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starter motor failure revisited
Posted to Technical Discussion Forum on 1/3/2014 21 Replies

In reply to the thread I will discuss a quick and very accurate way to test not only the starter, but the complete starting and charging system by using a lab scope, a high amp current probe and a voltage test lead.

For this test, hook the scope voltage lead to the positive and negative lead of the battery, hook the current probe around the ground wire to the battery. If there is more than one ground lead, hook your current probe around all the ground wires. You want your scope to measure all the current going in and out of the battery.

This quick and simple test will tell you about the integrity of the cables carrying the current to and from the battery, the condition of the starter solenoid contacts, the starter armature, relative compression of the engine, condition of the battery and the condition of the charging system, all with only two scope hook ups, and less than 5 minutes stop to finish of the test.

If you are using a Pico scope, put 5 minutes of time on the screen, if you are using a Snap On scope (Modis, Verus, VP) put abut 2 seconds of time on the screen. Set the voltage trace to 20 volts, and the current trace so it will capture 600 amps (if you are using a 600 amp probe). Set a trigger so the trace will be started when the starter is turned on. You should see a scope trace something like this. sulfated battery starting and charging system test Starting at the left side of the traces, I can see the starter solenoid contacts closed nice and clean, the starter got the engine running in short order, then the generator started charging the battery. Look at the right side of the trace and notice the charging current the battery is still absorbing after a three minute charge. It is still taking in over 50 amps. This is putting the hurt on the generator, and the battery will get very hot after it charges for a while at this rate.

This is something I found on a vehicle that came in for a routine service. In this case the battery is sulfated. There are no starting complaints at this time, but just wait, there will be several starting and charging complaints if this problem is not fixed.

Step 2 in the testing is to zoom in on the starter turn on. For you people using a Snap on scope, this will be where you started, and to be able to display the previous waveform, you will have to scrunch your waveform up. If you start with a waveform like the one displayed, you will not be able to zoom in for the detail of the starter turn-on. sulfated battery zoom

Now to use a carbon pile and load test the battery. sulfated battery test battery load test The battery load test shows nothing wrong with the battery, but the dynamic starting system test found the problem in short order.

Now on to a defective starter motor. This test found a starter solenoid with defective contacts. starter current test starter current test bad starter solenoid contacts . In this case, the starter motor started turning the engine a little slow. Here again, here were no customer complaints, but the scope found the lurking problem. In a case like this, load testing the battery or volt dropping the circuits with a DVOM will not find a problem like this. A picture is worth 2000 words in a case like this. Now a zoom in on the starter turn on To capture this waveform, I had my right foot on the throttle to put it in clear flood. The engine was cranked for about 10 seconds, then the throttle released and the engine started. I just performed a relative compression test, a complete starter system test, battery load test, then a charging system test, all with only two scope hookups, in less than 5 minutes.

Sometimes a Tech must think out of the box to find problems and make some easy cash.

Albin from Washington

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