Since the engine in all modern cars has some form of computer control, there are times when a volt meter will be required to properly diagnose a problem. Volt meters come in many shapes and sizes, and vary in price from as little as $20 to over $400. There are also two major styles available: analog and digital. A digital Volt Ohm Meter (VOM) will usually have a selector switch to tell the meter what it is measuring and a digital display. An analog meter has a needle gauge readout and the same selector switch to change the measuring modes. No matter which meter is chosen, the way they are used are the same. Listed below are the most common measurements and how to properly make them.
1. Voltage Check
The most used feature of a VOM is its ability to measure voltage. Although most meters can measure both AC and DV voltage, most cars only use DC. The exception is that the alternator creates AC voltage but converts it to DC internally, and some hybrids use AC in their systems. Voltage is considered a potential to transfer current, and requires a positive and negative source to be present. To measure voltage, the positive lead is placed on the sensor to be measured, and the negative lead is placed on a ground. Most cars have the negative side of the battery grounded to the frame. This can be used as a ground reference, but if it is accessible, the negative post on the battery is preferred. Be sure to set the meter’s selector switch to the DC position. Consult the owner’s manual if needed to perform this task. Once the switch is in the correct position, and the leads are where they need to be, read the display screen. This is the current voltage present at the leads.
2. Resistance or Ohms
When resistance is present in a wire, it reduces the current in proportion to the amount of resistance encountered. It does this by changing the current into heat. Most sensors use this principal to supply the computer with operating information. Take for instance the coolant temperature sensor. When the motor is cold, it provides the computer a voltage signal dependent on the voltage drop across it. As the motor heats up the resistance inside the coolant temperature sensor changes, therefore changing the voltage input to the computer. The computer reads this voltage and determines the engine temperature based on pre-programmed factory parameters. To measure resistance first switch the selector switch to the proper position. Then place the leads on either side of the sensor, and read the display screen. Compare this reading to the one in a manual to determine if the sensor is good or bad. Because the meter develops an internal voltage (approx 9 volts) that it passes through the sensor being measured, and electricity flows through the path of least resistance. Whatever is being measured has to be disconnected from the main wiring harness.
3. Current or Amps
This is the hardest measurement the meter will have to perform. Most people avoid the current measurement because if it is done incorrectly it can damage the VOM. Current has to be measured in line with the sensor. To do this, one side of the sensor has to be unplugged from the wiring harness. After the meter’s selector switch has been set to the proper measurement, one lead is hooked to the wiring harness side, and the other on to the sensor. The amount of current measured will be displayed on the screen.
With the advent of computer controls for the modern car, a VOM has become a necessary tool for the average mechanic. Be sure to follow all directions in the owner’s manual that came with the meter before performing any checks, and any safety rules spelled out in the maintenance manual for the car.