Topic: Follow-up: Engine Wiring - Seamaster 813/BMC 1.8

AndyH    -- Apr-18-2007 @ 3:59 PM
  I solved the charging problem that was the subject of my earlier post. Turned out to be a very straightforward but well hidden cause. The sense/field wire was fractured and separated from the crimp connection - even though the cable was securely held in place by the insulation of the crimp connector!

Was well chuffed to sort that out without parting with any money Playful

In my earlier posting I also asked about the panel of related electrical components mounted on the starboard side of the engine. I've now figured out most of this with one exception. There's a small cylindrical (slightly torpedo) shaped component (about 2.5in long and about 1in diameter) with three spade terminals. There no clue as to what it is and the only labelling I can see on the housing is "Do Not Drop" - helpful huh! I've tested between all three terminals with a multimeter and they all appear to be open-circuit. ANY IDEAS WHAT THIS COMPONENT IS PLEASE? The fact that I get an open-circuit reading suggests it's faulty but no obvious effect....

Finally, my appeal for a wiring diagram didn't get anywhere - not surprised. However, since solving the problem (in case it helps anyone else) it seems there's enough common information in the Thorneycroft 1500/2500 manual available at - except it doesn't answer the question above Frown

billmaxted    -- Apr-18-2007 @ 4:28 PM
  Cor You ask the hard ones when you can't see what you are talking about, at a guess it sounds like a starter relay (with a square back plate?) it could have been bypassed and replaced by another perhaps hard mounted to the hull to avoid vibration.


AndyH    -- Apr-18-2007 @ 8:05 PM
  No I don't think it's a relay - it's too light for one thing. The starter relay is mounted the other side of the alternator control unit.

Casper    -- Apr-19-2007 @ 7:20 AM

any chance of a photo of the article, we should be able to identify it for you then, someone on here will have one I am sure - and probably know what it is and does.


Alan & Gill

Antares_9    -- Apr-19-2007 @ 7:47 AM
  It may well still be a relay, some are very light and encased in an aluminium cylinder, not heavyweight enough for a starter but certainly enough for a bilge pump, stop solenoid or some domestic piece of kit long since removed. A previous owner may have wished to fit a lightweight switch or sensor that would not have carried the current required to operate it, or the resulting cable run may have caused a voltage drop that was unacceptable. Truth is , with old boats and disconnected bits you may never get to the bottom of it. If it is still connected you should be able to trace the cabling to the related components.

Meus stilus es pelagus testes

Cat philosophy:
Life was sweet for a broads ships cat

Dr-Diesel    -- Apr-19-2007 @ 11:54 AM

I doubt it is a relay unless the casing is earthed. If it is not, it will have four terminals on it, an earth which connects to the internal coil, a switched live which also connects to the internal coil. It will then have a permanent live, which will normally be a heavier cable than the switched live (otherwise you are defeating the object). And finally you should have an equally heavy wire from the relay which goes to the component it controls.

It may be a control for the heater plugs but if you can send a picture it will clear it up.


Paul H

Antares_9    -- Apr-19-2007 @ 12:09 PM
  Being mounted on the side of the engine would be earth enough, unless I misunderstood the post.

"vertically mounted metal plate on the starboard side of the engine", no, it appears I didnt

Meus stilus es pelagus testes

Cat philosophy:
Life was sweet for a broads ships cat

This message was edited by Antares_9 on Apr-19-07 @ 12:13 PM

BroadAmbition    -- Apr-19-2007 @ 4:37 PM
  I reckon it's a flasher canister for the indicators!! Evil Grin

Sorry, Sorry,  Yes I know, I'm leaving via the back door head held low in shame  Blush   -  Griff

One of these decades we WILL finish Broad Ambition !!

Antares_9    -- Apr-19-2007 @ 6:40 PM
  It does sound a lot like one Ba, especially the "do not drop" ledgend, it can't be....... can it? Smile

Meus stilus es pelagus testes

Cat philosophy:
Life was sweet for a broads ships cat

AndyH    -- Jul-7-2008 @ 9:37 PM
  In reply to Dr Diesel on 19 April 2007 (or anyone else that knows)!

>> It may be a control for the heater plugs but if you can send a picture it will clear it up.

It was a very long time ago but happened to think of taking a picture whilst crawling around under the cockpit floor on Sunday...

The component in question is the one with a red ring around it. Any ideas?


Ellaboat    -- Jul-8-2008 @ 2:04 PM

It sounds very much like the "6RA" relay unit that is paired with your "4TR" Alternator Control Unit.
Electrically, when at rest all 3 spade terminals would read open circuit to each other (but one of them would give a resistance reading to the case).
It's purpose is circuit isolation (not higher current switching) so all the cables would be the same size.

If it does turn out to be the 6RA, removing any of the spade terminals (or lifting the case off the metal mounting) would result in loss of alternator output.

