The top dead center, sometimes referred to as TDC, corresponds to the highest point reached by the piston of the first cylinder of the engine during the compression phase. You may need to locate it to install a new distributor in the right orientation, to connect the spark plug leads in the correct location, or for many other maintenance projects. You can do it simply with common tools, but using a specific detector you can get the most precise measurements.
Steps
Method 1 of 3: Install the Detector
Step 1. Disconnect the battery
Before starting work, use a wrench or socket to loosen the nut securing the ground wire to the negative battery terminal. Unplug the cable and clip it between the battery and the body to prevent electrical contact from being restored before finishing the job.
- By doing so, you don't get shocked and make sure you don't blow the fuses.
- The engine does not start when the battery is disconnected.
Step 2. Disconnect the spark plug lead from the first cylinder
Consult the vehicle maintenance manual to identify it and, once you find it, grab it at the base where it fits on the spark plug itself; pull it up to remove it.
- Remember to read the manual relating to the model, year of production and equipment of the car.
- Do not pull the wire anywhere along its length, but grab it by the base when you need to disconnect it from the spark plug.
Step 3. Remove the spark plug from the first cylinder
Connect a specific bush and an extension to the key to unscrew the spark plug housed on the first cylinder; turn the key counterclockwise until you succeed in your intent.
- The candle stays in the compass thanks to the rubber ring located in the latter.
- Inspect it for possible damage and store it in a safe place.
Step 4. Install the detector on the first cylinder
Insert it into the socket you used to remove the spark plug and carefully screw it onto the piston in the spark plug housing by turning it clockwise.
- Proceed carefully to prevent dirt from falling into the hole as you insert the detector.
- You can buy this device in most auto parts stores.
- There is no need to over tighten the detector, you can tighten it by hand.
Method 2 of 3: Find the Top Dead Center
Step 1. Use a wrench to slowly spin the motor
Locate the first pulley that is near the base of the motor. It is a circular element that gives movement to some accessories, such as the power steering and the air conditioner, through a poly-V belt. In the center of the pulley there is a nut on which you have to engage the wrench of the right size, to be used to rotate the motor counterclockwise.
- Make sure the socket or wrench is the correct size, otherwise you could damage the bolt.
- It may take a good amount of force to rotate the motor; large models require more effort than small ones.
Step 2. Never use the choke for this operation
While trying to locate the top dead center, you need to rotate the whole motor mechanism; do not turn the ignition key to start the starter, as this could cause serious damage if the piston hits the detector you have inserted.
- If you have disconnected the battery cable, you cannot electrically start the engine.
- Never try to get it started when it is partially disassembled.
Step 3. Make a mark on the pulley as the piston touches the detector
Keep turning the wrench until you feel the contact and draw a notch on the harmonic balance wheel that surrounds the pulley at the point where the pulley stops; use a marker for this.
- Make sure you can clearly see the mark you made.
- A regular or paint marker is perfect for leaving this landmark.
Step 4. Rotate the motor in the other direction
Once the first mark is drawn, use the wrench or socket to turn the pulley clockwise until the piston touches the detector a second time.
- Draw another reference mark on the harmonic balance where the pulley stops.
- Check that both notches are clearly visible before continuing.
Step 5. Find the center point between the two marks you just identified
Measure the distance that separates them and divide it by two; you should be able to take the measurements starting from one of the two lines and easily identify the midpoint, which corresponds exactly to the top dead point.
- Remember to take out the detector and reinsert the spark plug before starting the engine.
- When finished, connect the ground wire to the negative pole of the battery to restore the power supply.
Method 3 of 3: Finding the Top Dead Center Without a Detector
Step 1. Remove the spark plug from the first cylinder
Instead of pulling it out to insert the detector, you can use your thumb to locate the PMS to a good approximation. This measurement is accurate enough to allow a distributor or spark plug leads to be fitted, but it is not sufficient to align the camshaft.
- Remember to remove the spark plug using the special bush, otherwise you just unscrew it without taking it out of its housing.
- Proceed with extreme caution to prevent dirt from falling into the open hole after pulling out the spark plug.
Step 2. Place your thumb on the hole left by the spark plug
As the engine rotates, the piston moves up the cylinder, so you should be able to feel the pressure build up. Insert your thumb into the spark plug housing to assess this pressure change.
Check that the hole is perfectly sealed by your finger
Step 3. Ask a friend to turn the camshaft with a wrench
Keep your finger on the spark plug housing while a helper rotates the first pulley clockwise using a wrench of the right size. Make sure you keep doing this until the pressure building up inside the cylinder is enough to push your thumb away; this means that the piston is very close to the top dead center.
- Be very alert during the process to instantly sense when your finger is being pushed out of the hole.
- As your thumb moves, the internal pressure decreases and you can re-insert your finger.
Step 4. Light the inside of the hole with a flashlight to find the TDC
When the thumb has moved away from the pressure, inspect the hole to figure out the distance from the piston to the opening itself. Have the helper rotate the engine very slowly while watching the piston come as close to top dead center as possible.
- This process has a margin of error of 15 ° and you shouldn't use it to install a new camshaft.
- Remember to connect the battery again after inserting the spark plug.
