Alternator pulling amps at idle

Ok guys this has me stumped my 125 amp alternator on my gu td42t has started to show - 3amps at idle on my redarc gauge and only shows battery’s voltage.
When I get the revs past 1100 it starts to show positive amps and normal alternator voltage of 14.2-14.4 volts.
Before this at idle it would show 0 amps and 13.2 - 14 volts.
My Hall effect sensor is on the battery wire off the b+ post of the alternator.
what could be causing this drain at idle ?
Thanks guys done all the testa on alternator and results say alternator is good.
Do I need a new alternator ?

It should show at least a few positive amps, even at idle. Alternators are better above 1500 rpm. The B+wire is only for battery voltage, not as an output for other systems. When was the sensor connected?

The B+ post is the charging wire off the alternator to the battery.
The sensing is in the plug that excites the windings

Quote:

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Originally Posted by Patrolking

The B+ post is the charging wire off the alternator to the battery.
The sensing is in the plug that excites the windings

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Brain in neutral when typing. It should read that the b+ is only for charging the battery. Not as an auxiliary output. Usual practice is to run other systems from the Battery terminal. I have edited my post.

Are there any other dash lights on? you mention the excite circuit, so are those excite lights showing? Can you check your results on the guage with a Multimeter on the battery too simultaneously?

I've got this same redarc gauge and hall effects sensor installed.
Mine is monitoring the AUX battery and I have it fitted around the earth (negative) connection.
It still shows me 0, negative draw or positive charging as it's measuring the flow through the circuit.
I have my AMP, fridge and UHF connected to the AUX battery so there is always some draw on it at idle.
Once the revs increase around 1000rpm, the charging kicks in.
I have also fitted an after market Jaylec 125amp alternator.

The sub tank and water in fuel flashed instantaneously a couple of times yesterday morning on the way to work.No battery light.
What gets me is above 1200rpm it is making 14+ volts and + amps but as soon as the revs come off back to battery voltage and - 3-4 amps

Have pulled old alternator and done diode and continuity test and same on new alternator. Results will surprise you what was wrong. Post tomorrow with photos of tests.

New alternator fixed issue, 14.3 volts and + ve amps at idle.
Now have the auto choke / bulb check relay hum. Have batt volts at terminal 1& 2, so thinking just new relay.
Thoughts everyone.