Darren
12-10-2009, 02:56 PM
I typed this up on another forum in response to a question on how to use a multimeter. I've also added a continuity test to the write up which is a handy one. It's nothing official, just a bit of writeup based on my own experience with auto electrics & multimeters. Thought it might benefit some members here too who are fairly new to digital multimeters. They are an extremely handy diagnostic tool when it comes to car electrics. Often a little know how is all that is needed to save yourself a lot of expense taking your electrical problems to an auto electrician.
to test a V6 coil pack secondary circuit
black lead in 'COM' socket, red lead in the socket above. Select the 20K ohm range on the dial. Put the two leads across the spark lead terminals on the coils (ignition off). Polarity doesn't matter when measuring resistance so leads can go either way. From memory V6 coils should read no more than about 14K or 14,000 ohms. But usually when they are stuffed they go open circuit and meter will just display 'OL'
To use as a test light, or to test voltage on a circuit.
Leads connected to the meter as above, select the DCV range (direct current voltage) if using on car electrics. AC (alternating current) is mainly used for devices connected to the 240V mains in your house. Connect black lead to the negative side of the circuit to be tested and the red lead to the positive. If you get the leads back to front, no dramas - the screen will just show a negative '-' sign in front of the voltage reading. Note a multimeter will not replace a test light for all tests. Some diagnostic tests require the internal resistance or load of a light to operate. So really the multimeter can only test the voltage of the circuit 'unloaded' so to speak.
to check current consumption in a circuit
be careful with this one, it can stuff your meter if you aren't careful. Don't connect the leads to the circuit until you follow the procedure below - or you risk frying your meter - particularly if you have the resistance range selected when testing current draw.
Leave your black lead in the COM (common) socket on the meter. Now you need to determine what sort of current you are measuring. Up to 200mA (milliamps) you fit the red lead into the socket above the COM socket. Above 200mA and up to a max of 10A, you fit the red lead into the top socket (10A DC). Be aware that most circuits (apart from the engine management/sensor circuits) in a car draw at least 1 amp, so the lower 200mA range is going to be unsuitable, you will need to use the higher 10A range. However, any circuit that draws more than 10A may damage your meter. So look at the fuse used in a circuit before you rig up the multimeter, the fuse will give you some sort of indication of the maximum current the circuit is designed to draw.
Once you determine which amps range you are going to use, ensure your red lead is in the correct socket (as above) turn the dial to select that range. If the circuit is less than 200mA, choose the DCA range and select either 20mA or 200mA, dependant upon what sort of current draw you expect from the circuit. You can't do any damage selecting the wrong range, providing the current draw is less than 200mA. If the reading is too high for the selected range the meter will simply read 'OL' (overload) and alternatively if the reading is too low for your selected range, it will either display 'OL', 0 or just a figure like 1. IF you are going to test a circuit that is expected to draw over 200mA but less than 10A, then select the 10A range. Once again ensure the red lead is in the correct socket.
Now for connecting the leads to the circuit. With a current draw test, you need to connect the leads in series (unlike the other tests which are connected in parallel). In this example we'll test the current draw at the battery terminal. You can test either the +ve or -ve terminal and you will get the same result. However, best to choose the -ve (black) terminal to avoid any accidental shorts to the chassis. Disconnect the -ve lead from the battery, put one multimeter lead to your battery lead and the other to the -ve battery terminal (once again polarity is not an issue, so leads can go either way). You should then get some sort of reading on the screen - security system alone draws current. If not, try turning on your park lights with the leads still connected, and you will see a reading. The reading on the screen displays how many amps the circuit is consuming. As said though, be careful measuring current draw. There are many circuits in the car that draw more than 10A. It's a quick way to bbq your meter!
continuity/resistance test
This is used if you want to check for a break in a circuit, whether it be a broken wire, a bad connection etc.
Similar procedure as the coil pack test at the top, except you choose the lowest ohm range. Some meters come equipped with a continuity buzzer on the lowest range. On my meter the lowest range is 400ohms and that setting also includes the buzzer. A very simple example of this test, is to put the 2 multimeter probes across a good fuse and then a blown fuse and note the different readings. Another practical example of where you might use this test is where you want to test for a break in a wire which is part of the main wiring harness which winds itself from one side of the dash to the other (very common problem when a wiring harness rubs through on the dash and cuts through a wire) Stick one multimeter probe on the connector at one end of the harness and the other probe at the opposite end of the harness where the wire comes out. If the wire has no breaks your meter buzzer will sound or you can read the display on the meter which typically should read less than 0.1 ohm - depends how long the wire is of course. But even a few meters of wire should register much resistance. If the meter does display either a higher reading or remains 'OL' then you either have a bad connection somewhere (high resistance) or an open circuit (OL).
