The big change from then to now is computers and it’s not as if a modern car has just one – a Mercedes-Benz S-Class from a few years ago has 64 ECUs (Electronic Control Units).
Recently, Jaguar issued a warning bulletin that said you could damage the whole wiring harness if you try to jump-start one of their cars. In my opinion that warning is applicable to most vehicles built within the past 10 years.
A five-year-old Audi was recently taken to a motor repairer with an electrical system so badly damaged by an attempted jumpstart that, in the end, it was actually cheaper to write the car off rather than repairing the damage.
But what has driven this destructive change?
The simplest answer is … computers.
There are ECUs that control systems such as engine control, traction and body control and safety. They operate via a binary code that relies on a stable voltage from the battery/vehicle.
Zapping them with a random spark or short from jumper leads is like shocking the human heart: an interruption of the correct level and rhythm is deadly.
Always check the vehicle’s handbook before using jumper leads. All manuals include general advice on jump starting and some include model specific procedures. If this is the case you must follow the manufacturer’s procedure rather than the steps described below.
Jump starting a vehicle can be a safe procedure however and damage can be avoided if a few simple rules are followed:
- Engage both parking brakes as well.
- Place both vehicles in Park or Neutral and shut off the ignition in both cars.
- Attach one of the red clips to the positive terminal of your battery.
- It has “POS” or “+” on it, and the + terminal itself usually is the bigger of the two.
- Attach the other red clip to the positive terminal of the donor car.
- Attach one of the black clips to the negative terminal on the donor battery.
- Attach the last black clip to an unpainted metal surface on your car that isn’t near the battery. Many vehicles give you a specific earthing point.
- With both leads connected wait three minutes for the voltages to equalise before starting either engine.
- Start the engine of the donor car and allow it to run for a minute.
- With the donor car engine still running, start the engine of the donor car and leave both running at a fast idle for ten minutes.
- Don’t remove the jump leads while the engines are running as this can cause serious damage to the electronics on either car.
- If the jumper leads feel hot to the touch switch off both engines and allow the leads to cool to avoid a possible fire.
- Turn off the ignition on both cars and then disconnect the leads carefully in the reverse order to the way that they were connected. Be careful not to touch the clips against each other or against the car bodywork.
- Start the car that had the dead battery using its own battery power.
NB: If it won’t start, make sure that the cables are properly connected and have the donor vehicle run for five minutes. Then try to start your car again. If it still won’t start, your battery may be beyond help or this could indicate a more serious problem with the charging/ignition system that will need investigating by a professional.
An example image shows the correct way to hook up the 2 batteries