How to Find a Tach Wire with a Multimeter

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There’s no doubt that having a faulty tachometer in your vehicle can be extremely risky. You could end up without your driver’s license and not be able to go to work if you have no other way of commuting than by car. 

You could also drive at speeds, which aren’t adjusted to present road conditions and end up on the side of the road whenever it rains as you thought you’re going slower than you are. Car mechanicals could stop all of such things by checking your car frequently. However, if you like to save some cash, you can do something as well. 

What a Tachometer, by the way?

A tachometer is a gauge to quantify mechanical speed in units of Revolutions Per Minute or RPM. On a car, a tachometer measures the engine’s speed. Since the middle of the 1980s, tachometers became the standard equipment for vehicles and were installed alongside the speedometer. 

Often referred to as revolution counter or RPM gauge, a tachometer is used to measure the engine crankshaft’s overall rotation. 

Mechanics use this tool to set the lowest engine rotation that ranges from 500 to 1,000 RPM when the air conditioner is turned off. If the AC is turned on, the engine rotation should be increased to at least 100 to 200 RPM or greater to achieve the power to turn the AC compression.

Further, the engine speed is increased to drive air conditioning. Mechanics typically change how high the idle speed is when the AC is turned on and turned off. 

You can use a tachometer to ensure the engine’s speed is not greater than the permitted (red line). Passenger cars with maximal engine speed are normally limited to 6,000 RPM. Other vehicles have an engine that could reach 7,000 RPM. For truck, pickup, and jeep with diesel engines, their RPM could be about 4,000 RPM. 

The engine on a formula one race car could reach as high as 18,000 RPM. Take note that high engine speed normally happens when the vehicle is slowed down or accelerated using compression. 

When the engine maximum revolution is exceeded, that will cause more fast wear and tear on the engine parts, often may trigger engine jam. Other considerations that identify the maximum limit of engine speed are the following:

  • Component composition
  • Component balance
  • Component mass
  • The ability of engine lubrication systems 
  • And more

Typically, diesel engines have a lower limit because of the limited revolution of the fuel injection system. Further, the motorcycle engine has a high-speed limit as it has lighter-weight parts, particularly on reciprocating motion components like valves, connecting rods, and pistons. 

Other vehicles are geared with an engine speed limiter that will lower the amount of fuel injected to prevent engine over speed. 

What are Tachometers Used for? 

The tachometer’s purpose is to measure the machine’s rational speed. For instance, a tachometer is normally shown on the car’s dashboard within the automotive industry. That informs the driver when to change gears, considering the rotational speed of the shaft.

This tool can also be utilized to measure a wide array of performance and object metrics such as:

  • Length
  • Traveling speed
  • Rotational speed 

How Does It Work?

Even though you will find numerous types of tachometers available in the market, many instruments work similarly. They could be categorized differently, but the key types to be considered when operating are mechanical and electronic tachometers.

Many modern tachometers are electric tools that feature data storage and numerical readings. With electronic tachometers, the ignition system activates a voltage pulse every time the spark plug fires. The electromechanical part corresponds to the standard voltage of the series of pulses. It presents the average voltage of the pulse train as proportional to the engine speed. The signal is then sent to the indicator through a twin screened cable and shown to the user through an LCD or LED readout. 

On the other hand, a mechanical tachometer works by using the centrifugal force’s dependence on a rotating mass on the rotation speed. That force could stretch or compress mechanical springs. 

Resonance tachometers are a mechanical tachometer’s subtype. Such tools utilize a series of consecutively tuned reeds to determine the engine’s speed. Such instruments work by identifying the machine’s vibration frequency. 

How to Find a Tach Wire? 

Did you know there’s a simple way for you to find a tach wire? You can achieve that by using a digital multimeter. Below are the important steps you need to follow:

  1. Get a standard multimeter. 
  2. Set it to ACV or AC voltage, but 12V or 20V is just fine. 
  3. Connect the negative probe of the digital multimeter to the chassis ground. 
  4. Start and run your engine.
  5. Probe the wire you suspect of being the tachometer wire with the red probe of the digital multimeter. 
  6. If that’s the right wire, your digital multimeter will read between 1 volt and 6 volts. 

How Do You Test a Tachometer?

With your digital multimeter in hand, fit the tachometer probe with it to test a rev counter for your vehicle. The procedure of using a tachometer in an automotive application is as follows:

  • Set your digital multimeter on the AC setting.
  • With the vehicle’s engine on, touch the multimeter to the tachometer wire, which leads to the breaker box.
  • Recruit the help of another person to sit in the car and record whenever the rev counter dial reaches the specific point that is crucial to the collaboration process. 
  • Your digital multimeter will display exact readings at the same time. 
  • Compare both readings to determine the accuracy of the tachometer. 

Final Thoughts

Often, the cause of a faulty tachometer could be the pure fact that your vehicle is low on gas. We will not go into the details of it as it’s very challenging. But you need to ensure you have a full tank of gas before doing it. 

We hope this post has helped you determine and find the tach wire and how to test it with a multimeter. Also, we hope it helped you save a few dollars at the mechanic as you already understand what’s wrong with your car or tachometer.

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