February, 2010

Under the Hood

Troubleshooting the Starting System

Mark Lemay

The starting system includes the battery, starter and starter solenoid, ignition switch, safety switches and in some late model vehicles the PCM is also part of the starting system. Starting systems fall into 2 categories. Ignition switch controlled (Fig 1) and PCM controlled (Fig 2). Diagnosis of both systems is very similar.

Isolating the problem

Problems with the starting system show up in one of 2 ways. The starter will not engage and the starter will engage but can not turn the engine. I all cases where the starter will engage but the engine will not turn or turns too slowly, start by checking the battery.  Once you have determined that the battery is in good working order, you can move on to circuit tests to determine where the actual fault is.

If the starter engages but the engine will not turn or the engine turns too slow, begin by making sure that the engine spins freely on its own. You can do this by using a wrench on the crankshaft and then try to turn the engine. If the engine is hard to turn remove the serpentine belt. If removing the belt allows the engine to turn freely, look at the accessories such as the A/C compressor and the alternator.

Voltage tests

If the engine spins free you will have to do voltage tests to determine the failure area. For all voltage tests outlined in this article, the battery negative post is used as the ground point for your voltmeter. All tests should be performed with the key in the crank position and the test points are outlined in Figures 1 and 2.

With the key in the crank position, check the voltage at test point C. You should be within 1 volt of battery voltage. If you have more than 1 volt difference you will have to test back to the battery to determine where the fault is. Next, move your test lead to test point I. If you measure more than .75 volts, then the starter is not properly grounded. If you measure less than .75 volts, then the ground is good. If these readings are good and the starter engages but will not crank the engine, the starter is no good.

When working on an ignition switch controlled system and the starter will not engage, check for battery voltage at test point D. If the voltage at test point D is OK, then the starter solenoid or starter is defective.

If you do not have the correct voltage at test point D, you will have to work back through test points E, F and G to determine if the fault is with the neutral safety switch, wiring or the ignition switch.

PCM equipped systems

With a PCM controlled system which has no voltage at test point D, start by checking that the PCM is getting the correct data from the ignition switch and any safety switches (Fig 3). If any of the scan reading are incorrect test at points K and L to determine the fault. If the data is correct then determine if the PCM is grounding the relay at test point J.

Following these simple tests should quickly identify any problems with the starting system.