Basic knowledge streamlines electrical repairs A STEP-BY-STEP APPROACH CAN TAKE THE HEADACHE OUT OF COMPLEX ELECTRICAL SYSTEM DIAGNOSIS
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lectrical issues can be some of the most difficult problems to isolate and repair. Given all the electronics on today’s vehicles, electrical issues can be challenging and frustrating for even some of the best technicians. In this feature, we’ll discuss some issues that can occur and how to employ a diagnostic strategy using a variety of tools to help find the root cause of the problem.
ISSUE 1 — INTERMITTENT NO CRANK, NO START VEHICLE: 2014 JEEP WAGONEER This Jeep came into the workshop for an intermittent cranking issue. There was no real pattern to the issue but it had been occurring about fortnightly over the past few months. The vehicle had had a battery replacement about two weeks earlier; and the starting issue had occurred since then. The first step was to test the battery and charging system, using our battery/charging system tester. We found no faults and both appeared to be functioning correctly. Our next step was to scan the Jeep for codes. There were none, but we noticed that many of the monitors had not completed. Incomplete monitors
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Auto Channel Issue #41 November 2021
can occur either by someone clearing codes or by a loss of power to the PCM. We decided to monitor the charging system to determine any signs of an intermittent issue. Using our scan tool, we monitored the generator duty cycle and voltage sense. In the course of monitoring, we noticed that the voltage would drop to under 12 volts and the duty cycle would drop to zero. We also noted that the voltage would occasionally rise to around 15 volts, slightly higher than normal. We suspected some high resistance somewhere in the charging system was causing the computer, which controls charging output, to intermittently undercharge and overcharge the system. One of the best ways to test for resistance in the circuit is to perform a voltage drop test using a DMM in its voltage min/max setting. A voltage drop test is much more reliable than using an ohmmeter because the results will also show the ability of the circuit to deliver enough current. For example, if you were to use an ohmmeter to test a single strand of 16-gauge wire, it would show continuity and zero resistance, just as a length of 16-gauge wire will show continuity and no resistance. But if you tried to pass any kind of load through the single strand of wire, it would most likely burn.
No resistance but a single strand cannot carry any appreciable load
Performing a voltage drop test will show you the amount of voltage that is available at the circuit when current is flowing. A good rule of thumb for passing a drop test is a drop of less than one-half volt for most circuits.
