Starter motor wiring

[jimmyk]

It all works but still trying to make sense of it all. I'm making a wiring diagram of the boat. I see on mine that big thick cable runs directly from the starter motor to the battery switch. It's a 1,2 both switch. A thick negative cable runs from the back of the starter solenoid. And earth cables from both batteries join it on a big earth bolt/bus.
Is this correct? It works and has done for many years but I can't make sense of it because I'm a bit thick probably. The engine is an old petter AV
Thanks for any replies

 

[seaangler23]

Possibly, the starter doesn't need directly earthed as long as the engine is, it may have been wired this way. There will be a feed to the solenoid from the start switch too. My old petter is positive earth which is great for screwing with my head when i start thinking about the wiring

 

[jimmyk]

Possibly, the starter doesn't need directly earthed as long as the engine is, it may have been wired this way. There will be a feed to the solenoid from the start switch too. My old petter is positive earth which is great for screwing with my head when i start thinking about the wiring

Yes it looks like it's always been that way. I just thought that the earth came from the starter and the live from the solenoid. It looks the wrong way round to me. But I really don't know an awful lot about it

 

[seaangler23]

If your possitive runs from the battery switch to the body of the starter and a neg to the solenoid terminal it's running a positive earth, can check with a multimeter pretty quickly

 

[VicS]

It all works but still trying to make sense of it all. I'm making a wiring diagram of the boat. I see on mine that big thick cable runs directly from the starter motor to the battery switch. It's a 1,2 both switch. A thick negative cable runs from the back of the starter solenoid. And earth cables from both batteries join it on a big earth bolt/bus.
Is this correct? It works and has done for many years but I can't make sense of it because I'm a bit thick probably. The engine is an old petter AV
Thanks for any replies

All makes sense.

Heavy positive from battery switch to solenoid.
Common heavy negative

The smaller control wire from the starter switch to the solenoid, which Seangler mentions


Also what you have not noticed, and may not even be visible externally, is a heavy positive connection from the solenoid to the starter motor.

 

[jimmyk]

If your possitive runs from the battery switch to the body of the starter and a neg to the solenoid terminal it's running a positive earth, can check with a multimeter pretty quickly

That's what I thought but don't think it is. The alternator and regulater (Lucas 4tr). Are negative earth I think. I did wonder if the engine was built as positive earth then converted when installed and the leads on the starter turned round. Or is that a silly thing to say. I've been looking on google but a little knowledge is a dangerous thing

 

[jimmyk]

All makes sense.

Heavy positive from battery switch to solenoid.
Common heavy negative

The smaller control wire from the starter switch to the solenoid, which Seangler mentions


Also what you have not noticed, and may not even be visible externally, is a heavy positive connection from the solenoid to the starter motor.

The heavy battery switch wiring goes to the starter and not the solenoid though. On the switch I have engine battery on one. House battery on two. Then the main house cables going to fuse box and starter cable on the common. I think I should just ask a proper electrician to have a look. Too much for my tiny mind

 

[Lon nan Gruagach]

Starter motors dont use magnets, they have both stator and rotor coils. The upshot of this is that they will run the same direction regardless of supply polarity. The solenoid is also insensitive to polarity, so the solenoid/starter pair will function correctly regardless of -ve or +ve earth wiring. So, in your case connecting the solenoid control wire to +ve will energise it which will then connect the -ve to the starter, which already has +ve connected.

Edit add:
This is non standard wiring though, and risks burning out the solenoid on long crank times.
Since, once the solenoid is engaged, the hold current is very small compared to that needed to energise it,it is normal to connect +ve to the solenoid and -ve to the starter.
Does your start wire in fact connect to -ve? (this would have the same effect)

 

[VicS]

The heavy battery switch wiring goes to the starter and not the solenoid though. On the switch I have engine battery on one. House battery on two. Then the main house cables going to fuse box and starter cable on the common. I think I should just ask a proper electrician to have a look. Too much for my tiny mind

Draw a diagram of it and post the diagram. I am sure all will then become clear.
Just a hand drawn diagram and a photo of it should be adequate if you take a bit of care with it.

I am not sure at the moment if you are describing a positive earth system as Seangler suspected at first or if you merely have the battery isolation and selection switch in the negative supply.

