How to know if a starter motor is faulty

Unfortunately, there are no easy ways to test a starter while it is still mounted in a vehicle. The problem is that some of the symptoms of a defective starter motor can be caused by faults and defects in parts other than the starter. Here are some examples-

Wiring issues
Wiring issues could include poor ground connections between the engine and the chassis, or between the battery and the engine.

In some cases, the ignition switch may be defective. In other cases, the security/anti-theft system might prevent the starter motor from working. In still other cases, low battery voltages, blown fuses, or defective relays may prevent the starter from working.

Emission system issues
On diesel vehicles that use DEF (Diesel Exhaust Fluid) to reduce exhaust emissions, some faults in the DEF injection system will disable the starter motor. These kinds of fault codes cannot be cleared without repairing the fault, and so the starter will not work until the fault is fixed.

Therefore, it might be a mistake to condemn the starter out of hand if it stops working. The wiser thing to do is to have the vehicle inspected by a professional mechanic that specialises in auto electrics. Auto electrics is a specialised field, and you need a specialist to see if the starter motor is not locked out by one or more other systems.

Thus, assuming that the starter is not locked out by another system, how would a mechanic test the starter motor? The actual steps vary between vehicles, but below is a simplified test procedure a competent mechanic will follow-

  • Check and verify battery voltage
  • Check for blown fuses or defective relays
  • Check for signs of poor ground connections
  • If the battery voltage is OK, check to see if the starter motor can crank the engine
  • If the starter can crank the engine, but the current draw is too high, the most likely cause is a short circuit. In some cases, the armature shaft may be binding
  • If the current draw is below specified values, but the motor rotates, the most likely cause is a failed bendix. This is usually accompanied by loud grating or grinding mechanical noises
  • If the motor does not rotate, the most likely cause is a failed starter motor solenoid or starter motor relay

Depending on what the above tests and checks reveal, the mechanic will remove the starter from the vehicle. Note that depending on the vehicle, removing the starter could involve removing and/or disassembling other parts and/or components.

Such parts and components could include parts of the exhaust system, engine mountings, the A/C compressor, or A/C hoses. The mechanic may also have to remove power steering hoses, and in some cases, the alternator, and drive belt(s). More to the point though- simply removing a starter can take several hours.

With the starter removed, the mechanic will disassemble it to check for damaged or broken starter motor parts. Depending on the damage found, the mechanic might replace one or more individual parts, and reassemble the starter.

The last step is to test the starter on a special test bench. This test involves engaging the starter motor with a device that measures the power the starter motor develops.

The purpose of this test is to see if the motor can develop enough torque to crank an engine without overheating. Most starters used on petrol vehicles usually develop about 3 kW of power, or a little more on diesel starters. If the motor does not develop enough torque, the mechanic will likely recommend a starter motor replacement.

In practice, none of the steps outlined here is recommended for the average DIY home mechanic. If you suspect that your starter motor has failed, or is failing, we recommend that you select the Find a Mechanic button below to find a professional mechanic near you. Mechanics you find here have the tools, specialised test equipment, skills, technical knowledge, and experience you need to have your starter repaired professionally.

If there are no faults that mimic the symptoms of a defective starter, the first step is to remove the starter motor from the vehicle. Depending on the vehicle, this can take several hours.

If the starter is removed and stripped down, some faults can be identified just by looking at the parts. This is true for all starters- from tiny starters used on compact cars, to large V8 starter motors used on LS1 and LS2 engines. Here are some examples-

  • If the gear on the bendix rotates freely in both ways, the bendix is defective
  • If the gear on the bendix does not rotate freely in one direction, the bendix is defective
  • Any sign of discolouration on the field coils or armature is evidence of serious overheating. Discoloured parts might still work, but they could stop working at any time
  • If the solenoid does not retract the spindle, the starter motor solenoid is defective and it must be replaced
  • Visible scoring on the armature shaft where it is supported by bushes is evidence of worn bushes. Depending on the degree of scoring on the shaft, replacing the bushes might resolve the problem
  • If the starter has a reduction gearbox, simply cleaning the parts and inspecting them will reveal signs of mechanical wear or evidence of failure

NOTE #1: All automatic vehicles have a starter lockout switch on the gear selector as a safety precaution. When this switch works, it will prevent the starter from working if the gear selector is not in the “P” or “N” positions. If this switch fails, the starter may not work even if the gear selector is in the “P” or “N” position.

NOTE # 2: Many Asian vehicles have a similar switch on the clutch pedal, which needs to be depressed to enable the starting circuit(s). When this switch fails, it can keep the starter’s circuit(s) from working even if the clutch pedal is depressed.

While the above signs of trouble are easy to detect, other issues, like short circuits or open circuits in the armature are sometimes not visible. However, there is a way to test an armature, and the most commonly used method works like this-

Since the armature windings are wired in pairs, one way to test the windings is to induce a magnetic field in each pair of windings. Interpreting the test results is easy-

For instance, if a magnetic field can be induced in a pair of windings, that pair of windings is OK. If a magnetic field cannot be induced in a pair of windings, that pair of windings has an open circuit as the result of a break in the windings..

If a magnetic field can be induced in multiple pairs of windings at the same time, there is a short circuit between two or more pairs of windings. This will usually be confirmed by some degree of discolouration on the visible part(s) of the windings.

As a practical matter, it is relatively easy to diagnose starter motor parts. However, you need a special tool to induce magnetic fields in the armature’s windings. You also need a starter motor diagram to be sure you assemble everything correctly, and that you get the starter motor wiring connections right. Then you need a special test bench to do a definitive torque test on the motor’s operation.

Therefore, instead of spending thousands of dollars on buying equipment and tools to diagnose your starter, we highly recommend that hit the Find a Mechanic button below. Here, you can find a mechanic near you that will diagnose your starter professionally.

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