BMC 1.8 Wiring Diagram or Pre-made loom needed

[darbon]

Hi all,

 

I'm putting a BMC 1.8 into my boat, the engine was bought without any wiring and the boat is a shell.

 

I've been looking everywhere for a basic wiring diagram for connecting starter, alternator, ignition, to the battery and required cable sizes. Can't find anything for BMC or anybody who sells pre-made wiring loom.

 

Can anyone point me in the right direction please?

 

Dan

[RLWP]

I doubt such a thing exists. It's something I'm going to have to do for Tawny Owl soon

 

Richard

[Tony Brooks]

The problem is that all the critical wires will need their size calculated to avoid excess voltdrop and for that you need the length. On NO ACCOUNT take any notice of the current capacity stated on the drum or in a catalogue. Work out the size for less than 0.5V voltdrop on most and 0.3 voltdrop (0.25 is better)for the starting, glowplug, and charging circuits.

 

Generic diagrams for individual circuits on www.tb-training.co.uk in the electrical notes. It may well pay you to spend some time studying the whole lot before you start on this project.

 

Do not bundle the wires into a loom using insulating tape. It will make the whole thing as stiff as a board. Use non-adhesive PVC looming tape or spiral binding.

[RLWP]

Ah yes, your generic wiring diagrams are an excellent place to start Tony

 

Richard

[RobinJ]

Do have some loom info for 1.5, based on AMC setup with ISS instruments?

I found it easier to start from scratch and calculate lengths, sizes and connections, based on location, and power required, especially as I was fitting different oil pressure and alternator etc,!

[RLWP]

Do you remember how many different wire sizes you ended up with? Ignoring the starter cables, I think you generally end up with three different sizes

 

Richard

[Bod]

.........

Do not bundle the wires into a loom using insulating tape. It will make the whole thing as stiff as a board. Use non-adhesive PVC looming tape or spiral binding.

Great stuff spiral binding........it can be removed......and reused, wires can be taken through the sides.

 

Bod

[RLWP]

I prefer the corrugated sleeving myself, each to their own though. Anything's better than black tape which in a engine bay somehow manages to fail to stick to itself, but still manages to stick to everything else

 

Richard

[Tony Brooks]

Do you remember how many different wire sizes you ended up with? Ignoring the starter cables, I think you generally end up with three different sizes

 

Richard

2 sq mm for everything but the:

 

Ignition switch feed

Glowplugs - I would calculate the run from the battery to the plug and use that for the switch feed (assume 50 amps for a 1.5).

Alternator positive

Ditto neg if fitted.

 

These need calculating.

 

If the engine is a long way from the panel I would use the same cable size as for the glowplugs for the solenoid energising wire.

[RobinJ]

Do you remember how many different wire sizes you ended up with? Ignoring the starter cables, I think you generally end up with three different sizes

 

Richard

 

I agree, mostly to simplify things, you round sizes up, so you end up with about 3 sizes overall!

[RLWP]

I agree, mostly to simplify things, you round sizes up, so you end up with about 3 sizes overall!

 

If memory serves me right, 2.5, 4 and big

 

 

Richard

[darbon]

TB Training! Exactly what i was looking for, thanks. Working my way through it all now.

 

Engine, battery and control panel are very close thankfully.

[Tony Brooks]

TB Training! Exactly what i was looking for, thanks. Working my way through it all now.

 

Engine, battery and control panel are very close thankfully.

If the cables will only be about 2m or so long then just use the current capacity on the drum or in the catalogue.

[darbon]

 

If the cables will only be about 2m or so long then just use the current capacity on the drum or in the catalogue.

Thanks Tony,

 

As my engine has a mechanical stop the only high amp draw from the ignition is the glow plugs. I think i'm going to opt for a relay system for the glowplugs. However i'm unsure of a couple of things.

 

If the max draw from a BMC 1.8 is 75amps, why do you recommend a relay rated to 180amp? Surely a 100amp unit would suffice?

Is it ok to bolt a relay unto the engine? (ie between plugs and positive terminal on solenoid) or is engine vibration an issue?

