Electriacal ground problems

[Bevdent]

I have a steel boat fitted with two 24v battery banks. With the domestic batteries disconnected and the engine batteries connected and the ignition switch off, the panel lights come on and the temperature gauge and rev. counter dials flip to the top whilst the oil pressure gauge on the fly-bridge panel does likewise. When the engine batteries are disconnected and the domestic batteries are connected, all appears to be OK. until the nav. lights are switched on when exactly the same thing happens to the the dials. I have tested the domestic circuit in the usual way and all seems to be in order. All systems are working and none show a break when tested with an ohm meter (except the cabin lighting which I don't really think has any bearing here) Can anyone tell me where to start looking? The fault appe eared after having the starter motor overhauled by a qualified mechanic but we were doing other jobs on the boat so that may not be relevant.

 

[neilf39]

Difficult to say without more info on your wiring but one way I could imagine it is if the negative from your nav lights could be broken/disconnected from earth and shorting across the ignition switch. Or it could be the positive cable from the nav light switch going to the nav lights is broken and shorting across the ignition switch. That way with the engine batteries connected the panel is powered via the short across the ignition switch with negative return via the power panel negative. When the nav lights are on the domestic is then providing power via the short and again return to ground via the power panel negative. if you disconnect all the battery positives then with the ohmmeter across the battery positives do you get very low resistance when the nav light switch is on?

Sounds a bit weird but I guess it depends where/how your wiring is all run.

 

[john_q]

A fellow Steel boat owner, Just a thought,

Does your vessel have a negative floating DC supply, e.g. negative not connected to Salt water, (The negatives on the motor go to the battery and the motor is NOT earthed through the block)

If it has, then maybe this was compromised when the starter motor was worked on

 

[Bevdent]

Thanks Neil. Yes I am getting a reading of 1500 - 2000 when the nav. Lights are switched on. I am also getting a reading of 0.5 volts across the positive terminals.