Another use of this test is if you want to test where a circuit is shorting to ground, blowing a fuse or giving the ECU a incorrect sensor reading. Disconnect any connectors from the circuit so that nothing is connected to the circuit you are testing. Hold one probe to the chassis/ground and then the other probe goes into the wire/connector. If the continuity buzzer goes off and/or the display registers a reading then that section of wire is shorted to ground.
there are also some other handy test like voltage drop tests, but these are some of the most commonly used.
to test a V6 coil pack secondary circuit
black lead in 'COM' socket, red lead in the socket above. Select the 20K ohm range on the dial. Put the two leads across the spark lead terminals on the coils (ignition off). Polarity doesn't matter when measuring resistance so leads can go either way. From memory V6 coils should read no more than about 14K or 14,000 ohms. But usually when they are stuffed they go open circuit and meter will just display 'OL'
To use as a test light, or to test voltage on a circuit.
Leads connected to the meter as above, select the DCV range (direct current voltage) if using on car electrics. AC (alternating current) is mainly used for devices connected to the 240V mains in your house. Connect black lead to the negative side of the circuit to be tested and the red lead to the positive. If you get the leads back to front, no dramas - the screen will just show a negative '-' sign in front of the voltage reading. Note a multimeter will not replace a test light for all tests. Some diagnostic tests require the internal resistance or load of a light to operate. So really the multimeter can only test the voltage of the circuit 'unloaded' so to speak.
to check current consumption in a circuit
be careful with this one, it can stuff your meter if you aren't careful. Don't connect the leads to the circuit until you follow the procedure below - or you risk frying your meter - particularly if you have the resistance range selected when testing current draw.
Leave your black lead in the COM (common) socket on the meter. Now you need to determine what sort of current you are measuring. Up to 200mA (milliamps) you fit the red lead into the socket above the COM socket. Above 200mA and up to a max of 10A, you fit the red lead into the top socket (10A DC). Be aware that most circuits (apart from the engine management/sensor circuits) in a car draw at least 1 amp, so the lower 200mA range is going to be unsuitable, you will need to use the higher 10A range. However, any circuit that draws more than 10A may damage your meter. So look at the fuse used in a circuit before you rig up the multimeter, the fuse will give you some sort of indication of the maximum current the circuit is designed to draw.
Once you determine which amps range you are going to use, ensure your red lead is in the correct socket (as above) turn the dial to select that range. If the circuit is less than 200mA, choose the DCA range and select either 20mA or 200mA, dependant upon what sort of current draw you expect from the circuit. You can't do any damage selecting the wrong range, providing the current draw is less than 200mA. If the reading is too high for the selected range the meter will simply read 'OL' (overload) and alternatively if the reading is too low for your selected range, it will either display 'OL', 0 or just a figure like 1. IF you are going to test a circuit that is expected to draw over 200mA but less than 10A, then select the 10A range. Once again ensure the red lead is in the correct socket.
Now for connecting the leads to the circuit. With a current draw test, you need to connect the leads in series (unlike the other tests which are connected in parallel). In this example we'll test the current draw at the battery terminal. You can test either the +ve or -ve terminal and you will get the same result. However, best to choose the -ve (black) terminal to avoid any accidental shorts to the chassis. Disconnect the -ve lead from the battery, put one multimeter lead to your battery lead and the other to the -ve battery terminal (once again polarity is not an issue, so leads can go either way). You should then get some sort of reading on the screen - security system alone draws current. If not, try turning on your park lights with the leads still connected, and you will see a reading. The reading on the screen displays how many amps the circuit is consuming. As said though, be careful measuring current draw. There are many circuits in the car that draw more than 10A. It's a quick way to bbq your meter!
continuity/resistance test
This is used if you want to check for a break in a circuit, whether it be a broken wire, a bad connection etc.
Similar procedure as the coil pack test at the top, except you choose the lowest ohm range. Some meters come equipped with a continuity buzzer on the lowest range. On my meter the lowest range is 400ohms and that setting also includes the buzzer. A very simple example of this test, is to put the 2 multimeter probes across a good fuse and then a blown fuse and note the different readings. Another practical example of where you might use this test is where you want to test for a break in a wire which is part of the main wiring harness which winds itself from one side of the dash to the other (very common problem when a wiring harness rubs through on the dash and cuts through a wire) Stick one multimeter probe on the connector at one end of the harness and the other probe at the opposite end of the harness where the wire comes out. If the wire has no breaks your meter buzzer will sound or you can read the display on the meter which typically should read less than 0.1 ohm - depends how long the wire is of course. But even a few meters of wire should register much resistance. If the meter does display either a higher reading or remains 'OL' then you either have a bad connection somewhere (high resistance) or an open circuit (OL).
Another use of this test is if you want to test where a circuit is shorting to ground, blowing a fuse or giving the ECU a incorrect sensor reading. Disconnect any connectors from the circuit so that nothing is connected to the circuit you are testing. Hold one probe to the chassis/ground and then the other probe goes into the wire/connector. If the continuity buzzer goes off and/or the display registers a reading then that section of wire is shorted to ground.
there are also some other handy test like voltage drop tests, but these are some of the most commonly used.