strange 12v wiring fault - advice?

[chris in SG]

I'm posting this more out of curiosity than need for solution. Some 12v experts might understand immediately but for me it's just strange..

Cabin lights and fresh water pumps on separate circuits (obviously).

When water pump cycles, LED cabin lights get significantly BRIGHTER, although regular brightness seems adequate/normal.

This is not a battery voltage issue.

Any ideas?

[GordMay]

I cannot answer your specific question.

But, FWIW:

A loose/faulty neutral can cause lights to get brighter, when a heavier load is turned on, on a 120/240VAC system, due to an unbalanced voltage.
The lights get brighter [or dimmer], according to the balance of loads between the two 120V [red and black] legs. When the load on one leg is greater than another, the voltage drop causes lights to dim. At the same time, the lights on the other leg get brighter, because the effective voltage is greater than 120 volts.

[glenn.225]

In my experience, almost all problems like this are caused by wiring problems; bad grounds, poor connections, corrosion in wiring, broken strands in multi-strand wire, or most likely in this case one circuit grounded through another.
LED's requires very little current and could be getting ground through the pump, but the wiring is not supplying enough current to operate the pump.
I've seen lots of cases where there is enough current getting through corroded connections to show voltage on a multimeter but not light an incandescent taillight bulb.

Sent from my SM-A536W using Cruisers Sailing Forum mobile app

[Tin Tin]

Similar happens on boat trailers, indicators light up when brake is pressed - usually a ground fault.