The terminals are usually marked C1, C2 and W2 (or W1) and voltages to check would be as follows:-

C1 =  Constant +12 volts DC
C2 =  Switched +12 volts DC
W2 =  +12 volts DC when ignition switch is on
Case (W1) = Negative

On the other hand, the square box on the other side of the 4TR could be the relay described above (with a 4th terminal instead of the case) Smile


E/boat Steve

This message was edited by Ellaboat on Jul-8-08 @ 3:13 PM

Ellaboat    -- Jul-8-2008 @ 2:20 PM
  Another check is that the following would all be linked together:-

4TR terminal "+"
6RA terminal C2
Alternator terminal "F+"

E/boat Steve

Ellaboat    -- Jul-8-2008 @ 2:53 PM
  When you see it on paper it's a really basic circuit (see attachment)

E/boat Steve

wildfowler    -- Jul-8-2008 @ 5:27 PM
  Hi Guys, I hope I have not missed any of the posts about this very interesting thread, I would like to add my tuppence worth of knowledge if I may.
Years ago in the early days,  Alternators(10 AC 11AC) had separate regulators and separate relays.
The regulator was a 4tr unit, 4 or 5 terminals  red or black Cased.
The relay was a 6 Ra unit silver if for a car, or black water proofed for marine use.
The unit you are refering to and unable to identify is a 3AW warning light control and originally had a green label on it.
It picked up  a feed from the alternator
and through a by/metal circuit switched the ign w/light off as the voltage at the alternator terminal increased and on again when the alternator stopped rotating.
The terminals were I think marked E.  alt. and w/l ???? not too sure of this point as it was 35 years ago!!!!!
I hope this has been of help and look foward to somebody actually replying to my posting ( Make a nice change)

Jesse       M.S.A E.T  (Retired)


Ellaboat    -- Jul-8-2008 @ 5:56 PM
  Hi Jesse,
I didn't think that they bothered with the 3AW on a boat.
We certainly haven't got one, but if that's definitely what it is, I can add it to that earlier sketch.

E/boat Steve

wildfowler    -- Jul-8-2008 @ 6:24 PM
  Hi E Boat/Steve, thanks for you quick reply, much appreicated.
All the previous info was from memory, not bad when you think I started my apprenticeship in 1959!!
Can't remember things I did yesterday but ----well you get my drift.
Now, I don't want anybody thinking i'm getting too clever but Iv'e just remembered the part number for the 3 AW control !!!!! Its ----- 38706A.  Phew!!
I think I need a lie down now!!

Kind regards



Ellaboat    -- Jul-8-2008 @ 6:36 PM
  And just try to remember with the same clarity from last month. Such is life Frown

Here's a new dwg:-

E/boat Steve

wildfowler    -- Jul-8-2008 @ 7:26 PM
  Hi E/boat Steve,  Diagram attached, any good ??



Ellaboat    -- Jul-9-2008 @ 8:12 AM
  Thanks Jesse,

Info is the same with a different layout.
Trouble is, my last dwg opens up so huge that you can't see all of it unless you right click it and use "save as."

Whatever happened to AndyH who asked the question that prompted all this?

All the best,

E/boat Steve

wildfowler    -- Jul-9-2008 @ 8:44 AM
  Hi Steve, you are right, where Has Andy got to???
Perhaps we should all join hands and say the magic words:- "Are you out there Andy H" who knows it might work !!!


Ellaboat    -- Jul-23-2008 @ 8:53 PM
  Found this while trying to buy brushes for a Lucas 11AC:-

It's nice to see all these items pictured and positively identified, together with the relevant original part numbers.

E/boat Steve

AndyH    -- Jul-25-2008 @ 10:01 AM
  Firstly, thanks to everyone that has replied on this.

The explanation of the bi-metalic switch sounds very sensible to me and also explains the red warning light on the helm control panel - the purpose of which I hadn't worked out either!

Ellaboat wrote:
>Whatever happened to AndyH who asked the question
>that prompted all this?

Yes apologies for not following up - I was away on the boat Smile  for more that a week - then spent a week catching up with all sorts of things Frown

This leads me to another question I'm afraid....

I've now noticed that when the batteries are charged the ammeter, rather than ramping down to a low charging current, "pulses" between 0 and (about) 30A. I'm thinking this indicates the 4TR is faulty. Any views?


Ellaboat    -- Jul-26-2008 @ 6:41 AM

You don't have a problem.

On a healthy (but discharged) battery, the 4TR regulator will maintain a constant output from the alternator, but when the batteries have taken sufficient charge for the terminal voltage to rise above a preset level it will start pulsing as you describe.

The 4TR is just a voltage dependent switch and relies on the time it takes for the battery terminal voltage to rise and fall coupled with the time it takes for the alternator's magnetic field intensity to establish and collapse to prevent it switching far more rapidly.

This simple system works well enough (eventually) but you can get a higher charge into your batteries (and quicker too) by fitting one of the new alternator charge controllers on the market.

There's no need to disconnect the 4TR if you do fit a "smart" charge controller.
The only real issue is maintaining sufficient airflow around the alternator to keep it from overheating in a hot engine compartment (when it's working harder than before) but "smart" charge controllers take this into consideration by incorporating a rest period into the charge sequence. This rest period is important for the batteries too.

E/boat Steve

This message was edited by Ellaboat on Jul-26-08 @ 2:39 PM

The Norfolk Broads Forum :