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Symptoms of a Bad or Failing Top Dead Center (TDC) Sensor

top dead center

For more than 100 years, the four cycle combustion engine has utilized the same method for creating power. The intake stroke will draw air and fuel into the combustion chamber, the compression stroke will increase cylinder pressure, the power stroke will ignite the air fuel mixture and force the piston down, and finally the exhaust stroke will expel used gases through the tail pipe. However, in order for all the internal components of an engine to work together, there must be a starting point. This is known as Top Dead Center (TDC) and in today's modern engines is monitored by a top dead center sensor.

TDC is often defined as when the first cylinder to fire (usually number one cylinder) has the intake and exhaust valves both closed at the same time. In older engines, TDC was marked on a harmonic balancer as zero degrees and allowed a mechanic to assemble an engine and adjust cylinder head valves to ensure smooth operation. Today's engines are assembled with the same precision, however, the TDC sensor continually monitors the firing sequence of all cylinders. This sensor is vital as today's modern ignition systems are frequently adjusted to variable driving conditions.

When everything works as planned, the TDC sensor should last forever. However, since the sensor is an electrical part, it can fail. Issues such as wear and tear, cracking, and even corrosion can take over and cause a malfunction of the TDC sensor. If a problem exists with this sensor, certain warning signs will alert the driver to a potential problem. Listed below are a few of these indicators that if noticed should inspire you to contact a certified mechanic to inspect, diagnose and replace the TDC sensor if needed.

1. Check Engine Light comes on

The first indicator that a problem with the TDC sensor exists will be that the Check Engine Light comes on. All sensors are monitored by the ECU in any car. When the TDC sensor sends inaccurate information to the ECU, an error code will be created and cause the Check Engine Light to illuminate on the dashboard. Once this occurs, a certified mechanic will need to be called to complete a diagnostic check with a specialized computer that plugs into a port under the dashboard. This will download the error codes and let the mechanic know what area of the vehicle triggered the warning light so they can inspect and repair any damage.

The Check Engine Light is not something that should be ignored. If you see this light on your dash, contact a certified mechanic as soon as possible as it may be a serious problem.

2. Engine will not start

As we described above, the internal combustion motor needs to have a precise ignition timing setting in order for all the cylinders to fire in the correct order and at the right time. If the TDC sensor is damaged, it will not send this information to the onboard computer. As a safety measure, the ECU will shut down the ignition system and you will be unable to start the motor. Depending on the vehicle, the engine will either not crank over or the engine will crank over but not produce a spark. In either case, if your car won't start, contact a mechanic so they can determine why this is happening to you.

3. Engine seems to misfire or runs rough

Another common symptom of a bad or worn out TDC sensor is when the car seems to run rough or the engine misfires. If the TDC sensor is not working properly, the motor usually will shut off quickly to avoid damage to internal components. However, this doesn't always occur. If your motor seems to run rough or sounds like it's misfiring, you should pull the car over to a safe place or head home. Once you arrive home, contact a local mechanic so they can come over to your home or office and inspect the problem.

The top dead center sensor plays a vital role in today's modern engines. This component is commonly used on vehicles manufactured after 1993. If you notice any of the symptoms indicated above, or if the engine of your car just doesn't seem to run happy, contact one of our local ASE Certified mechanics from YourMechanic.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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