Lewmar Bow thruster control circuit (2024)

There are 5 wires come out of the back of the Lewmar 589001 touch panel controller.
Single black wire, not used in my set-up
4 wires into a connector, black 0v, red 12v, blue port and grey starboard.
These 4 wires are routed to a "black box" mounted on the thruster, which activate the main contactors.
I'm trying to understand what's in the black box, I guess it must be 2 relays with diodes to minimise arcing.
I can't find anything in the Lewmar manuals online.
Does anyone have circuit diagram of the black box, it’s not as simple as it seems at first sight, the system is working on negative switching, so the main terminals on the thruster are live all the time and the 0 volt side of the main contactors is connected to the ground to power the thruster.
The electrical motor is serially wound, and the motor is reversed by swapping the polarity of either the rotor or the stator, it’s not clear which from what I can see. But this doesn’t matter to get it to work.
The thruster isn't working, and I'm trying to work out a "real" circuit diagram of the black box, the ones online just show a "black a box".

Appologies for the very rough sketch, I'm working on the boat, at anchor waiting for the wind to moderate and settle down, but bouncing around a bit, NW Scotland.

Offhand I can't understand the need for 4 wires unless there is an indicator lamp on the control. All that is needed is the live wire connected to the central pin of a two-way switch, spring loaded to central off. Pressed to the left, it should send the positive voltage to the contactor relay that activates the thruster to turn the bow to port and vice versa for pressing to the right. Very little current is needed to activate the relays. Also no negative connection is needed as at the thruster end of the system the heavy cables for the thruster will supply the negative.

My own system is of rather low power and does away with relays as far as I can see, at least I have not found any. The relays in the compartment are for the anchor windlass. I have had no problems with the thruster, but I have had problems with the windlass, many of which have been traced to bad contacts in the link between the remote control and the relays.

Bodach na mara said:

Offhand I can't understand the need for 4 wires unless there is an indicator lamp on the control.

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Thruster controllers generally have an indicator LED, and some sort of timer circuit to turn them off after a period of inactivity. Hence the need for positive and negative connections.

Ian_Edwards said:

I'm trying to understand what's in the black box, I guess it must be 2 relays with diodes to minimise arcing.

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Thrusters usually have timer circuitry to prevent rapid reversals of thrust, that's probably what's in the box.

Ian_Edwards said:

Hi pvb,
We've been through this before, all the control for this version of the Lewmar Thruster is in the co*ckpit touch panel control.
The co*ckpit control panel is powered from the thruster, but has it's own on-off switch, and logic to prevent both button being pressed at once, a delay to prevent rapid switching between port and starboard, a maximum operational time of 3mins and an auto shut down of after 20min of in activity.
I can probably work out the circuit diagram of the black box, it can't be complex, given time; I'm just making up a test harness. I was just hoping that someone else had worked out, these units are both notoriously unreliable and expensive.

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Which version do you have?

Ian_Edwards said:

185 5.0 kW-12 V although it's a 4.5kW motor Lewmar part number 591803
The black box is a Lewmar part number 559003
The co*ckpit controller is a Lewmar part number 589001, which I believe is no longer made.

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From one of your previous posts, it seems you're aware of the "problem solver" PDF file which Lewmar published. This should enable you to determine whether it's the controller, the black box or the motor which is at fault.

If it's the controller, fit a different make; Lewmar controllers seem to be fragile, the controller is only a switch, so isn't brand-specific.

If it's the black box, why not take it apart and see what's inside? You'll no doubt need to buy a new one anyway, so attacking the old one isn't a problem.

If it's the motor, well, you'll need a new motor.

I spent the morning working on understanding the bow thruster circuit, working on the saloon floor.
I brought a bench power supply with me, because I though it would be useful to have controllable power soucre with a current limiter.

I have 2 black boxes from previous skirmishes with the bow thruster, I wasn’t sure if either of them work. I decided to work out a truth table, all the input against the output. I used the bench power supply to provide a nominal 12 volts, and a multi meter to measure the outputs.

There are 8 possible combinations of input/output. Six of those show about 0.5 volts, which I guess is the reverse bias of the diodes wired in parallel with the relays. 2 outputs are 0 volts, indicating that the relay has closed and pulled the output down to 0v. The 2 truth tables for the 2 black boxes I have are identical, which tells me that both black boxes probably work.

The next objective was to work out a schematic, I found this quite challenging to draw. The circuit consists of 3 sets of nested relays, although the I think that the co*ckpit switching is done with electronics. Take care when looking at the sketch, the centre section, the black box has the 0volts at the top, and the motors is only shown as a token on each leg of the contactor. The motor is serial would, so more complex than I’ve drawn.

Armed with this tenative knowlege, I'll try replacing the black box this afternoon, assuming that it stops raining.
The usual warnings apply, drawn by an enthusiastic amateur, and may well be wrong!

I spent much of yesterday afternoon fitting a replacement “black box”.
I first need to remove the anchor and 80m of 10mm chain, the bow thruster is housed below the anchor locker, then open the manhole hatch and lower myself down, well over 2m from deck level, swapping the box isn’t difficult, just difficult to access. To add to the joy of the task I was joined by some west coast midges looking for a meal.

After cleaning up some corrosion on the connector between the co*ckpit controller and the black box, the bow thruster now works, and I have a much better understanding of the circuit, to the point where I’m confident I could replace the black box with two small relays (they are protected by a 5amp fuse) and two diodes.

Close Out.
I've just tested the black box I removed yesterday.
This is the truth table:

The reverse bias for the diodes is all over the place, generally when I measure with the DVM, the voltage is unstable and increasing. However, when the relays should be close (0.0v) they are closed. This indicates that the relays are OK, but the diodes have failed, the failed diodes explain why the fuse was warm and a mysterious 2amp load on the 12v system I couldn't trace.
I just wanted to check that I hadn't taken out a perfectly servicable black box.
I'll take the faulty black box home with me and hit it with a lump hammer to see what is inside
I hope some will find this write-up useful in the future.

Lewmar Bow Thruster - Black Box – post-mortem
Now that the boat is safely tucked away for winter, I decided to have look inside the Black Box which is located between the co*ckpit control and the thruster motor. Lewmar don’t say what’s in the Black Box, and I’d assumed that it was a couple of relays.
This afternoon I took a known faulty Black Box and decided to explore what’s in it.
My first attempt to break into this encapsulated lump was with a 1kg lump hammer, I’d assumed it was encapsulated in Epoxy. Repeatedly hitting it with the lump hammer on a large block of very dense hardwood had absolutely no effect. The hammer just bounced! The potting is obviously some form of potting compound, not epoxy.
The next approach was with a mortis chisel and mallet, that made some impression, but it was impossible to split the box open as I’d hoped. I then tried hacksaw and cut into the sides of the Black Box. I hit something solid with a few mm of the edge, but no sign of the bock splitting.
Final I used a sharp chisel and mallet to cut way the potting compound, in the same way you’d carve a lump of wood.
After a bit a whittling away, I discovered a couple of capacitors and eventually a semiconductor.
The semiconductor is a Z44NS, an N channel MOSFET switch.
https://www.infineon.com/dgdl/irfz44nspbf.pdf?fileId=5546d462533600a40153563b43a5220f
I haven’t fully uncovered the circuit board, I might have another got tomorrow, I’d like to see if there is any form of logic on the PCB, which could introduce time delays.
So, the mark 2 Lewmar Black Box is essentially a semiconductor switch which drives the contractors on the DC thruster motor.