BMC 1500 glow plugs

[TimmyT]

I am replacing the glow plugs. Power is supplied through 4 fuses and two relays so that plugs 1&2 have separate power from plugs 3&4. 

Strangely all 4 are then linked with 3 solid bars across the plugs. Has been working ok but I wonder if I should remove the bar between plugs 2&3. Why split them and then link them.

Be grateful for som advice. Thanks

TT

[Jen-in-Wellies]

Welcome to the forum. The glow plugs on BMC1500's are very low resistance, leading to high currents when operating. The fuses are to protect the wiring and two wires and fuses are there on your boat presumably so thinner wires and smaller fuses can be used, or a style of fuse that can't take the higher total current. That they are all linked by bus bars at the plugs makes no difference. It is working OK, leave it alone. If one fuse blows, the other will blow almost instantly from the doubled current.

[TimmyT]

Many thanks Jen for your speedy reply most helpful.

I did have one fuse blow, 30 amp , relays 40 amp.

Couldn't find fault, cleaned all terminals and checked wiring with multimeter.

Working ok now.

Thanks again, TT

[Tony Brooks]

If I were to do anything to it, I would rewire with a single cable of the correct size and a single fuse. Doing that ensures that it will be what most pope would expect, so making fault-finding easier. But as Jen says, if it is working, you can leave it.

[TimmyT]

Thanks Tony, if I have any further problem I will do as you say.

Does that mean I can omit the relays.

TT

[Tony Brooks]

23 minutes ago, TimmyT said:

Thanks Tony, if I have any further problem I will do as you say.

Does that mean I can omit the relays.

TT

 

I think the relay may be why your system is twinned. High current relays are not as easy to come by as 10 to 40 amp ones. The relay takes a lot of load off the ignition switch, so I would use one, but not a physical relay. I would use a starter solenoid, as used on older petrol-engined cars. Mount it on the boat, not the engine because constant vibrations tend to reduce a relay or solenoid's life. using this system allows you to use a thinner cable from the ignition switch and a much shorter length from starter main terminal (B+) to solenoid to glowplugs.

[Jen-in-Wellies]

40 minutes ago, TimmyT said:

Many thanks Jen for your speedy reply most helpful.

I did have one fuse blow, 30 amp , relays 40 amp.

Couldn't find fault, cleaned all terminals and checked wiring with multimeter.

Working ok now.

Thanks again, TT

Just out of interest, what style of fuses are they? Modern blade fuses, continental ceramic type, or the old style glass ones? It might explain why it has been wired like this. If I was going to rewire this circuit with just one wire, I'd probably use a suitably rated midi fuse in the appropriate holder. Much more substantial. If the present system works though it might as well stay.

[TimmyT]

Thanks again Tony.

Attached photo of the installation. Should leave alone I suppose but couldn't find out why one glow plug fuse blew when turning on heater at the switch. There are 2 relays

[Tony Brooks]

Fuses can blow through age fatigue caused by constant heating and cooling, I suspect that might be the cause. It looks far too complicated with far too many terminals to me.  Bear in mind that many marinises use just one fuse to protect the whole of the engine electrical system.

 

I go with Gen, a bolt down midi fuse is far less likely to corrode, but the short length of high amperage cable runs and very short time it is in use gives some justification for not fusing the glow plug circuit.

[TimmyT]

Thanks Jen, I will investigate you suggestion and give it a go when out of water end of season. Be good to lose many connections and wires as Tony says. Thanks again  to you both.

[TimmyT]

Tony/Jen, Reading through your post again, I think I might try installing a single wire from the heater switch, through a 30 amp midi fuse and then to the glow plug rail disconnect the existing system at the switch. 

I could then easily go back to existing if necessary.

Does that sound ok

Thanks again. JB

[Tracy D'arth]

3 minutes ago, TimmyT said:

Tony/Jen, Reading through your post again, I think I might try installing a single wire from the heater switch, through a 30 amp midi fuse and then to the glow plug rail disconnect the existing system at the switch. 

I could then easily go back to existing if necessary.

Does that sound ok

Thanks again. JB

They draw over 40A when cold, you need more than a 30A fuse.  Use a 140A Durite relay, powered from the ign switch and a 60A fuse, 10mm cables.

