Stern thruster runs spontaneously!

[trialframe]

We were in our Nimbus 340 moored in the marina when the stern thruster suddenly started running. It took me a few moments to realise what was happening and then I was able to stop it by switching off the 'side power' switch on the panel. Now when I switch this back on usually the stern thruster runs spontaneously - but sometimes it doesn't. This is leading me to think that there is a fault in a control box somewhere because the switch that tells the thruster to go right or left only has thin wires supplying it. Presumably these thin wires go to a control box where there may be a solenoid or similar high current switch which actuates the thruster motor right & left.
It is a Sleipner stern thruster. I need to know where to look for the offending control box because it is very difficult to trace the thin wires away from the control switch. Might the control box be built into the actual thruster or would it possibly be a separate part mounted near the stern thruster fuse? Any help with this would be gratefully accepted.
By the way the switch that actuates the thruster to go right or left is a conventional one and not a touch panel so it is unlikely to be faulty.

 

[tinkicker0]

Look to the signal side first (thin wires = low amps) and the bits connected to it. Sounds like it is receiving a spurious signal.

 

[volvopaul]

Joystick probably corroded or has water in it causing a contact to be made across the terminals.

 

[rosssavage]

Generally, the high current switching is done by a pair of contactors mounted in a box on the side of the thruster itself. Not sure which model thruster you have, but being on a 34' boat almost certainly as described above.

Unsual to have spurious uncommanded operation of the contactors due their heavy physical construction, more likely to be an erroneous command from the switch or a low current wiring fault.

Start by disconnecting the low current wiring from the thruster (should be a multiplug near the thruster somewhere..) and see if that stops it. Yes = low current switch / wire fault. No = contactor fault.

 

[DAVIDO]

Speaking with some recent knowledge and a background in switchgear manufacturing. A friend recently had exactly the same problem his bow thruster came on in the middle of the night and could only be turned off with the isolator.
This was a Sleipner unit on an F33. On investigation there was no spurious signals from or too either joystick what had happened was that one of the pole pieces in the forward/reverse contactor had snapped which allowed one of the moving contacts to close therby actuating the thruster.
I fixed it by obtaining the service kit for the contactor and all was well.
The contactor is an Albright unit which you will find on most thrusters but suitable service kits can be bought from Shaltbau Machine Elecrtrics (01633 877555) in Cwmbran.

 

[rosssavage]

OP said: "It took me a few moments to realise what was happening and then I was able to stop it by switching off the 'side power' switch on the panel".

Unlikely to be a snapped pole piece or contactor fault if switching off the control side stopped it.

I do, of course, stand to be corrected!