Digital Ammeters

A series type ammeter is the type of meter that is built into every DMM. This meter is designed to measure relatively small current flows (below 10 amps). Most meters measure in either milliamps (mA) or Amps (A). Before connecting the meter into the circuit, make sure the circuit draw is within what your meter can handle. It is a good practice to initially set the meter to the highest range available, and lower the range while the current is being measured. Most ammeters are fuse protected to prevent damage from short-to-grounds or overload conditions. The series type ammeter is best suited for measuring current flows below 1 amp.

We have been using clamp type ammeters for years on starting/charging system testers such as the Sun VAT- 40. This type of ammeter is also available as an accessory that you can use with any DMM. These battery-powered clamp type ammeters (sometimes referred to as "inductive-type" ammeters) measure current flow by sensing the strength of the magnetic field produced around the wire while current flow is present. These clamps then convert this amperage reading into a voltage which is read with the DMM set to measure millivolts. Due to a lack of accuracy below 1 amp, these accessories are best suited for any amperage measurement except normal parasitic loads. It can be used to troubleshoot a high parasitic load problem if the "draw" is above 0.5 A, depending on the model of "amp clamp" you are using.

The correct connections for each type of ammeter is shown below.

The correct connections for each type of ammeter is shown below.

Digital Ohmmeter All ohmmeter measures the amount of electrical resistance between two points. The digital ohmmeter has several significant advantages over its analog counterpart:

• Easier to read—the sweep doesn't go "backwards"

• "Zero" resets automatically

• Extremely accurate

When connecting an ohmmeter, make sure that the circuit or component is isolated from parallel branches or other voltage sources. Most good quality meters are "forgiving" when accidentally connected to voltage, but analog meters and low priced DMM's may not be.

Digital Ohmmeter Display

If you are using the meter in auto-ranging mode, be sure to look at the units (Kii or O) at the side of the display or on the range selection knob.

Look at the Display or the Range Selector

Look at the Display or the Range Selector

Additional Features: Diode Check


In the past, an ohmmeter was commonly used to check diodes. The operation of the diode could be verified by checking for continuity in one direction, and for no continuity in the other. However, the voltage that a digital ohmmeter uses to make its resistance measurement is usually less than 0.2 V. This low voltage is not enough to "forward bias" the diode, so the diode will show no continuity in either direction.

Most good quality DMM's have a diode check function. This function (on the better meters) will tell you the forward bias voltage drop of the diode—the amount of voltage required to turn ON the diode so that current will flow through it. For the silicon diodes found on the car, this voltage should be around 0.5V.

Some low priced meter's diode check function do not measure the forward bias voltage drop. Instead, these meters simply raise the voltage used by the ohmmeter to allow you to check for continuity in one direction and no continuity in the other. The number on the display is not a voltage drop.

Diode Check

Use the diode check function to check the condition of a diode. Besides in the alternator, diodes are used frequently in the wiring harness to provide circuit isolation. Look for about 0.5 V with the diode check function.

Introduction In step #3 of the six-step troubleshooting plan, you analyzed all the symptoms that were confirmed through your preliminary checks. Based upon these symptoms, you could make a conclusion as to the type of electrical problem that the circuit has:

• An unwanted parasitic load or short-to-ground

• A high resistance problem

• A feedback from another circuit

In this section, we will concentrate on diagnostic strategies and techniques that should be used to isolate each of these problems. Youll find that using the "right" tool for each type of problem will save you a lot of time when working to pinpoint location of the circuit problem.

Diagnosing Open Circuit Problems

Of all the types of electrical problems, open circuit problems are the most common. Open circuits are typically caused by:

1. Disconnected connectors

2. Bad switches

3. Poor terminal contacts

4. Cut wires

5. Blown or defective fuses

You can assume that you have an open circuit problem whenever there is no visible sign of operation. You can use a number of tools to find the location of an open circuit. Each of the tools has its advantages and disadvantages, so it's probably best to use a combination of the three, depending on the situation.

Using a Voltmeter On Open Circuit Problems

An open circuit voltage (positive probe at terminal, negative probe connected to a known good ground) will verify continuity in the circuit to the B+ source. If the negative probe of the meter is grounded through the ground wire of the circuit (meter is connected in series to the circuit), it will verify continuity of the ground side as well.

1. Use the EWD to determine where to make the checks and if any switches/relays need to be closed.

2. Connect the negative probe of the voltmeter to ground, and use the positive probe to check the various pin voltages with the circuit ON. Remember that the EWD will not tell you how much voltage you should have at every pin in the circuit. You need to apply your knowledge about circuits to determine what the correct voltage should be.

Inspect the connectors/locations that are the easiest access, then check the harder ones, if necessary.

Keep in mind that even if your voltmeter indicates near battery voltage at a terminal, it tells you only that there is a connection between B+ and the inspection point, and not how good the connection is. With high circuit resistance, the open circuit voltage would stay about the same. The only way to detect this resistance would be to measure for a voltage drop around the load or the suspect area of the circuit or to check the resistance with an ohmmeter.

Voltmeter Advantage: Easy to use, cannot cause circuit/fuse damage Advantages and

Disadvantages Disadvantage: Cannot detect a high resistance problem with open circuit voltage check; would have to disconnect the ground point to check the continuity of the ground side wiring. (It would probably be easier to use an ohmmeter to check the ground side.)

Using the Voltmeter for Open Circuits

Using an Ohmmeter On Open Circuit Problems

Ohmmeter Advantages and Disadvantages

An ohmmeter can also be used to check continuity in the wiring on both sides of the circuit.

1. Use the EWD to determine the appropriate test points. Be sure that the circuit is OFF while making the measurement, and that there are no unwanted parallel connections in the section of the circuit you are testing.

2. Connect the ohmmeter probes on each end of the section of the circuit you want to check.

Advantage: Checks for resistance problems

Disadvantage: More difficult to connect to the circuit, requires power to be turned OFF. Usually need to disconnect more connectors to isolate the portion of the circuit being tested. On high current flow circuits (starter motor or load which draws above 4 amps), the amount of resistance that can cause a problem (in the tenths of an ohm) is very small and difficult to detect. A voltage drop check is more useful in this case.

Using a Jumper Wire Use a jumper wire to by-pass sections of the circuit.

1. Use the EWD to determine sections circuit which can be by-passed with a jumper wire

2. Connect the jumper wire by backprobing connectors Advantages: A quick, simple means of eliminating parts of the circuit

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