Auto Service Professional

JUN 2018

Magazine for the auto service professional

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Page 16 of 69

J u n e 2 018 A S P 15 Starting system diagnosis At this point, we move on to our starting system diagnosis. One of the few things besides normal wear and tear that will shorten the life of a starter is the lack of proper voltage going to the starter. Issues can be caused by either a loose connection or excessive resistance in the circuit. Battery ca- bles that have a high resistance can dramatically reduce the available amount of amperage that the starter needs to properly start the engine. One important test in starter diagnostics is to perform a voltage drop test on the battery cables, both at the positive and the ground side of the starter. You need to make sure that the proper voltage required and the starter ground side have good clean connections. Any circuit that has ex- cessive resistance will not properly operate the component on that circuit. Testing a starter motor for the correct amper- age draw can be done in a couple of ways. One approach involves using an electrical system tester like the one shown in this article or by us- ing a standard DVOM and a high amps current clamp. When using a meter and a clamp you can set your meter to the voltage scale and hook your amp clamp to one the battery cables. Disable the fuel or ignition so the engine will not start during the test. Crank the engine as you observe the tester's ammeter and voltmeter. Never crank the engine for more than 15 sec- onds and allow the starter to cool for two min- utes between cranks. Slow cranking and high current draw typi- cally indicate worn bearings or bushings inside the starter. Worn bushings result in an off-cen- ter armature. is can result in poling and can throw off the alignment of the starter's magnetic fields. Observe the voltage and the amperage at the same time. e voltage should remain at or above specifications while cranking the engine. High current draw and low cranking speed indicate a faulty starter or possible engine prob- lems. Low cranking speed and low current draw indicate excessive resistance in the starter circuit. Always check specifications; the bigger the engine the higher the current draw. Typical read- ings for a four cylinder engine is 150 amps, six cylinder is 200 amps and eight cylinder is 250 amps. Remember, these specs are only approxi- mate. You need to consult your service informa- tion system for the proper spec for the vehicle you are working on. Also, remember that any of these testing procedures are null and void if your battery isn't working properly. Automotive electrical diagnostics play a very important role in today's increasingly sophisti- Figure 7: Shown here is a screen shot of a battery sensor monitored by a scan tool. This sensor is showing the battery current sensor in amperage.

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