DVOM, Part 2June 6, 2018
Diagnostics using a DVOM
In Part 1 the basics of a DVOM were covered. I will now provide an over view of the most frequently used circuit and component diagnostic tests.
It is imperative you get familiar with your meter PRIOR to having to diagnose a problem —- a tool that you do not know how to use has little value. It is always best to have a schematic of the circuit you are testing. Do not assume that you know how it is wired.
Keep in mind that for every test the proper meter range/scale will need to be determined as represented in Part 1.
Disconnect the power source from the component or circuit being tested. Connect one lead (an ohmmeter has no polarity) to one side of the component or circuit and the other to the other side. Read the display.
If a sensor or solenoid is being tested it will have a specific value that will need to be referenced to see if it is in range, such as 208 ohms. When checking a switch there should be continuity (zero ohms) when closed and out-of-range or infinity when it is off. If it is a multi-position switch (low. medium, high) keep one lead of the DVOM on the input terminal and then move the other lead to the different poles after moving the switch to the each position.
When testing a diode which is a one way electrical check valve, take a resistance reading with the leads one way and then reverse them. A diode should read continuity one way and open with the leads reversed.
To check continuity in a circuit, place one-meter lead at the end of the circuit and the other at the beginning. If there is continuity it should read zero ohms or very close to that. If open it will read infinity or over-range. Some meters have an audible continuity test mode that will allow you to not have to look at the display to check a circuit. This is called “ringing out a circuit.”
To check an ignition coil do the following:
Primary: Ohmmeter leads across the two primary terminals. If the reading is not in specification the coil needs to be replaced.
Secondary: Ohmmeter lead to the connector where the coil wire attaches with the other lead to the negative terminal. Compare to specification.
Supply voltage: Unplug the component and place meter positive lead to the supply wire. Attach meter negative lead to a good ground. Turn on the circuit. In most applications other than a microprocessor controlled circuit there should be battery voltage present. Most computer circuits use sensors that work on 5 volts.
Charging circuit: With the engine off connect the meter with proper polarity to battery. The voltage should be slightly higher than rated battery voltage (this is called surface charge). Start the engine with the meter attached. The voltage should go up to the alternator output — just below 15 volts in most applications.
Cranking voltage: Disable engine so it does not start. Connect voltmeter across battery. Crank engine for ten seconds and record lowest battery voltage. Reading should be above 9.6 volts for most applications.
Voltage drop test (most accurate way to check a ground): Place the voltmeter positive lead on the ground to be checked and the negative lead on a chassis or engine ground. Activate circuit. There should be less than 0.2 volt (2/10) on the ground circuit. If the voltage is 0.2 or higher the ground is weak/defective.
Current draw of component: Place the positive lead in the side of the circuit that is providing the power and the negative lead going toward the load. Activate the load and read the display.
Current draw when the engine/equipment is off: Disconnect the negative cable from the battery. Place one lead on the battery negative terminal and the other on the end of the ground cable (If reading is negative reverse meter leads). The draw if any will be read on the display. If the reading is in milliamps that is the memory for any controller the equipment has or for the radio or clock. That is normal. The draw needs to be up near one amp for it to kill the battery over time. To locate the source of the draw, remove fuses until it goes to zero. That circuit is the one with the problem. Also unplug the alternator— a bad diode will cause a draw.
When diagnosing a problem with a DVOM it is best to apply what I coined as SAT, for stop and think. The problem is going to be found by checking voltage, ground or amperage. Often the most difficult part is gaining access to the component or circuit. When it comes to weather pack connectors used on modern equipment it is important that you do not violate the seal or wire when testing. Most tool dealers offer a set of jumpers and pin removal tools so these circuits can be confirmed without being damaged. These along with a set of traditional jumper wires are essential tools that compliment your DVOM.