 

[seaangler23]

I have a lucas positive earth alternator and the regulator is a lucas as well, when I researched the only part that is different to a neg earth system is the alternator (must be something to do with the diodes) it looks the same. My alt has just died so I will be rewriting it to neg earth.
Take a multimeter, set to vdc, red lead to battery cable where it terminates on the solenoid and black lead to the block, if you get a - reading it's positive earth.

 

[jimmyk]

X

I have a lucas positive earth alternator and the regulator is a lucas as well, when I researched the only part that is different to a neg earth system is the alternator (must be something to do with the diodes) it looks the same. My alt has just died so I will be rewriting it to neg earth.
Take a multimeter, set to vdc, red lead to battery cable where it terminates on the solenoid and black lead to the block, if you get a - reading it's positive earth.

Thanks. Will do

 

[jimmyk]

Draw a diagram of it and post the diagram. I am sure all will then become clear.
Just a hand drawn diagram and a photo of it should be adequate if you take a bit of care with it.

I am not sure at the moment if you are describing a positive earth system as Seangler suspected at first or if you merely have the battery isolation and selection switch in the negative supply.

Here is my rather crude drawing

 

[VicS]

Here is my rather crude drawingView attachment 54797

Draw a diagram of it and post the diagram. I am sure all will then become clear.

Perhaps not so clear after all.

 

[jimmyk]

Perhaps not so clear after all.

I know. Unless I have done something stupid. Which is not beyond me. And have not looked properly. The earth definitely comes off the solenoid though. Its the thick wire to the common on the switch I cant get my head around. I am assuming it comes off the starter but cannot really see underneath where it is. It does all work at least

 

[LadyInBed]

Download a copy of 12v handbook -
www.Montymariner.co.uk/downloads

 

[VicS]

I know. Unless I have done something stupid. Which is not beyond me. And have not looked properly. The earth definitely comes off the solenoid though. Its the thick wire to the common on the switch I cant get my head around. I am assuming it comes off the starter but cannot really see underneath where it is. It does all work at least

I looks from your diagram as though your house electrics are fed via the selector/ isolator switch from battery +ve. ie are negative earthed asi s uaually the case

but the engine appears to be positive "earthed" ??????

If that is so it may well all work satisfactorily but could be a recipe for disaster if one was not aware of it.


I would have expected the positive and negative connections to the starter and solenoid to be the other way round, i.e. +ve to solenoid terminal , negative to starter motor casing and engine block.


I have to accept your diagram ........ but if I had access to the boat I sure would check it!

 

[seaangler23]

Looks like pos earth then, I wired up my house electrics neg earth on a separate fuse/ distribution box as well as I'm the only one who touches the boat it's fine and it's always been the plan to change the wiring on the engine.
It was interesting when I first put a battery in it when I got it, luckily my old man was there who our me right, I'd never heard of it

 

[jimmyk]

I looks from your diagram as though your house electrics are fed via the selector/ isolator switch from battery +ve. ie are negative earthed asi s uaually the case

but the engine appears to be positive "earthed" ??????

If that is so it may well all work satisfactorily but could be a recipe for disaster if one was not aware of it.


I would have expected the positive and negative connections to the starter and solenoid to be the other way round, i.e. +ve to solenoid terminal , negative to starter motor casing and engine block.


I have to accept your diagram ........ but if I had access to the boat I sure would check it!

I know exactly what you mean and it has confused me totally. The solenoid wire definitely goes to earth. It has worked for the last 50 years but i really dont understand how. As seangler says it looks positive earth. but looks negative earth too!

 

[VicS]

it's always been the plan to change the wiring on the engine.

But as you have alraedy noted requires a different alternator or some major alterations to te existing one.

Actually an option that may be open to you is to get your defective alternator repaired and converted to negative earth operation at the same time. Perhaps that's your intention anyway

 

[jimmyk]

But as you have alraedy noted requires a different alternator or some major alterations to te existing one.

Actually an option that may be open to you is to get your defective alternator repaired and converted to negative earth operation at the same time. Perhaps that's your intention anyway

Yes the whole thing needs sorting. I just wanted to understand it. I think I will have to learn to live without the understanding. It will be like being married