An open electrical circuit means that there is a break in the continuity of the circuit. Circuits that are complete are considered closed. When a break occurs in the wiring of a vehicle's circuit it can be difficult to find because of the number of wires used in the electrical system. While time consuming, using an Ohmmeter allows you to trace open electrical circuits on a car. The job becomes easier if a wiring diagram is available as this will show the colour codes for the OEM wiring and which accessory is attached to the wire.
Disconnect the cable from the negative (-) post on the battery using a socket wrench to loosen the locknut on the terminal and pulling the cable off. Wait 20 minutes for the stored charge in the vehicle's electrical system to dissipate before continuing.
Start at one end of the fuse box, using the fuse diagram in the car's manual or the diagram that is affixed to the fuse box cover; identify the first wire coming out of the fuse box. Put a small label on the wire that is marked with the wire's identity (e.g. headlights).
Connect the two probes to the Ohmmeter and turn the meter on. Make sure the meter reads 0.00 with the probes not touching. Touch the probes and the meter should read between 0.00 and 0.03.
Touch a probe to each end of a short piece of wire simultaneously. Note the reading on the meter. Turn the dial on the meter, slowly, through the various levels of resistance ranges until the lowest range is found that still gives a reading. The higher range of resistance will be at the top of the dial, or for meters with a needle gauge, the higher range is to the right side of the meter.
Scrape a spot of insulation off the first wire to be tested (Spot #1) to reveal the inner copper core as close to the fuse box as possible. Scrape another spot of insulation off 8-inches away from that (Spot #2).
Place a probe tip on each spot simultaneously. If the meter gives a reading higher than 0.03 then there is no break in the wire between those two points. Remove the probes. Wrap Spot #1 with a piece of electrician's tape to reseal the insulation.
Scrape the insulation off the wire 8-inches away from the Spot #2. Touch the probes to the wire and take a reading. Continue working down the wire in this manner until the meter does not return a reading. The space between the probes (where there is no reading) contains the break in the wire that is causing the open circuit. Reseal the insulation on the spots no longer being used in the test. Label the wire every two feet so it is easier to keep track of which wire has already been tested.
Trace and test each wire coming from the fuse box.
Follow the wiring diagram, if available, to more quickly identify the path of the wire and its purpose. Wires are colour coded. If the vehicle still has its original wiring the diagram will make the job that much easier.
Never use an Ohmmeter to test wires with the vehicle's battery still connected, the charge will damage the meter.