When using a "volt Ohm meter" as they are sometimes called, there are a few basics that you should know. When checking resistance with the meter, you are checking for continuity when in the Ω Ohms setting.
Good continuity is when you have little to no resistance on the circuit. This means on a digital meter, the reading should be around 0 Ohms. When using an analog meter, the needle will move from the left side to the far right side. This shows that there are no breaks on the circuit.
● If you Ohm a circuit and get no movement of the needle or get O.L. on a digital meter, that means you have an open. An open means that you have no continuity or a break in the circuit somewhere, and you've found a problem.
● Unless you are testing the resistance of a diode, it doesn't matter which lead (red or black) is on a particular side of the circuit.
● Before you start any resistance check, always touch together the two leads on the meter and make sure you get somewhere around zero Ohms. Note: most digital meters will produce around 0.1 to 0.3 Ohms when you do this.
● Another thing to do before testing the resistance of a circuit is to always make sure to disconnect power before taking any reading. If you don't, it can send current into the meter and destroy a perfectly good meter.
● Checking a ground wire. We'll use a car engine bay for example. This means the ground is secured to the metal portion of the frame or body. You can leave the ground wire hooked up if it is secured by itself. When you Ohm this, put one lead on the end of the wire to be checked and the other on any metal that is also grounded in that area, usually the unpainted body or chassis will work. When you take the reading, it should be read continuity (0 Ohms). Basically you are reading from one end of the wire, through the metal frame, and then back to the meter to make the circuit. That brings me to the next tip. It's a short one. You must be touching metal to get a correct reading, so touching the outside of your car's paint job will not do it, it must be bare metal.
● Now say you are checking two wires that run parallel to each other, but do not connect anywhere. If you put each lead on the two wire ends, you should get an open, because the wires are not connected to each other. If you get any continuity at all between the two, then that means you have a "shorted" circuit. Somewhere along the wires, the metal wire from them is touching together. This can cause circuit breakers to pop, and if no circuit breaker is installed, it could start fires or destroy the component it is connected to. If you find a short, immediately discontinue use and unplug the appliance or component.
● When you are checking resistance with a meter, make sure the wire or circuit you are testing is isolated from all other circuits. This means, for example, if you are testing a wire that is spliced into another wire, you would first want to remove the wire to be checked from the splice point. This way you are checking only that portion of the circuit, and could easily narrow it down. Note: If you do not know how to splice wiring
, then leave it connected and find another point to disconnect the circuit.
● When disconnecting wires, always mark your wire with where it was connected to, this way when your done, you don't have to guess where it went.
● Diodes. When testing diodes, which allow current to flow only in one direction, you want to set your meter to the diode setting
if it has one. You should disconnect at least one side of the diode if it is still in the circuit. First check resistance across the diode, then check again but swap your leads to the opposite side they were on for the first reading. If you get an open (meter doesn't move) one way, but some kind of reading the other way, then the diode is good. If you get a reading (meter moves) both ways, then the diode is shorted. If you get an open (no movement) both ways, then the diode is open.
● Fuel injectors. Most fuel injectors will have a resistance of approximately 10 to 18 Ohms, check your service manual for exact number. To Ohm, you can leave installed or remove for ease, putting your meter leads on the two terminals, you should get around the same resistance on all your injectors. If not, the odd one is most likely bad.