Starting system in a car: how it works, common problems
October 29, 2022
The starting system in a car consists of the starter motor, starter cables and the battery as well as the starter motor control circuit. When you are starting your car, it happens in several steps. Let's consider these steps for the starting system with a conventional ignition lock/key as well as the Push Button Start system.
Starting system with conventional ignition lock/key
The car computer called PCM checks through the sensor built into the ignition lock if the code in the ignition key matches the code stored in the PCM (immobilizer). If not, the security warning light will stay on or flash on and off and the starter circuit will be disabled.
Car starting system with an ignition key/lock.
The computer verifies if the transmission is in Park or Neutral or, in case of a manual transmission, if the clutch pedal is depressed.
In case of an automatic transmission, it's the transmission range switch that sends the signal to the PCM if the transmission is in Park or Neutral. In cars with a manual transmission, the clutch pedal position sensor performs the same function.
If the above-mentioned checks pass, the computer operates the starter motor through the control circuit and the starter relay, see the diagram. Of course, the battery has to have enough charge for the starter motor to work
Starting system with Push Button Start
When you enter the vehicle with the fob, the security system communicates wirelessly with the fob and checks if the code stored in the fob matches the code stored in the PCM.
Push Button Start starting system.
If the fob battery is too weak, the fob will have to be held against the start button or placed into a special slot to perform this check.
The PCM verifies that the automatic transmission is in Park or Neutral (through the transmission range switch) or that the clutch pedal is depressed in case of a manual transmission.
In cars with an automatic transmission, the PCM checks the signal from the brake light switch to verify that the brake pedal is depressed.
If all above checks pass, and the battery is sufficiently charged, the PCM operates the starter motor through the starter relay and starter control circuit.
Starter motor control circuit issues
In order for the starter motor to operate in either of these two systems, the starter motor control circuit must be in working order. For example, if the control circuit connector at the starter motor is corroded or loose (which is very common), or the starter relay is bad, the starter motor will not operate.
Mechanics verify if the control circuit is working by checking the 12-volt power at the starter control terminal. If there is a 12-volt power at the starter control terminal when starting the car, but the starter motor is not working, the starter motor is bad. That is, provided that the battery is sufficiently charged and battery cables and terminals are good. If there is no 12 volt power at the control terminal when starting the car, the starter relay is next to check. If the relay doesn't click when starting the car, the problem is before the starter relay, see the diagrams.
The starter motor requires very high electric current to operate
Corroded versus serviced battery terminal.
The starter motor is powered by the 12-volt car battery and will only work if the battery has enough charge. Read more: Signs of a weak battery
. The starter motor requires very strong electric current to work. This means that the battery cables must be in good shape and have solid electrical connections to deliver the current to the starter motor.
Often, the starter motor does not operate because the positive or ground cable connections between the starter motor and the battery are loose or corroded, see the photo.
Common problems causing the vehicle not to start:
The most common problem with either of these starting system we see is a dead battery. Even though the dash lights and other electric functions might still work, there is not enough charge to engage the starter motor.
If the vehicle starts after boosting it, in 99 percent of the cases the problem was indeed caused by the low battery. If the battery is more than 7-years old, it's probably time to replace it. Read more: When to replace the 12-Volt car battery?
If the battery is relatively new, it will need to be recharged. Read also: 3 Common causes of the car battery draining fast
The starter motor that has failed or is going bad is the second most common problem causing a car not to start. Read more: What are the signs that the starter motor is going bad?
Other common problems include a dead battery inside the key fob, bad brake light switch, issues with the transmission range switch (when the car starts in Neutral but doesn't start in Park) and problems with the ignition switch.
If the starter motor does not operate when starting the car (no crank, no start
), the battery must be tested first and the battery terminals and cables, as well as the starter control circuit and then the starter motor, must be checked.
If the starter motor does turn over the engine normally, but the engine doesn't start (crank, no start
), the problem is not in the starting system, but somewhere else (e.g. no spark, no fuel, low compression, etc.).