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Starter Motors - Testing

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The car will not start, the engine doesn't turn

This section will help narrow down the cause. Ideally you will have access to a multi-meter but a test light is OK for many tests.

 

1. Check the battery

Has the battery electrolyte over the plates?

  • Is it discoloured ?
  • Are the terminals  tight ?
  • Is it charged ?
  • Is it worn out ?

What is the battery voltage?

A fully charged battery in good condition has a standing voltage of about 13.2 v. Test this  by connecting a multi-meter between positive and negative terminals. 

Note - A vehicle that has just stopped may show a higher voltage than this. Switch on the lights for a minute or two then turn them off and leave the car for about 2 – 3 minutes before testing!

If the voltage is under 12 volts suspect the battery. At this stage it is worth turning the key if the voltage drops drastically it is almost certain that the battery is faulty. So charge – retest and only then replace. Hydrometer tests are also valid and are available from accessory shops. 

It is a sad fact that a faulty battery can cause the starter motor and the alternator to fail. It is worthwhile being prepared to change all items.

2. Check the wiring

Once the battery has been tested, check the wiring. Many faults are caused simply by bad connections. 

Check earths

To check earths you can test the resistance down the leads which should always be less than ½ ohm and ideally zero. The maxim is if in doubt clean or replace any earth connections – shiny metal is a must. Some cars still have braided earth straps, which has a tendency to become resistive just for the hell of it. If in doubt - chuck them out and fit new ones.

Check the signal

At the same time check that the starter is receiving a signal to the solenoid by connecting your multi-meter or test light to the smaller of the wires going into the solenoid. This wire is sometimes called the excitation cable). When the key is turned to the start position battery voltage should be indicated (or the test light should come on).

Check the engine

Finally before condemning the starter it makes sense to see if the engine has seized (or partially seized). This is not a common fault but it has tripped me up on a few occasions especially where the starter isn’t totally dead but just sluggish.

If it's all OK so far...

Remove the starter. Remember to disconnect the battery!

Hot tip. Whenever you disconnect a battery, take the keys out of the ignition, because when you re-connect it the central locking might work, and lock you out of the car. This is really embarrassing to all but the best car thieves.

The Alternator 

I haven’t mentioned the alternator, the component that completes the set.

If the alternator is faulty it can either over-charge the battery, or fail to charge it at all. Both problems can ruin the battery - sometimes the battery has been half ruined by an alternator fault and then hasn’t got the required power to turn the starter. 

As the starter can’t turn, (or it turns too slowly) the current available from the battery just overheats it. I liken it to asking your ageing granny to bump start your Escort - the chances are, all that will happen is she will get hot and fall over, however, give her a nice dose of some dodgy pick me up and the promise of a signed photo of Errol Flynn and there’s a good chance you won’t even need the engine. (Crap simile but I like it). 

In other words too little energy just causes burn out.

The starter motor is faulty

Once you are pretty certain the starter motor is faulty it is worthwhile removing and bench testing, before finally condemning it.

 

 

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