Alternator waveform question

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Alternator waveform question

Postby fisher4x4 » Tue Jun 07, 2011 2:23 am

I was just wondering if anybody could help me with a capture set up i tried today. I captured an alternator pattern, blue channel is ac voltage tested at alternator stud. Red channel is amperage taken from the charge wire to the battery. The vehicle is a 2007 ford f-150 witch will kill the battery intermittently about every two weeks. I believe it may have an intermittent draw, but want to verify charging system before i move on. I think i should have used a filter setting on the scope to remove unwanted interference, but i'm not sure what setting to use. Any help would be appreciated.
2007 f150 alternator
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Re: Alternator waveform question

Postby Alan » Wed Jun 08, 2011 6:35 am


For testing the alternator you will probably have an easier job if you use PicoDiagnostics and use the battery test option. This will test the battery, alternator and starter motor in one go.

For your waveform you have got some fairly large spikes so it might be worth checking you have a good ground connection and the wires are well away from injectors and ignition components. The spikes however are not from the alternator so as you suggest can be filtered out - try a 10kHz filter on channel A.

Channel A in your test is AC coupled so shows the alternator does not have any blown diodes. Channel B shows a current of about 25A. If the engine was running and this is a charging current then again this may be OK.

I suggest you next either use PicoDiagnostics or use the preset for testing Current / Volts / Starting (12V). This will show the battery DC voltage before, during and after starting. It will also show the peak current draw. Once you have saved this test it might also be worth keeping the same settings, switch off the car and capture again with a much longer timebase - say 200 sec/div so you can see if anything is still drawing current after the engine has been switched off.

With this waveform and the one you have captured we should have a clear idea of the problem.
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