Two relays too many?

[Skip 52]

I have a very strange (or at least, so it seems to me) wiring system fitted to the Nanni diesel/ Lewmar bow thruster setup in my 2004 Beneteau motor boat. There are two relays. The first of these is literally hanging off its wires toward the back of the engine. This first relay's activator is connected to the main starter motor +ve cable and to one of the terminals on the oil pressure sender, which seems to be connected to ground (this is what really puzzles me, why connect it to the oil pressure sender?). Thus, immediately when the power is on, this activates the switch… which connects a cable from the main ground on the engine… to post 1 of the activator on the second relay. I'm not impressed with the wire used here, by the way - it's a normal AC flex with the other two wires, the brown and the yellow/green, simply snipped off. So it's not exactly a highly professional job!

The second relay is on the bulkhead in front of the engine, about six feet away from the first one. It takes a +ve feed from the bow thruster fuse (via a smaller inline fuse) to both the activator (post 2) and the switch (post 5). Which then feeds a +ve current to two other wires which disappear into a harness Thus this second relay feeds +ve current immediately the power is on.

To sum it up, power on = both relays activated.

So, why have two relays at all? Surely I would get the same effect by simply connecting the +ve feed from the bow thruster fuse directly to the two other wires into the harness - unless I am missing something that the oil pressure sender is, or should be doing? The oil pressure gauge works fine.

The only reason I can think of for these relays is that it was a failed attempt to stop the bow thruster from being operated when the engine isn't running?

I am in no way an electrician and, although I hate to see a relay hanging off the back of the engine from a domestic cable, I am reluctant to mess with it in case it's simply a matter of my not understanding. Any suggestions or advice would be most gratefully received!

 

[William_H]

I can't really suggest help without any idea what the relays power or don't power.
You probably understand but a relay can be used to control a large current witha small current. ie multiply the current capability of a switch.
A relay can also be used to alternate the sense of a switch. So an oil pressure switch makes a contact when oil pressure is not present (light on) and opens when engine is running. This can control a relay where the relay operates with no oil pressure but releases with pressure. the relay probably has contacts which make in operated position but also contacts which break in the operated position. So the relay might be used to activate something when oil pressure appears ie anchor winch or domestic battery charging.
Now if you know all this then you will have start digging. You really should unerstand what is meant to happen. If you didn't know about relays this may help. (or at least bump the post) good luck olewill

 

[coopec]

I find it very hard to follow without a circuit diagram as I think a lot of us would. Any chance of one?
Cheers

 

[coopec]

William_H

DON'T GO AWAY! Please help!

(Originally posted under thread "Instrumentation")

I've decided to install 7 small flashing red LEDs for the alarms (5 taken plus two spare) in the instrument binnacle. To do a neat job I am considering using "10 Ways COLORFUL Flat Ribbon Cable " as you wouldn't notice that behind the headlining material. Being a novice in regard to electronics I ask the experts out there if that is acceptable.

 

[Skip 52]

I find it very hard to follow without a circuit diagram as I think a lot of us would. Any chance of one?

Thanks for coming back to me. Here we go...



Electrically speaking, is there any difference in effect between the two circuits?

 

[Dave_Seager]

I can suggest two possible uses for this arrangement. I think that it is using the oil pressure switch to prevent some circuit from operating until the pressure has built up.

It may be intended to prevent the engine from firing until it has cranked enough to build up some oil pressure. Alternatively, it may connect the domestic and starter batteries once the engine is running enough to provide oil pressure and will disconnect when the engine stops.

For either of these theories, the wire from the first relay to the second would need to be taken from the normally closed terminal, not the normally open as shown. Then the second relay would not be energised until the oil pressure had risen enough to extinguish the light.

If the relay is hanging on the wiring it sounds like a bodge. Most automotive relays have slots in the plastic cover to fit onto a metal bracket. Has it possibly fallen of its bracket?

 

[Skip 52]

That makes a lot of sense, Dave. Yes, it does look like a bodge (note the type of wire) and no, there is no bracket for the relay to have fallen off, it simply dangles.



The relay has, in fact, become disconnected from terminal 5 a number of times - space is a bit cramped down there and it is very vulnerable to being knocked. So it is possible that the wire attached to terminal 5 was originally attached to terminal 4. Would that disconnect when the oil pressure goes up? If so, that would explain it. The feed from the second relay eventually goes to the bow thruster control and this would prevent it from working when the engine was not running. Perhaps I should have marked terminal 3 on the first relay as being -ve, rather than ground, like this altered version of the circuit diagram?



Would this have the desired effect?

If so then, rather than regarding this as a bodge without any apparent purpose, I shall rewire it properly with tinned cable and fasten the relay safely out of harms way!