Join Date: Dec 2004
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alternator test - diesel engine
Pulling the cable off your battery will blow your alternator if it is working.The current has to go somewhere,even jumping it and running on a dead battery can damage it.If you need to check the alternator, all you need is a voltmeter. Make sure you have a good battery. Start the engine and set the voltmeter on DC voltage. Take the red lead of the voltmeter and put it on the large red wire at the alternator. Put the black lead on the alternator housing. With the engine at high idle, you should have a reading of 14.2 - 14.6 volts. Take the voltmeter to the battery and place the red lead on positive and the black on negative. You should be reading 14.2 - 14.6 volts. If not, you have a wiring problem to the battery. If you do have 14.2 - 14.6 volts and you still have problems, test the battery.Also remember that the diodes in the alternator can flow backwards when they malfunction,draining it when it isn`t running.