How to Find an Open Circuit in a Car's Electrical System

Updated on December 27, 2018
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

A failed load can create an electrical open in a circuit.
A failed load can create an electrical open in a circuit. | Source

Most car electrical opens are the result of a:

  • corroded connection
  • loose wire
  • disconnected wire
  • failed load
  • blown fuse
  • burned out bulb
  • damaged wire (frayed, chaffed or burnt)

So you might want to concentrate on these problems first when trying to diagnose the cause of an open circuit.

How to Make Your Open Circuit Diagnostic Easier

Also, there are a several steps you can take to speed up your diagnostic:

  1. Try to identify the number of components affected in the circuit.

    • If only one component is affected, start there. Make sure it is receiving power and has a good ground; and test the load, if possible. Then work your way towards the power or ground source as necessary.
    • If all the components in the circuit have failed, check your electric diagram (more on this later) and see where they share a common power source or ground and start there.
  2. Check if other circuits are affected and how all they relate to each other, if at all. The wiring diagram may tell you the most logical place to start.

  3. If you have an intermittent open, check for corroded or loose connections; pay attention to fail patterns, for example:

    • a broken coil in a motor (for example a fuel pump) or ignition coil may open (and fail) after warming up.
    • a circuit with a loose wire may only fail when the car turns or goes over rough road.
    • a component may fail during wet whether (for example, a leaking spark plug wire).

    If possible, warm up a load, wiggle or spray a wire with water to recreate the conditions that cause a load or circuit to fail. Then start testing.

Also, don't forget that a short circuit can cause a wire or wires to burn and separate from a load, connection or circuit, resulting in an open. If you suspect a short circuit, you can check this other post on troubleshooting for car electrical shorts. The guide has a practical diagnostic you can use to locate most electrical shorts in your vehicle.

Index
I. Preparation and Precautions
II. Checking the Power Source
III. Checking the Circuit for an Open
IV. Isolating a Suspect Circuit
V. Using a Jumper Wire to Test for Power
VI. Using a Self-Powered Test Light
VII. Making the Most of Your Diagnostic Time
An electrical diagram will guide through your diagnostic process.
An electrical diagram will guide through your diagnostic process. | Source

I. Preparation and Precautions

If you are not familiar with the circuit you want to test, get the electrical diagram. Having the electrical diagram for the circuit you need to test will make your diagnostic much easier.

An electrical diagram helps you identify loads, wires, switches and control devices in the circuit; how current is supposed to flow in the circuit, how the circuit is protected and the normally hot or open state of the circuit.

Aftermarket manuals have specific and simple diagrams you can use for key electrical circuits in your vehicle. If you don't have the diagram, you can buy an inexpensive, aftermarket Haynes repair manual for your particular vehicle make and model on Amazon.

Haynes manuals come with photos, drawings and step-by-step procedures for many maintenance, repair, and troubleshooting projects. So you'll recoup your small investment in a short time.

  • Study the diagram and see how the circuit is supposed to work. Pay special attention to connectors, splices, switches and components in the circuit where an open is possible to occur. These items are also your test points. They will help you to divide the circuit into sections to make your testing easier.
  • To avoid confusion, identify wires by their color and the terminals, switches or load they run to and from.
  • Unplug connectors properly to prevent damage.
  • Whenever possible, probe connectors from the rear side or wire side. If you need to probe from the front of the connector, be careful not to damage pins or short out the terminals.

Depending on the circuit you are dealing with, you may or may not have access to the test points described in the following steps. Still, you can apply and adapt the steps to whatever access you have in your circuit.

These are just ideal points in the circuit to keep in mind so that you can follow, as much as possible, a systematic approach to your car electrical circuit diagnostic, and help you isolate the problem area in the circuit.

This is a common troubleshooting technique that you can adapt to any circuit you need to test for an open.

Preventing Damage to Wires

If you need to pierce through wire insulation with your test light or voltmeter, seal the opening with regular or liquid electrical tape. This will prevent corrosion buildup in the wire that might interfere with circuit operation later on.

Locate the fuse box or power distribution center in your car and check the fuse or breaker for the circuit you want to test.
Locate the fuse box or power distribution center in your car and check the fuse or breaker for the circuit you want to test. | Source

II. Checking the Power Source

Although here we're using a voltmeter to test for specific voltage values, you can certainly use a test light to check for the presence of electrical current, or voltage, when necessary.

But remember, a test light only tells you that some voltage is present at a particular point in the circuit, not how much voltage is present. That's why it's important to use a DMM as you'll see in the following diagnostic procedure.

  1. Set your digital multimeter to 20 volts on the DCV scale or to auto-range, if your meter has this feature.

  2. Turn on the circuit you want to test so that there is voltage and current going through the circuit. You may need to turn the ignition key to the On position, or idle the engine.

    • If you need to start the engine, make sure your transmission is in Neutral (manual) or Park (automatic), and set the Parking brake. Also, be careful with moving components, if you are testing wires around the engine compartment.
  3. Connect your meter's black lead to the battery negative terminal or chassis ground; connect your meter's red lead to the circuit's fuse. Make sure you have power there. Test both test points on the fuse so that you know power is flowing through the fuse and the fuse isn't blown.

    • If power is present at the fuse, go on to the following section.
    • If there's no power present at the fuse, check the fuse itself, and, if necessary, the part of the circuit between the fuse holder connection and the battery. For example, there may be a broken fusible link in that part of the circuit or a bad connection to battery power.

Using a Digital Multimeter

When testing electrical or electronic components in your vehicle, use a 10 Megohm impedance digital multimeter to prevent damage to your car computer and other electrical devices.

