3.1.1 Introduction
The obvious objective of the starter circuit is to crank the engine at the required speed with only a nominal loading on the battery. The load that is placed upon the battery and the subsequent battery voltage and cranking speed can be determined by any of the following areas :-
- Any voltage drop within either the live or the earth circuit due to bad connections or poor electrical contact surfaces, will reduce the amps within the circuit, and reduce the effective cranking speed.
- Any internal resistance within the starter will also have the same effect as the above, for example, poor electrical contacts on the starter motor brushes.
- A starter motor that has an internal electrical 'leak', will cause the current to be taken to earth instead of completing the electrical circuit via the brushes and armature and providing the necessary momentum to crank the engine at the required speed. This 'leakage' will cause the current to be excessively high, and thus cause the battery's voltage to be drastically reduced.
- A battery with reduced capacity, that requires replacing, will also reduce the cranking speed and the effective time that the engine is able to be cranked.
It is therefore critical that the battery is of the correct size and Reserve Capacity (RC) rating and equally important that the battery is not too large as it will soon become sulphated, due to the fact that only a small amount of its capacity is being utilised.
3.1.2 Volt Drop Testing
If the engine suffers from slow cranking yet still maintains a good battery voltage it is essential to test the voltage drop in both the live and the earth circuits.
3.1.2.1 Live Circuit
To test the voltage drop in this circuit, a multimeter is used to assess the areas of electrical resistance. Select a suitable voltage range (0 to 2 volts) and place the red voltage probe to the batteries live terminal and the black probe to the starters live terminal (the starter side of the solenoid). Activate the starter and observe the voltage drop. If a reading above 0.5 volts is seen, move the black probe back along the circuit towards the battery, re-testing until the poor electrical connection is found.
3.1.2.2 Earth Circuit
This circuit is tested in a similar way to that of the live circuit except that the black probe is placed on the batteries earth terminal and the red probe onto the starter motor body. Activate the starter and ensure that while the engine is cranking, the volt drop reading is below 0.25 volts. Check and rectify any poor electrical connections that are found when the red probe is moved back along the earth path.
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Fig. 2.1 |
Fig. 2.2 |
Figures 2.1 and 2.2 shows how each circuit is tested.
3.1.3 Voltage and Current Readings
A typical 4 cylinder petrol engine will consume between 80 amps and 180 amps when the engine is cranking and the voltage on a good battery should not drop below 9 volts. Larger engines with a higher cylinder configuration and especially diesel engines, will produce a larger current discharge on the battery, but the voltage should still remain above 9 volts when cranking.
Fig. 2.3
Figure 2.3 shows a typical starter motor.
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Sample Waveforms : |
Petrol - Cranking Amps |
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Diesel - Cranking Amps |
3.1.4 Dual Mass Flywheel Failing
When replacing the starter motor on a vehicle fitted with a dual mass flywheel (DMF) the starter motor drive housing should be inspected for filings / metallic dust: it may also be possible to shine a torch into the bell housing recess to check this area. These filings are invariable evidence of DFM wear / damage. This is often associated with a noise from the bell housing and a vibration felt through the clutch pedal.
Many aftermarket parts suppliers are only offering a warranty on a starter motor on problematic vehicles if the clutch and DMF are replaced and the bell housing thoroughly cleaned. Proof of this work may be necessary to validate the warranty - instructions and procedures are normally supplied with the replacement unit.
Fitting a new starter motor to a vehicle with a faulty DMF will result in premature failing such as the unit illustrated in figure 3.1.4.
Fig. 3.1.4
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