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Video tutorial on how to diagnose the starter on your vehicle. If you are having an issue with the starter or starting circuit, the engine of your vehicle will not turn over. If the engine does turn over, then you are having an issue relating to somewhere else on the vehicle, therefore the starter and starting circuit is functioning correctly. As for starter replacement, you can rebuilt it yourself which is by far the cheapest method and I have a full in depth tutorial on how to do this, have a specialist rebuild the unit, purchase a rebuilt unit, or purchase a new unit. The option of a used replacement also exists, but I am not normally a fan of this as you are unable to determine it’s life expectancy.

Tools/Supplies Needed:
-multimeter
-socket set and ratchet
-battery load tester
-wrenches
-screwdriver set

Procedure:
-the ignition switch which is the main controller of the circuit
-next is the battery that provides the power to the whole circuit
-the solenoid acts as a relay and provides linear mechanical movement
-finally the motor
-the ignition switch and solenoid operate on the low amperage circuit
-the starter operates on the high amperage circuit
-the case of the starter motor assembly is ground or earth
-when the ignition switch is turned to the start position, this activates the solenoid that provides mechanical movement and turns on the high amperage circuit providing power to the motor
-either the power going into the ignition switch, the ignition switch itself, or the wire between the ignition switch and solenoid my have an extra switching device which can be relating to neutral safety switch, clutch switch, or some type of anti theft feature
-if it’s an anti theft issue, then most likely you will have some type of light flashing on the dash relating to a security feature
-first check the fuse
-use a multimeter or test light to determine if the fuse is faulty
-if your vehicle is equipped with a fusible link, this will be known by it’s wiring diagram
-a continuity test can be done between two points, but make sure your testing the wiring only with no electronic components in between
-the battery might be either low on voltage or it is unable to handle a load
-a load test can be done on a battery to determine it’s condition
-use a multimeter, also test to ensure the engine has a sufficient ground
-not all vehicles will have a relay in the circuit, so refer to your wiring diagram which will determine if the vehicle has one or not
-the relay itself can be tested to determine if the coil or contacts are faulty
-next the electrical connector can be tested to determine if it has voltage and a sufficient ground
-on some older vehicles, the solenoid might be separate, but the same testing procedure will apply
-test the voltage at the solenoid switching wire using a multimeter as you turn the key
-battery voltage should be present at the connection
-if not, there could be a fault with the wire or a faulty ignition switch if previous areas have passed
-test the main power able feeding the starter assembly which is the high amperage side & should have battery voltage present at all times
-the ignition switch connector can be tested for power, back probed when it’s operated, & also have a continuity test when disconnect from the circuit
-if it’s not receiving power, then there is an issue with the wiring or circuit between the switch and battery
-set the multimeter on the lowest ohms setting and test the resistance of the coil of the solenoid
-the solenoid also has an internal connection to switch the high amperage circuit
-the solenoid may need to be powered up if it doesn’t have a plunger, but do not hold the power on for an excessive amount of time as you can overheat the coil
-if all tests pass above, then you will have a faulty starter motor
 

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Although that's pretty well written (especially for TN), there are quite a bit of pieces missing - especially since this is a Toyota forum.

Toyota's have a specific starter circuit, and for essentially anything from the 80's and newer, the STA circuit isn't as simple as running straight from the key to the solenoid. This is the STA circuit. Older EFI systems incorporate the CSI and CSI switch directly off of the engine bay wiring for the starter solenoid - all of which is incorporated into the COR (circuit opening relay). As the vehicles get newer, and the piece which was mentioned - anti-theft devices, the incorporation of circuitry between the starter solenoid and the key becomes more complicated.

There is no mention of a quick test - to bypass the key'd system altogether, by jumpering power straight to the starter solenoid. This should be done with a remote start switch (can be purchased cheap), or can be done with something as simple as a length of wire with alligator clamps (be careful if you go this method).

Further - as vehicles, starters, and wiring get older, there are other ramifications that can cause starter or starting issues. As wiring degrades it's ability to carry the necessary voltage for higher amperages can reduce - causing a starter to turn over slow or not at all. This is also the case for grounds from the battery to the chassis/engine. Toyotas are especially sensitive to their grounding circuits. Simply finding that there is voltage to the starter and solenoid isn't enough of a test to determine that the problem absolutely is the starter.

There are also situations where multiple factors contribute towards a problem - and replacing one piece may only be a band aid and a temporary solution until another factor fails. For instance a starter that has contacts that are starting to fail - may fail on a mediocre battery, then when a new/stronger battery is installed, the additional amperage/capacity of the new battery may allow the starter to run a little longer, but could (or will) fail soon after.
 
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