How to diagnose a fault in the charging system

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banner001
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Track day God
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Joined: Sat Sep 24, 2011 3:27 pm
My Bike: ZXR400 L4
Location: Nuneaton

How to diagnose a fault in the charging system

Post by banner001 »

There are 4 main causes of this happening (most likely first):
1. Your battery is dead
2. Your Reg/Rec is dead
3. Your alternator is dead
4. All components work but there is a wiring fault

How to check your battery:
If the two tests above all come back fine, i.e. your alternator and reg/rec are all outputting correctly then the likely culprit is that your battery is old and is no longer holding a charge correctly. If you have an alarm or some other "off-ignition" drain this could be causing your battery to lose a charge overnight, or if you have a short circuit somewhere this can also cause you to pull a constant current.

You can check for this in two ways, first disconnect the terminals so the battery is isolated, allow the bike to stand for an hour, measure the voltage, then measure the voltage again after 24Hrs of not touching the bike, this will show if your battery is holding a charge or not (there should be almost no change in the voltage). Next connect the battery back to the terminals, measure the voltage and then measure it after 24Hrs, this will look for off-ignition drains on your battery.

How to check your Reg/Rec:
Plug the alternator back in, turn on the headlights and run the bike at 4000rpm, check the voltage in DC across the battery terminals, it should be 14.5V, anything less than 13.5 or more than 15V indicates that you have a problem with your reg/rec. This test also confirms that you dont have any wiring faults/breaks in the charging system.

How to check your Alternator:
Unplug the black connector going from your alternator to your wiring loom, there will be 3 pins (number them 1 to 3 in your head), you need to check the AC voltage across the 3 possible pairs of pins (i.e. pins 1 and 2, pins 1 and 3, pins 2 and 3 - polarity doesnt matter as its AC). Run the bike at 4000rpm, the AC voltage across each of the 3 sets of pins should be 43-45V, if its around this voltage then your alternator is good, if it's less than 40V or more than 50V you potentially have a problem with your alternator. Next with bike turned off check your continuities, the 3 pins from the alternator should all have continuity to themselves (a low resistance of 2-8 ohm), and none should have continuity to ground (i.e. the bike frame, the resistance should be infinity) a deviation from any of these indicates a fault with the alternator.

If the battery, reg/rec and alternator all check out fine, but the bike still refuses to hold a charge when running its likely that you have an intermittent break in your wiring that is affected by the bikes movement, or that your battery/one of your charging components is breaking down under load as it gets hot. If in doubt change the battery and see if the problem persists.
UK ZXR400 L3 (1993) - Fully restored and on the roads, my green beast!
JPN ZXR250 A2 (1990) - Revs to 19,200rpm... 'nuff said :smt003
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