Top Ten Multimeter Tests - Part 1 - Forums at Modded Mustangs
 
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post #1 of 1 Old November 26th, 2010, 12:28 PM Thread Starter
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Top Ten Multimeter Tests - Part 1

Yep... here we go, diving into the whole electrical scene. I know it probably gives you the willies, but face it - your vehicle can't get along without good electrics. For a surprisingly small investment, you'll be amazed at how much these little meters can do, as well as how much time (and $$) they can save, when you know what to do with them.

These testers are called multimeters because they measure a number of things. The most common measurements are voltage, current and resistance. More advanced meters may measure temperature and frequency, while multimeters intended for automotive use will also measure rpm and dwell angle.

Most multimeters today are digital meters that provide a numeric display of measured values. Older ones used a sweep hand and require you to read the value from a scale under the needle... like a speedometer. Modern multimeters are surprisingly inexpensive. Full-tilt automotive troubleshooting meters are typically less than a hundred dollars, while simple hobbyist meters can often be found for around ten dollars.

1: Your First Test - Measure Battery volts



Many meters are inexpensive because they require you to manually select the correct measurement and range for the test you want to do. More expensive auto-ranging meters are available to handle range selection. All car batteries provide DC (direct current) voltage, most often in the range of about 10 to 16 volts.

Look at the face of the meter and turn the handle to select DC volts, with a maximum range of 20. Other choices may include 2 or 200 DC volts. You could harm an inexpensive meter by selecting 2 volts, if it doesn't have over voltage protection. Choosing the 200 volt range won't blow anything up, by you won't get a useful reading.

The two probes that came with the meter are red and black. Red is always for the positive terminal and black for the negative, or ground, post when doing voltage readings. With the engine off and the key out of the ignition switch, touch the black probe to the ground post and the red probe to the positive post. The meter should instantly display a number, which is the battery voltage.

Expect to see something in the range of 11.5 to 12.6 volts. Anything less than this may indicate a bad cell in the battery, or one requiring water if it is not maintenance-free (unusual these days).

2: Really Important Simple Test - Ground Resistance



The next test could save you hours and hours of messing around, particularly on older vehicles. Modern car electrical systems use a 12 volt, negative ground configuration. This means that electricity is supplied from the positive terminal on the battery, while the negative terminal is connected to the car body and chassis.

This works well so long as the body and chassis are metal. It also works well as long as the body and chassis are electrically connected, usually by a ground strap or fusible link. Checking the resistance between various points on the body, engine and chassis will ideally show no resistance. This allows continuous flow of electricity and is called a continuity check.

Resistance is measured in ohms, so we're going to set the meter to its lowest resistance range to do our tests. With that done, touch the red probe to the negative terminal on the battery. Yes, I said the black goes to the negative last time, but we're not measuring volts any more.

Now, touch the black probe to some bare metal on the engine. The exhaust manifold is often a good place. Your meter should display a zero for the resistance value. It may also beep or flash a light, depending on its design. If it doesn't display zero or some very low value, check the connection between the battery's negative cable and the body or engine.

Follow the black cable from the battery to its destination. Look for a corroded or loose terminal. Look for a ground strap that connects the body and the chassis or subframe. The body is mounted on rubber isolators to keep the cabin quiet, but rubber is an insulator. A direct electrical connection is established using a flexible metal strap. Check the ground strap for corrosion, loose connections and overall good condition. Clean and tighten the connections, and/or replace the ground strap as needed.

3: Next Up - Cranking Voltage Test



You may need more than one pair of hands to do this, so if someone is around ask for their help. Even if the battery showed a good voltage in the first test, it may be worn internally and barely able to supply enough juice to start the car reliably.

Before anything else, make sure that the battery terminals are clean and the connections are tight and secure. Do the same for the starter motor and the starter solenoid connections. Now, we want to disable the ignition system so that the car won't start when we crank it.

On older cars, you can do this by disconnecting the ignition coil - either the high voltage wire going to the distributor, or preferably the low voltage positive (red) wire going to the coil. For electronic ignition, refer to the car's manual on how best to do this. You may need to disconnect the distributor pickup coil, or a crankshaft or camshaft position sensor.

Set the multimeter's range and connect it to the battery as done in Test 1. If you have a remote starting switch, use it or else have someone try to start the car for about 15 seconds. Watch the voltage displayed on the meter. The following chart shows the best result for the current temperature. What you saw on the meter doesn't correspond with these results, then you have a problem.

Temperature -> Volts
70F+/ 21.1C+ = 9.6 or more
60F / 15.5C = 9.5
50F / 10.0C = 9.4
40F / 4.4C = 9.3
30F / -1.1C = 9.1
20F / -6.7C = 8.9
10F / -12.2C = 8.7
0F / -17.8C = 8.5

If the previous step went well, or you cleaned up all the connections, the current problem lies either with the battery, starter motor or solenoid. You can take the battery to a shop to have a load test done. Further checks of the starting system are possible, but are beyond the scope of this article. Check the manual that came with your meter for more information that may help.

4 - Charging System Test



To see if the vehicle's charging system is healthy, set up and connect the meter as we did in Test 1. Start the engine and let it idle. Turn off all electrical loads, such as the stereo, fan, lights, defroster and so on.

On the multimeter, you should see a value between 13.2 and 15.2 volts. Values lower than this may indicate loose or corroded connections, a loose drive belt or a mechanical problem with the alternator. Higher readings are most likely due to an electrical problem with the alternator.

If you're good to this point, have someone sit in the car and run the engine up to 2000 to 2500 rpm and hold it there. Check the reading on the meter. It should not have changed by more than half a volt from the previous reading.

Now, have them turn on as much electrical load as you can. With the fan, lights, rear defroster and everything else running, the battery voltage should not drop below 13.0 volts. If you have installed a high end audio system, leave it off at first. If the voltage is at 13.0 or higher, but drops below when the audio system is on, it is drawing too much power and you need to upgrade to a higher output alternator.

5 - More Continuity Tests



The ground resistance test we did earlier is a special application of the Continuity test, which checks for a continuous connection between two points or electrical terminals. This is a very basic and also very handy test.

For example, you can verify the state of a fuse with a continuity test. If the resistance between the fuse's terminals is zero or very close to that, then the fuse is good. If the resistance cannot be measured because it is too high, then the fuse has blown and there is no continuity between the terminals.

The same thing applies to high voltage ignition wires. These will have some resistance to them. Just check the manufacturer's literature for the value. If you can't find that, then just keep the average value in mind as you go along. Touch one probe to each end of the ignition wire to measure the resistance. Wiggle the wire and see if any big changes in the resistance happen, which would indicate a loose connector or perhaps a break in the wire. If all wires but one have a similar resistance value and one is significantly higher, the oddball wire probably is defective.

Checking a mechanical switch is just about as easy. After disconnecting the wiring harness plug,touch one probe to each of its two terminals. Then actuate the switch manually. You should see the change. A normally open switch will show infinite resistance until you actuate it, then it should show zero resistance. A normally closed switch will show the opposite. If it doesn't change, the switch is shot.

(continued in Part 2)

Last edited by AF_Don; December 15th, 2010 at 03:30 PM.
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