 

cheers

 

dan

[Tony Brooks]

Thanks Tony,

 

As my engine has a mechanical stop the only high amp draw from the ignition is the glow plugs. I think i'm going to opt for a relay system for the glowplugs. However i'm unsure of a couple of things.

 

If the max draw from a BMC 1.8 is 75amps, why do you recommend a relay rated to 180amp? Surely a 100amp unit would suffice?

Is it ok to bolt a relay unto the engine? (ie between plugs and positive terminal on solenoid) or is engine vibration an issue?

 

cheers

 

dan

 

Bot sure if the question about 180 amp relay is directed at me or not. I think that you will find the 180 amo relay relates to the "cheap" split charge relays. If I were to use a relay to power glowplugs I would use a old car type starter solenoid - far more robust.

 

The 1.8 glowplugs puts a simple 100 amp clamp meter riogfht off the scale for a ery few seconds as they heat up.

[RobinJ]

If the max draw from a BMC 1.8 is 75amps, why do you recommend a relay rated to 180amp? Surely a 100amp unit would suffice?

Is it ok to bolt a relay unto the engine? (ie between plugs and positive terminal on solenoid) or is engine vibration an issue?

 

When fitting split charge relays, people always look at what the alternator is capable of delivering, they forget that when you connect 110A batteries together, under some circumstances you can get a lot more current (even if only for a short time), hence the bigger the better!

I found this out when I started the engine one morning and fried the wiring on the split charge relay (boat was like that when I bought it), if I hadn't fried the wiring, I would have fried the relay, now I have a bigger relay and the right wiring!

[Tony Brooks]

When fitting split charge relays, people always look at what the alternator is capable of delivering, they forget that when you connect 110A batteries together, under some circumstances you can get a lot more current (even if only for a short time), hence the bigger the better!

I found this out when I started the engine one morning and fried the wiring on the split charge relay (boat was like that when I bought it), if I hadn't fried the wiring, I would have fried the relay, now I have a bigger relay and the right wiring!

 

I bet the alternator was connected to the engine battery rather than the domestic.

[smileypete]

I found this out when I started the engine one morning and fried the wiring on the split charge relay (boat was like that when I bought it), if I hadn't fried the wiring, I would have fried the relay, now I have a bigger relay and the right wiring!

Sounds like the relay was closed while the engine was cranking, and pulling starter current from the domestic bank through the relay, not really what a split charge relay is supposed be for.

 

Having large and/or poorly crimped cables pulling on the terminals can be another source of problems.

 

cheers, Pete.

~smpt~

Edited May 3, 2013 by smileypete

[RobinJ]

 

I bet the alternator was connected to the engine battery rather than the domestic.

As with most split charge relay systems, aternator direct to starter!

 

Sounds like the relay was closed while the engine was cranking, and pulling starter current from the domestic bank through the relay, not really what a split charge relay is supposed be for.

I think it was more like the domestic's were flat and when the relay kicked in, more current flowed from starter to domestic than the cable could handle!

 

I had not considered the advantages/disadvantages on connecting the alternator direct to the domestic!

[Tony Brooks]

As with most split charge relay systems, aternator direct to starter!

 

Not really, only those installed for maximum ease/minimum work or by peopel who do not understand or think about it.

 

I think it was more like the domestic's were flat and when the relay kicked in, more current flowed from starter to domestic than the cable could handle!

 

I had not considered the advantages/disadvantages on connecting the alternator direct to the domestic!

 

if the alternator is feeding the domestic bank as soon as the alt. kicks in the voltage will rise to something above 13 volts on the domestic bank, even with well under 50% discharged batteries. Even with dead flat batteries it is likely to rise to a bit over 13 volts. As the engine battery open circuit voltage will be below 13 volts you will not get that heavy discharge through the relay. Also, as the engine battery is usually well charged the charging current passing through the relay will be relatively small.

 

This applies to relays energised by the alternator, not the ignition switch, but even so engine cranking will pull the engine battery voltage down so in normal circumstances the current flow to the domestic bank will be limited.

 

I've had the "Lucas" version of the typical Durite split charge relay on JennyB for over 10 years without problem.