My car won’t start: is it my starter or my alternator?

So let’s say you go out to your car one day and find that it won’t start.  You turn the key and all the lights come on on the dash but the car doesn’t crank over.  You know that you have a good battery so you start to wonder whether your problem is your starter or your alternator.  Here’s how you can figure out the answer to that question.

The first thing to think about is your alternator.  You know that you can rule that out as a problem because your alternator is not one of the components of your starting system.  The starting system it is composed primarily of the starter, the starter relay, the ignition switch, the battery, and the wiring that connects those components.  The starter also has three parts inside of it: a starter solenoid, a motor, and a gear that moves axially to reach out and mesh with teeth on the flex plate or torque converter.

The starter is a common point of failure so we’ll start by checking it. First, you want to pay attention to whether or not you hear a click when you turn your key in the ignition. If you do, that’s an indication that your starter solenoid is moving. The starter solenoid moves for two reasons. First, it moves to shift the starter gear to make contact with the teeth on the flex plate or torque converter. Second, it moves a contact plate that transmits energy from a hot wire to the motor of the starter. So, if you are hearing the click but the starter motor is not turning then you are getting closer to understanding the problem with your car. If you don’t hear a click when you turn the key in the ignition, that indicates that either the solenoid is not receiving power or that it is not able to move for some reason.

It’s a common and useful procedure in that case to power the solenoid directly. If the key is in the on position in the ignition switch then many cars will start up at that point. If that’s the case, then you should be thinking about a possible problem with the ignition switch or a problem with the wiring between the ignition switch and the solenoid. To power the solenoid directly, you would ideally use a tool called a power probe. If you don’t have a power probe then you can connect a wire directly from the positive post of your battery to the starter solenoid terminal. Bear in mind, however, that this procedure is fairly risky because it’s very easy to cause a short circuit by touching your hot wire to a different part of the vehicle, most of which serves as a battery ground, or even to the starter solenoid terminal itself if the solenoid has certain electrical problems inside of it. If the starter turns when you perform this test then the next thing to check is the ignition switch.

In order to check the ignition switch, it will be a necessity on many vehicles to unmount the ignition switch so that you can get the probes of a digital multimeter into its electrical connectors. Once you have the switch exposed, you’ll need to find the wire that goes hot when the switch is turned to the start position. Use the multimeter to determine whether it does in fact go hot. If it does not, locate the hot wire coming from the battery going to the ignition switch. Determine whether this power source is in fact providing 12 volts. If it isn’t, you’ll need to look for a problem upstream of that point such as a blown fuse or a damaged wire. If that wire is providing 12 volts then continue on.

You’ll need to disconnect electrical connector from the ignition switch at this point. Then, with the switch in the start position, use your multimeter to check the resistance between the terminal for the hot wire coming from the battery and the terminal for the wire that goes to the starter solenoid. The resistance should the near zero Ohms. If it isn’t, replace the switch and try to starting system again; it should work at this point.

If the switch does have no resistance then visually inspect the wiring between the switch and the solenoid. You’ll likely find an area of high resistance due to a severed wire or corrosion. Once you’ve determined the cause of those conditions, you can remedy them and the starting system should start working again.

If you’re still unable to find the problem at this point, the next thing you should check is the starter relay. Remove the relay and look for obvious signs of damage such as burned contacts. You can briefly connect a jumper wire from terminal 30 to terminal 87 for the relay in the power distribution block. If the starter motor starts to turn when you do this and the relay is bad and replacing it will fix the problem.

If these checks have turned up a problem then you should look for a seized engine. The procedure for checking for a seized engine can cause serious internal engine damage if it isn’t done exactly correctly. You must be able to determine the natural rotation direction of the engine in order to ensure that the engine will maintain proper valve timing when you complete the procedure. Using the crankshaft pulley bolt, rotate the crankshaft about 20°. That should be a sufficient amount of rotation to determine whether the engine is seized. If it is seized, you should feel substantial resistance against your effort to rotate the engine. If that happens, it’s not uncommon for the engine itself to the able to rotate freely but some accessory part such as a distributor or water pump is seized and preventing the engine from turning. Locating the source of the seizure and repairing it should allow the vehicle to start.

As you can see, many of these procedures are risky and difficult. Not all information that is necessary to provide for a technician’s or DIYer’s safety has been provided here so please use this information at your own risk.