The batteries are connected in parallel circuit (positive terminal to positive terminal, negative terminal to negative terminal). Hooking the batteries up in parallel circuit increases battery cranking power without increasing total battery voltage output (12 volts). On the other hand, hooking two 12 volt batteries up in a series circuit (positive terminal to negative terminal, positive terminal to negative terminal) increases total battery output to 24 volts (12 volts + 12 volts).
In the event that a dual battery diesel must be jump started, use the following procedure.
- Open the hood and locate the batteries. On GM diesels, the manufacturer usually suggests using the battery on the driver's side of the vehicle to make the connection.
- Position the donor vehicle so that the jumper cables will reach from its battery (must be 12 volt, negative ground) to the appropriate battery in the diesel. Do not allow the vehicles to touch.
- Shut off all electrical equipment on both vehicles. Turn off the engine of the donor vehicle, set the parking brakes on both vehicles and block the wheels. Also, make sure both vehicles are in NEUTRAL (manual transmission models) or PARK (automatic transmission models).
- Using the jumper cables, connect the positive (+) terminal of the donor vehicle battery to the positive terminal of one (not both) of the diesel batteries.
- Using the second jumper cable, connect the negative (-) terminal of the donor battery to a solid, stationary, metallic point on the diesel (alternator bracket, engine block, etc.). Be very careful to keep the jumper cables away from moving parts (cooling fan, alternator belt, etc.) on both vehicles.
- Start the engine of the donor vehicle and run it at moderate speed.
- Start the engine of the diesel.
- When the diesel starts, disconnect the battery cables in the reverse order of attachment.