Auto Service Professional

OCT 2018

Magazine for the auto service professional

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36 A S P O c t o b e r 2 018 E l e c t r i c a l S y s t e m F a i l u r e s Performing a voltage drop test on a circuit is a surefire way of checking that circuit for high resistance. Performing a voltage drop test is quick and ef- fective when testing the integrity of that circuit. For example, let's check a negative ground circuit such as a battery cable. Simply take your DVOM (digital volt ohm meter), place one lead on the negative battery post and the other lead on the end of the battery cable where it attaches to the ground connection on the engine block. Set your DVOM to the millivolts scale and op- erate the circuit. Crank the engine and watch your reading. is is best viewed on a graphing meter, if possible. e voltage reading should be very close to zero volts. If you are obtaining a voltage reading that's too high, then adjust your voltage scale on your meter so you can see what the actual voltage is. If it's excessive, then your ground circuit has too much resistance, either due to a bad connec- tion, loose connection or possibly a faulty battery cable. Remember, a circuit only has 12 volts. If you have two volts on your ground side then that only leaves you 10 volts to operate that circuit. You will probably start to see problems arise if that's the case. As a mobile tech, some of the issues I come across out in the field are created by unapproved testing methods. Anytime you are testing a cir- cuit it is imperative not to disturb the circuit. I prefer back probing a connector or disconnect- ing the connector if possible, but it is important to take great care of that circuit while testing. If you disturb that circuit too much you just might have temporarily repaired the circuit and you don't even know it! If you have to pierce a wire to check it or back probe a connector which has a moisture protect- ing seal on it, make sure you seal that test point with a sealer that doesn't allow moisture into the wire or terminal. If you have to apply shrink wrap to the area that will work as well. Sometimes you might even have to solder your test point with a new connection. Here is an example of a terminal connector that was probed with a test light and then le un- Figure 7: It's best to have a terminal test kit with probes that feature both male and female connections. They fit the terminals properly and work great when performing a terminal retention test.

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