Hypothetical and simplified vehicle electrical circuit with an open.
Hypothetical and simplified vehicle electrical circuit with an open. | Source

III. Checking the Circuit for an Open

  1. Turn on the circuit you need to test. You may need to turn the ignition key to the On position or idle the engine.

    • If you need to start the engine, set the transmission to Neutral (manual) or Park (automatic) and set the Parking brake.
  2. Connect your meter’s negative (black) lead to ground, and the positive (red) lead to the positive side of the load in the circuit you are testing.

    1. If you get 0 volts, move your meter's red lead to the output side of the control device (switch, relay).

      1. If now you get at least 10.5 volts, you got an open between the load and the control device.
    2. If you get 10.5 volts or more, move the meter's red lead to the ground side of the load.

      1. If your meter reads less than 1 volt and the load isn't working, most likely you got a bad load.
      2. If your meter reads about 1 volt or more but less than battery voltage, look for an open or too much resistance in the ground side of the circuit. There could be a corroded or loose connection or damaged wire preventing proper flow.
      3. If your meter reads battery voltage, there's an open in the ground side of the circuit.

        1. Make sure the load's ground connection is tight and free of corrosion. If necessary, use a jumper wire to connect a good ground directly to the load's ground connection to confirm the load works when provided with a good ground. If necessary, test different points at the ground side of the circuit to bypass a suspected open.
    3. If your reading is below 10.5 volts but more than 0 volts, begin testing the output and input voltage side of every connection, switch or splice on the positive side of the circuit, using your meter, gradually moving toward battery power. There could be a corroded or loose wire or connection.

      1. If at any point you get 10.5 or more volts, there's either an open or too much resistance between that point and the previously checked point.

      If you suspect there is too much resistance in a circuit, you can test for voltage drop to help you locate the connection or terminal with the problem. Check this other post on voltage drops.

Check loads, switches, relays and connectors for continuity.
Check loads, switches, relays and connectors for continuity. | Source

IV. Isolating a Suspect Circuit

You can also check for continuity within a suspected section of a circuit by isolating that portion from the rest of the circuit.

Make sure there is no load or a relay between the two points of the circuit section you want to test; otherwise, broke further that section at those points.

  1. Make sure the circuit is turned off. Then disconnect that portion of the circuit by unplugging a connector, switch or load from each end of the section you want to test to isolate it.

  2. Set your meter for continuity or to the lowest setting on the Ohms (resistance) scale.

  3. Connect one of the DMM leads to one end of the isolated circuit, and the other lead to the other end.

    • If the readout says "OL" (infinite resistance), then there is an open in that part of the circuit. Move one of the leads closer to the other one, testing at different points. Once you get a resistance value, or your meter beeps, the open is located between that point and the previous point you tested.
    • If the DMM beeps or the readout says .3 Ohms or a similar small resistance value, there's no open in the circuit. However, you make get a higher resistance value, which might indicate a loose or corroded connection or terminal or wire. If necessary, check for voltage drop in that part of the circuit to locate the problem.

A jumper wire can help you locate opens in a circuit.
A jumper wire can help you locate opens in a circuit. | Source

V. Using a Jumper Wire to Test for Power

You can use a jumper wire to test sections on the power (or ground) side of a circuit. For example, to connect a load directly to battery power.

However, keep this in mind:

  • Don't use a jumper wire with a smaller gauge than the wire used in the circuit you need to test; otherwise, the jumper wire may overheat and cause a fire.
  • if you are connecting a load directly to power, make sure the load is designed to work with 12 volts. Some components work with around 5 or 4 volts only. Consult your repair manual, if necessary.
  • Use a 5 amp fused jumper wire to test the power side section of a circuit, for better protection.
  • Connect one end of the jumper wire to battery power. Connect the other end to the power side of the load you need to test.
  • You also can use the jumper wire to bypass switches, relays, splices and connectors. But don't bypass a load or you'll actually create a short circuit.

Test lights and self-powered test lights can save you time when testing circuits.
Test lights and self-powered test lights can save you time when testing circuits. | Source

VI. Using a Self-Powered Test Light

Another useful tool to test open circuits is the self-powered test light. You can use it to test sections of wires and switches.

When using this tool, remember:

  1. Most self-powered test lights use a 1.5 volt battery to operate. Make sure the battery in your test light is good. You can touch the test light probe to the ground clip. If the battery is good, the light will shine brightly.

  2. Only test computer controlled systems with a self-powered test light if your repair manual says it is OK to do so, or the manual says a system or component you want to test works with a voltage equal or higher than the one used by your self-powered test light.

  3. Disconnect the battery or unplug the harness connector in the circuit to isolate it for your test.

  4. Connect the test light ground clip to a good ground.

  5. Start testing at either end of the circuit.

    • If the test light doesn't come on, the open is located between the ground and the point in the circuit you are testing with your probe.
    • If the test light comes on, the open is located between the point in the circuit you are testing with your probe and the other end of the circuit towards the power source.

A MAF sensor, other sensors and loads can fail and create opens in a circuit.
A MAF sensor, other sensors and loads can fail and create opens in a circuit. | Source

VII. Making the Most of Your Diagnostic Time

Testing for opens can be tricky, and sometimes scary, especially when you have poor or no access to sections of a circuit or components, splices or connectors you may want to test.

But the surprising thing is that most electrical opens are caused by disconnected or loose connections at switches, relays or loads; corroded terminals or a failed load, as shown in the video below. So you stand a good chance of finding and repairing an electrical open when it occurs in your vehicle. Concentrate on these sections first.

Having said that, don't be misled by simple electrical plugs, switches and otherwise accessible loads and their terminals and connectors. Modern vehicles have complicated circuits under computer control. If you are not careful when testing even simple connectors, you may cause unintended damage to a circuit.

A Failed Load in a Circuit

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

    © 2018 Dan Ferrell

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