Mysterious short circuit

[Smithyyy]

Ideas wanted please - what's going on here? I seem to have a major short circuit in the electrics around the A-frame and engines but can't work out how its happening.

The boat has twin 50hp 2-stroke Mariners, and twin batteries, wired so you can swich each engine to battery 1 or 2 via the usual rotary switches. On the A-frame are nav lights, VHF aerial, a horn, a pair of floodlights, and a couple of solar panels. The house electrics (including nav lights, floodlights and horn) are connected to the port engine switch.

I've been replacing the solar panels, and had to remove the horn to do so. I changed the floodlights at the same time - in fact the lights were done first, tested, all working fine. Then, with the solar panels still off, I started mounting the horn back onto the (aluminium) a frame. I then realised there was a lot of smoke coming from the port engine. The battery switches were both on although all other switches - ignition, lights, horn, instruments, etc - were off. So I dived for the port battery switch, as I did so the port tilt motor started running, but turning the switch off stopped that and the smoke started getting thinner. On removing the engine cowl, the tilt/trim motor relay and wires to it were burnt and half melted, obvious where the smoke had been coming from.

So there's clearly a short happening somewhere. But all switches (nav light, floodlight, horn) were off, so there was no power going to any of them on the A-frame. Solar panels were still off, wires to them disconnected at both ends. VHF aerial is not connected to any of this and had not been touched. So I assume the problem must be, not 12V from one of the A-frame items going astray, but a short to earth around the A-frame. In fact, the horn I was just remounting has a metal case, and may well have earthed the whole A-frame as I reattached it. But - and I think this is what this longish post comes down to - is why would earthing the A-frame short out the tilt/trim relay? That seems to have quite separate wiring

Or is there some other possiibility I've missed?

Any ideas please? Don't know how to fiix this until I work out what caused it!

Thanks

[OSPREY RIBS]

I would start by getting a multimeter and using the continuity setting to work out where you have a short... (start across the engine battery terminals) It might be a case of gradually disconnecting items until you work out what is causing the issue.... It may have been something in the engine that shorted when you were moving wiring around if you were dragging wires through the various conduits etc you may have disturbed something....

If you have not already disconnected the battery(ies) I would do so as a precaution.

[Smithyyy]

Thanks. I've been trying that this afternoon - problem is that it all checks out, resistances/voltage on the supply to the starboard engne and those for the port side are the same, except that the tilt motor engine on the port engine now starts up (one direction only - down is default) as soon as you connect the red wire which is the main supply to the relay, even if the blue and grey wires which feed the relay the up/down signal are disconncted. I'm putting this constant running of the one relay down to it shorting earlier. Makes it tricky to troubleshoot (although having two identical engines, I also have the option of comparing engine vs engine, or just swapping componenents between the two, and so on). But what's still bothering me - and what might provide a lead, if you'll forgive the pun, is how anything to do with the A-frame nav lights, floodlights, horn, aerial & solar panels can be affectng - to the point of burning it out - the trim relay. The wiring to that runs through the main "tube" of wiring from the console and then to the engine, and this section has bot been disturbed. The A-frame wiring goes through the same main "tube" first, then goes into the frame tubing - the only pieces movrer have been at the top of tis pieve. So the mystery is how the trim motor is being affected at all, seeing all its wiring is self-contained. Any suggestions welcome! Thanks

[Chris]

It's going to be a painful process of checking all the wiring for breaks / chaffing.

[boristhebold]

Sounds nasty and will be a pain to sort out. Normal set-up or rather recomended set up for twin engines would be 3 batteries, one 'house' battery for running electrical kit and two batteries one for each engine start with appropriate switches for each battery and a voltage regulator to sort out the charging.

Connecting stuff direct to batteries isnt a good idea, ideally should run all electrics through positive fused bus bar or breaker bar and a ground bus bar, all with appropriate fuse protection and switches where needed.

If it were me I would remove all the wiring from the battery in question, strip out all wiring to A frame completely, then sort out the engine wiring and test then draw a wiring diagram and re wire a frame stuff and use fused bus bar etc etc. Test each thing in turn before connecting the next thing.

The solar panels I dont have much experience with but take a close look at the instructions for wiring something doesn't sit right in my mind about them and how you have things connected direct to that battery.

[Fenlander]

I don't have experience with outboard wiring as such but decades of elec fault finding on vehicles and trailers.

I'd agree with those above that the most important thing is to be 100% sure *all* earths and feeds related to the A-frame are removed from the engine wiring and fault find the tilt motor on its own.

You mention in the second post that the tilt motor operates as soon as you connect the power wire to the relay even if the trigger wires are removed?? Is this a replacement relay after the other was involved in the burning wires? That aspect of the fault you're seeing sounds like an internal short in the relay or perhaps the power feed contacts fused in the closed position??

Also I wonder how your horn is/was wired. They can be powered 12v to the horn full time with the button in the return to earth, the 12v feed might be only sent to the horn when the button is pushed or it could be on a relay??

[Aquaplanning]

Best way to find a short like this is to connect a 12V lightbulb in series with your battery. If the light is bright: short present. As soon as the light goes dim or off you have found the problem.

I wish you good hunting, these things can take some time....

[Nightfisher]

Retrace all your all your initial work. Something was NOT done right on re-assembly or a wiring (Pos v Neg) was revered and just so happed the tilt was on and received the full reversal-current ... though why no fuses?? Everything's supposed to be fused at the panel and between each battery and battery selector.

[Smithyyy]

Thanks for the suggestions. To answer Fenlanders question, no this is still the same relay that burned - I'm assuming it’s now toast, with an internal short as you suggest. Replacement on its way

Boristhebold, thanks, yes I'm dealing with an inherited setup here, ie the boat came (several previous owners) as it is now. The solar panels have been great in keeping the batteries trickle charged during storage so I'm reluctant to lose them, both they and a tracker mean cables to the batteries bypassing the main breakers but they are at least fused as close to the batteries as possible, while the solar panel wiring has diode protection against current going the "wrong way" down it. I didn't know that what you describe is the preference for twin motor setups although as clearly it's a complete rebuild to be quite honest its not something I have time to undertake right now unless the problems persist and I have to - could be a good future project though! But thanks

Nightfisher, there are fuses on everything except – to Boristhebold’s point – the battery to selector switch and selector switch to starter solenoid cables. In fact there are contact breakers in the switches that control the nav lights, flood lights and horn AND fuses in the lines running to them. While as above the solar panel cables are fused close to the batteries

Fenlander, interesting thought on the horn possibly being +12V all the time with the switch earthing out the wire coming back from it, will check that out - one of the mysteries here is where the 12V came from to burn the tilt relay as everything was turned off (except the engine’s own 12v supply, but that doesn’t attach to any of the A-frame wiring) - that could be an answer. I don't think the A-frame is used as an earth by any of the equipment on it, they all have wires back to a common earth point and then the batteries -ve terminals. So if the horn wiring took the A-frame live while the horn was being refitted, maybe that could have found it way somehow to the engine cabling (I know there is no voltage now between A-frame and boat earths, I have checked that, )
And thanks to all for not telling me the batteries should have been isolated/disconnected during such electrical work, I know...