Using jump leads

Introduction

It is important to check the vehicle handbook before using jump leads. It will include general advice on jump starting but some also include model specific procedures. If this is the case you must follow the manufacturer’s procedure NOT the steps described here.

Figure 3 Jump leads

Safety

Jump starting a vehicle is a safe procedure if some simple safety rules are followed (in fact these rules are appropriate for most work in the engine area):

  • Keep metal objects a way; this includes rings, watch straps, tools, etc. Touching a battery post can cause a spark, possibly exploding the battery and releasing acid.
  • Don’t attempt to jump start a battery that is leaking or looks damaged because it could explode.
  • Avoid smoking or naked flames. Batteries give off flammable gases (hydrogen and oxygen) so an explosion could result.
  • Keep your hands well away and avoid loose fitting clothing such as scarves or neckties. When the engine is running these items become trapped in pulleys or belts and serious injury can result.
  • Don’t use damaged jump leads. Broken conductors or damaged clamps can overheat and may cause a fire.

Step by step

Before connecting any leads make sure the vehicles are the same voltage and that they’re parked with their handbrakes on and ignition off. The vehicles must not touch as this can cause a short circuit.

Figure 2 Do not let the vehicle bodies touch

Procedure

  1. Connect the positive terminal of the donor vehicle’s good battery to the positive terminal of the flat battery using the red lead.
  2. Connect the negative terminal of the good battery to a suitable earthing point on the engine or chassis of the other vehicle using the black lead. This earthing point must be away from the battery and fuel system.
  3. Now wait three or four minutes for the voltages to equalise before starting either engine.
  4. Start the engine of the donor car and allow it to run for a few minutes.
  5. With the donor car engine still running, start the engine of the other car and leave both running at a fast idle for ten minutes.
  6. Do not remove the jump leads while the engines are running. This can cause serious damage to the electronics on either car.
  7. If the jump leads get hot switch off both engines and allow the leads to cool to avoid a possible fire.
  8. Turn off the ignition on both cars and then disconnect the red (positive) lead being careful not to touch the clips against the other lead or the car bodywork. Now disconnect the black lead.
  9. Start the car that had the dead battery using its own battery power.

Note: If the car will not start this could indicate a more serious problem with the starting, charging or ignition system that will need further investigating.

Figure 3 Jump leads correctly connected to the battery

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