What could be wrong if you installed a new battery but the car will not start?

Answer 1

No modern vehicle with electronic systems will be able to start if you have fitted the battery the wrong way round - i.e. it should not have its positive terminal connected to the car's chassis and its negative teminal connected to the main 12 volt terminals on the starter and fuse box - so it is worth making certain that you know exactly which post on the battery is positive and which is negative and which cables have to be connected to each post.

Assuming the engine usually started reliably before the battery failed and that you made sure the new battery was fully charged-up before before you installed it - and that the battery has been fitted the right way round - then bad connections are the most likely reason the engine won't start.

Safety Advice

To avoid getting accidental sparks or bad shorts, which could easily melt your tools as well as give you a nasty burn - or even start a very dangerous situation if ever the battery's leads catch on fire - it is highly recommended that you fully disconnect both sets of leads from the + and - battery posts before you disconnect and re-connect any other leads elsewhere around the engine.

The next thing to do is: undo, clean and re-connect tightly all the terminal posts and terminal eyes for the ground leads you can see connected between the battery, the starter motor and the car's firewall and frame/chassis. Then clean and re-connect the leads to the battery posts.

If the engine still won't start, again fully-disconnect both sets of leads from the + and - battery posts and undo, clean and re-connect tightly all the other (non-grounded) terminal posts and terminal eye connections for all the other leads going between the battery and the starter and all other ignition and injection devices around the engine. Then once again clean and re-connect the leads to the battery posts.

If all those connections are definitely good - cleaned and tightened - but the car still won't start:

• if the starter motor is definitely turning:

• either the fuse feeding the spark-ignition system has blown

  or, if that fuse is ok:

• there could be problems with the spark-ignition system (ignition coil and/or electronics) if it is a gas/petrol engine

  OR

• either the fuse feeding the fuel injection management system has blown

  or, if that fuse is ok:

• there could be problems with the fuel injection management system

  (e.g. for a diesel engine or a gas/petrol engine having fuel injectors and no carburettor)

  or

• there could be problems with the engine control/management unit (ECU)

• if the starter motor is not turning:
  • either (if you don't get any "clicks" from the starter motor's solenoid) the fuse feeding the starter solenoid has blown and needs to be replaced

    or, if that fuse is ok:

  • (if you only get "clicks" from the starter motor's solenoid) the starter motor and/or its solenoid may need to be repaired or replaced.