: Modern trucks may have intelligent energy management systems. Before connecting an external source, check the instruction manual (Owner's Manual). Some systems require disconnecting the “mass” or carrying out the procedure in a special service mode.
Questions and answers (FAQ)
Is it possible to power a 24-volt truck with two 12-volt car batteries?
Yes, this is possible, but only if you connect two car batteries in series (minus the first to the plus of the second) to get 24 volts. However, the capacity and starting current of passenger car batteries (usually 60 Ah) may not be sufficient for a large diesel engine, especially in winter. Car batteries can quickly fail from overload.
How long does it take to charge a dead truck battery from a donor?
For a surface charge sufficient for one start, 10-15 minutes of running the donor engine at high speed is usually sufficient. It takes several hours to fully charge, but in the field the task is to get the minimum charge to start.
What should I do if my truck stalls after lighting up?
This may indicate a faulty generator or voltage regulator. If the alternator does not charge the battery, the engine will run only on the residual charge. The problem may also be in the air filters, fuel system or glow plugs, especially in cold weather. Diagnosis required.
Is the lighting process dangerous for the truck's electronics?
If the polarity and sequence of actions are followed correctly, the risk is minimal. The danger is represented by voltage surges during sudden connection/disconnection under load and polarity reversal. Using high-quality equipment and following the instructions reduces risks to zero.
Can a booster (portable jump starter) be used on a truck?
Yes, if the booster is designed for a voltage of 24 volts and has sufficient starting current (usually from 2000A peak). This is often safer and more convenient than looking for a donor, as it eliminates errors with connecting two machines. It is important to monitor the charge of the booster itself.