Chapter 9 Basic Electrical Tests 171 Copyright by Goodheart-Willcox Co., Inc. A grounded circuit is caused by a ground wire shorted to ground. Th is usually has no noticeable aff ect. Th e short just provides another path to ground. See Figure 9-8B. Figure 9-9 shows how a short in diff erent locations in a circuit can cause diff erent symptoms. As shown in Figure 9-9A, a short aft er a load does not blow the fuse. Th ere is still a resistance unit (bulb) between the power source and short to limit current. However, since the short is between two loads, it shunts current away from the second load. Th e second load (bulb) will not function. Th e current seeks the path of least resistance through the short to ground. Figure 9-9B shows how a short before a switch can prevent a circuit from being shut off . A short before the grounded switch allows current, even with the switch off . Figure 9-9C shows an example of a wire-to-wire short. Th e conductors in two wires are touching because of dam- aged insulation. In this case, the short could make both loads (bulbs) function even when the switch to one of the loads is turned off . Intermittent Problems An intermittent problem is one that occurs only under certain conditions. Th e problem may be present one time but may not show up later. Th is makes testing dif- fi cult because the symptoms may not occur when you are troubleshooting. Figure 9-5. Opens in a parallel circuit may cause different symptoms, depending on where in the circuit they occur. A—An open in the main wire feeding to the parallel branches disrupts power to both branches. B—If the open is on one of the parallel branches, the other branches still function normally. Open at main feed wire Both branches dead Bulb Out A B Open in parallel branch Fuse Other branch still functions Figure 9-6. A high circuit resistance is similar to an open, but some current can still exist in the circuit. In this example, high resistance in the top connector reduces current through that branch. The bulb in that branch therefore burns dimly. Normal resistance Normal illumination Dim bulb Connector has high resistance Figure 9-7. Some sensors, such as this oxygen sensor, produce an output of less than one volt. Any resistance in the circuit lead- ing to the sensor could produce false signals for the computer and upset engine or other system operation. (Love the Wind/ Shutterstock.com) Figure 9-8. Examples of short circuits. A—When a hot wire is shorted to ground, it causes very high current that blows the fuse. Both bulbs fail to function because of the open fuse. B—When a ground wire is shorted to ground, it does not affect the circuit. The short just provides a second path for ground. Both bulbs off No resistance in circuit Circuit not affected A B Short in ground Fuse blows Hot Wire Short to Ground Shorted Ground or Grounded Circuit