Testing a starter motor is straight forward with our 'How too' guide.
Is your starter motor doing nothing when you turn the key to start the engine?
Or is it turning but not engaging? Turning really slowly? Only making a click noise?
This blog will help you figure out why, and what is wrong/what needs to be repaired or replaced?
To successfully test a starter motor in place, you will need a DC voltmeter, and what ever tools are required to remove things so you can get to the starter motor with your test leads.
The first thing you need to know is the condition of your battery?
12.7 volts is a fully charged battery, 12.0 volts is flat. If you do not have a battery tester, and have not had your battery checked, it is a good idea to go have your battery checked at your local Repco, Supercheap Auto, or mechanical/auto electrical garage.
Once you know you have a good battery, (or you have a good set of jumper leads, or a charged up, working jumpstarter pack connected) Be careful of my little cheat though. If the battery you are jumping off of, your jumper cables, or your jumpstarter pack are no good, that will throw the test off and have you replacing a starter motor, only to have to replace the battery as well, once you find out the new starter didn't fix the problem. No thanks!
So, where was I? Ah yes, you now know your battery is good.
The engine is cranking over, but really slowly and it won't start..
First step,
Test your battery voltage from post to post while trying to start the engine. I say post to post for a reason-not battery terminal to terminal. The voltage across the battery should not drop to less than 10.5 volts.
If the voltage drops well below 10.5 volts when holding the key on the start position. e.g 4 volts, 6 volts, 7 volts etc, the likely causes are, from most likely to least likely,
Bad battery (our battery test has ruled this option out)
Bad starter motor
Seized engine
You may laugh at the seized engine option, I can tell you, I have diagnosed many starting faults as seized engines, it does happen. To check for this, get a socket and a big bar on the crankshaft bolt, and make sure you can turn the engine over by hand, if you can, you have a bad starter motor.
Next step,
If the voltage stays above 10.5 volts, but the cranking speed is really slow, the likely causes from most likely to least are,
Bad starter motor.
Bad connection (high electrical resistance) between the positive battery post and the battery stud on the starter solenoid, or between the negative battery post and the body of the starter motor.
Bad battery cables, the big thick ones from the positive battery terminal to the starter solenoid, and the negative battery terminal to engine block.
To test for bad connections and or cables, we take our DC volt meter and test from the negative battery post to the body of the starter motor while cranking the engine. And from the positive battery post to the battery stud on the starter solenoid while cranking the engine.
Both of these should read less than 1 volt. If they read more, you have a loose, dirty, or corroded connection in the line. Or a bad battery cable.
Your starter motor sounds like its spinning when you turn the key, but it doesn't engage, and the engine doesn't turn over at all.
This one is easy, it's the drive clutch in the starter motor slipping, no further testing required.
Replace the starter or replace the drive clutch in the starter.
Your starter motor is silent, or only makes a click noise when you turn the key, but nothing else happens.
First step,
Test the battery voltage when trying to start the engine, it should be no less than 12.0 volts.
Next step, test the voltage from the negative battery post to the positive battery stud on the starter solenoid while trying to start the engine. This should be no less than 12.0 volts.
If the voltage is dropping well under 12.0 volts, you have a bad battery.
Next step, test the voltage from the negative battery post to the small energize wire where it joins onto the starter solenoid. This should be no less than 12.0 volts. If it is dropping well below 12 volts, you have a wiring, and or ignition switch fault.
If the test voltages stay above 12.0 volts, you have a bad starter motor.
When I say bad starter motor, this could mean, burnt solenoid contacts, bearings seizing up, worn out bronze bushes, an internal short circuit within the field coils or armature of the starter motor, worn out brushes, open circuit solenoid windings, slipping drive clutch.
There are many ways a starter motor can fail, but for this blog we are focusing on diagnosing the cause of a starting fault on an engine. We will address starter fault diagnosis in another blog.
Cheers
Paul
StartnCharge