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Author Topic: Using a Multimeter  (Read 1292 times)

Dan Cluley

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Using a Multimeter
« on: July 17, 2010, 12:27:29 AM »

Disclaimer:  I am not a mechanic or an electrician, just a guy who usually knows which end of the screwdriver to use.  If you feel that you are likely to electrocute yourself or burn your car to the ground get help with the project.
There are lots of things you can do with a multimeter, but I'm just going to cover a couple of the basic ones.  However these are by far the most common things you will need to check for most problems.
First, a quick look at how things are wired:
A basic electrical circuit has two wires, one running from the + terminal of the battery to the thing you are powering (lightbulb, motor, etc) and the second runing from that item back to the negative or ground connection of the battery.
To save weight, and space, and especially MONEY, cars use the metal of the body, frame, dash, etc instead of actual wire for the ground connections.
So the most basic circuit in your car would be something like this.

Now, along the path of that red wire, add some connectors, a fuse to protect things, and a switch to turn it on and off, and you've got an actual circuit from your car.

1.  Voltage

To check voltage, set your meter to measure DCV (direct current voltage) Most meters will have several DCV settings, Each is identified by the maximum voltage it will read.  Pick the smallest one that is more than 12.

To check the battery voltage, connect the Black meter probe to the negative terminal, and the Red meter probe to the positive terminal.


With nothing turned on (draining the battery) a good battery should be pretty close to 13 volts.
Lets say that bulb isn't lighting up:
Turn on the power to whatever you are checking.
Move the Red meter probe to the positive wire at the bulb socket, it should still be pretty close to the same voltage that you had right at the battery.  You can also do this at the other connections along the path (bulkhead connector, switches, etc)

Checking the ground connections is basically the same.  Keep the Red meter probe attached to the positive battery post (or other + power spot that you know is good) and use the Black meter probe to check the voltage at the ground connection.  Should still be 12 volts.

2. Continuity
Unlike voltage, you check continuity with the power off, or even with components unplugged
The multimeter is designed to give exact measurements of resistance, but in a lot of cases we just need to know if a wire is intact, a switch is working, or connections are good.
For these purposes basically no resistance (or close to it) means things are all connected, and infinite resistance means there is a break somewhere.
Like the voltage settings, most meters will have several options for measuring resistance.  Again, if you are looking for the most accurate  measurement pick the closest number of Ohms that is larger than what you are checking.  For just checking continuity it shouldn't matter which Ohm range you pick.

With the probes not touching each other (or anything that will conduct electricity) notice what the meter reads. 

This one shows 0.L indicating "No Load"  Analog meters will point to the infinity symbol, and other meters may have other indications.  Whatever it shows at this point will be the indication of a lack of continuity when you are checking things.
With the two probes together, the meter should read 0.  If it doesn't check to see if there is a small knob or wheel that adjusts the zero setting, and turn it until it is reading 0.  If this doesn't work, check the battery in the meter.

To check the continuity of something, simply touch one of the probes to each end of the wire, switch, etc that you are checking.

Most components should have zero resistance, but some things like lights, or resistors will measure some Ohms.  
If you see the infinite resistance or no load indication then there is not continuity.
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Using a Multimeter
« Reply #1 on: July 17, 2010, 07:54:55 AM »

I made this a Sticky
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