Battery light

[Trevor]

Got a problem here I'm trying to troubleshoot. On my Defender the battery light on the dash is illuminating (brightly) when the vehicle is off. Here's a bit of background.

- went out to start it this morning, it was dead.
- boosted it, got it started, then noticed I had left the rear interior light on several days ago...hence the dead battery
- charge is showing as very good, drove it into the office, shut it off and started it again, no problem
- it was then that I noticed the battery light was on.

Since this is happening the only thing i can think of is that the diodes are shot in the alternator and the charge is working back via the alternator when it is turned off...but I'm not sure about that even because shouldn't I be seeing the battery light illuminated when the vehicle is running still if the diodes were gone? Or perhaps a broken wire off the alternator causing a short somewhere...er, but again, same thing as before, wouldn't I see that battery light illuminated when it was running in that case too?

Any thoughts on this one folks?

[Red90]

The light is on with the ignition off?  If so, you will need to troubleshoot as that is not normal.  It should come on with the ignition before the alternator is working..

Here is the 300TDI wiring schematic (yours is a 300TDI, right?):
http://alre.club/Manuals/Defender%20300%20tdi%20-%20Wiring%20Schematics.pdf

See page 4.1  The light should only be able to get power with the ignition switched on.  The normal way it works...  When you turn on the ignition, it supplys 12V to one side of the light.  The other side goes to the voltage regulator on the alternator.  When it is not producing power, it goes to ground and the light comes on.  Once the alternator starts to produce power, you get 12V at the voltage regulator and the light goes out.  If you have a bad diode, the alternator produces a AC power and the light will light up a bit as it goes to ground each revolution.  There is a diode to help reduce the AC signal getting to the vehicle.  With the ignition off, nothing should happen.  If that is happening, you will need to trace the power as there must be a strange fault.

[binch]

sounds like a bad ground or failed switch.   But it's electrickery and not my friend >:(

[Trevor]

Ok, that diagram does help, and your explanation does make sense John. Regarding the diode failure, the fact that I'm seeing nothing at all when the vehicle is running does seem to rule that out. Could this simply be a fault in the ignition switch, wherein it is letting the 12v through without the swithc on...with the other side of the switch going to the voltage regulator (which wouldn't be seeing power then, right?), that would cause the light to illuminate if I understand this chart properly.

One thing I do have in the system that is not covered by the wiring diagram is a marine grade battery isolator. But I'm not seeing how it could be a factor here.

The ignition switch seems to be the gatekeeper here for current to that light. If its working properly, the light should not be able to illuminate when the vehicle is off, right?

[Matt H]

My money is on the diode trio/rectifier bridge in your alternator. They can "leak"!

I've seen this quite a few times. It can happen when a vehicle is boosted with a very flat battery. Voltage spikes can bias the diodes to allow current flow in both directions. This has a less dramatic effect on older Diesel engines than it would on a spark ign engine but the result is the same. The Battery is discharging all the time through the alt.

In fact, I just did the exact same thing to myself recently boosting my JD tractor!  ::)

You can often smell a burning wire smell at the alt and if you charge the battery off the vehicle, when you reconnect the terminals the ground wire arcs brightly as soon as you get close to the post.

You can test the rectifier bridge with a ohm meter. Dissconnect the rectifier bridge and test each diode separately. It should allow current flow in one direction and not in the other with the test leads reversed.

My 2c.

[Trevor]

Great info Matt, thanks. I gotta get the alternator out of there and do some testing, clearly it's a candidate here. I didn't realize a boost would put those diodes at that much risk. I'm guessing the best way to avoid that risk (short of not letting the battery go flat to start with) is to use a trickle charge if possible to recover the battery rather than doing a straight across boost?

[Matt H]

Electrical current will always follow the path of least resistance. So all you need to do is give it a 2nd  path to ground though a more robust or current protected circuit when boosting.

If you turn on your headlights before you give the car a boost you give that 2nd path to ground and as headlights are protected by a self resetting type 1 circuit breaker. This is actually a very good practice in modern Computer controlled cars as voltage spikes can cause havoc with the ECM.

[Red90]

Where I was going is that the failure is not obvious.  You need to get a test light or volt meter and find the source of the power.

1) Go to the alternator and check for power at the brown/yellow wire.  If you have power there, then both the alternator and the external diode (G126) have failed.

2) Check for power at the white terminal on the ignition switch.  If you have power there, then you need to keep hunting.  Could be the switch or power back feeding from something else that has failed.  At that point you need to start isolating the fault as the various switched power items can cross feed through the switch.

[Trevor]

Good points John, I will do some investigating with the vm.