[Tony Brooks]

4 minutes ago, TimmyT said:

Tony/Jen, Reading through your post again, I think I might try installing a single wire from the heater switch, through a 30 amp midi fuse and then to the glow plug rail disconnect the existing system at the switch. 

I could then easily go back to existing if necessary.

Does that sound ok

Thanks again. JB

 

Apart from the fact that the glow plugs tend to draw 50 amps dropping, for a few seconds when you turn them on. I think I have come across some modern ones that draw more than that. you will also be passing the full current through the ignition switch. Many boas do, but it is not ideal in the long term.

[TimmyT]

Tracy, thanks for the information. The high ratings for the fuses and relays you suggest may explain why I have a duplicated supply of 4  fuses and 2 relays. I originally wondered why all glow plugs were then linked. The installation has been working for many years before me.

Perhaps I should leave it if 'it ain't broke'?

Many thanks all for your interest and support. I'll let you know if it blows up!😊 TT

[TimmyT]

Tony, Gen, Darcy. 

I have two weeks of grief with this wing and decided to go with the single system  now. Another BMC 1500 boat nearby has just a wire from the switch and he doesn't know if it's fused! Been ok for 10 years.

I woul be very grateful if you could confirm Gen's installation with a midi fuse and also the fuse rating and wire size. Trying to avoid delays or solenoid as I don't think I could wire them. The distance from the switch to the first glow plug is less than 1 metre.

I will disconnect the power to the existing from the switch.

Sorry to be a pain but I need to get going and stop blowing fuses in my head as well!

Many thanks. TT

[Tony Brooks]

A fuse to protect the cable is best practice, but hundreds or thousands of boats do not have one specifically for the glow plugs so a single cable will work, but make sure it is at least 4 so mm CCSA to allow for the high initial current demand as the plugs eat up.

[TimmyT]

Thanks Tony for your always speedy reply. I have some 6mm wire so will use the 30 amp midi fuse, maybe 400 amp. I'll let you know how it goes. Thanks again.

Sorry  40 amp not 400!!?

[Tony Brooks]

If you can it may be worth measuring the currant into each individual glow plug in case one or more have shorted. However, I think that you will need a DC clamp type ammeter because I doubt the 10/20 amp range on a normal multi-meter will be enough. Just a wire from the battery pos. and push against each glow plug in turn.

 

I have just checked some measurements I made some time ago and find the BMC 1.5 pin type plugs traditionally drew about 6 amps each and steady, BUT I have seen some modern versions starting with a very high currant and dropping to 6 amps each as they heat. If you have that type then a 40 amp fuse may not be enough. However, it is a suck it and see situation.

[TimmyT]

Tony, I have checked all plugs for resistance and  all reading near zero. I also found that one wire to plugs 3&4 was not getting any voltage. Wire to 1&2 getting 12.4v.. I gave up then. Decided to try the single wire system. I could put in a relay but I don't think I would know how to wire it in!

[Tony Brooks]

15 minutes ago, TimmyT said:

Tony, I have checked all plugs for resistance and  all reading near zero. I also found that one wire to plugs 3&4 was not getting any voltage. Wire to 1&2 getting 12.4v.. I gave up then. Decided to try the single wire system. I could put in a relay but I don't think I would know how to wire it in!

 

Checking a heating device for resistance when cold can easily give very confusing readings. "Near zero" Ohms could easily mean the plugs could draw high currents and blow fuses. That is why I suggested measuring the current drawn. You just might have shorting plugs.

 

Have you found this: http://www.tb-training.co.uk/MarineE07.html#GLOW PLUG CIRCUITS

 

Wiring a relay for glow plugs:

 

1. use a high current relay, a slit charge relay should do.

 

2. Mount relay close to the engine, but not on it because the vibrations may wreck it.

 

3. 2 sq mm CCSA wire from ignition switch to one small terminal on the relay

 

4. 2 sq mm CCSA wire from other terminal to any battery negative cable/connection, even the engine block or negative stud on the starter motor.

 

5.  Heavey cable from the main battery terminal (nut type) on the starter solenoid to a large terminal on the relay.

 

6. Heavey cable from the other large terminal to the glow plugs.

Edited July 24, 2023 by Tony Brooks

[TimmyT]

Thanks Tony, I'll try the single wire and fuse to see if the plugs short. Be at the end of the week so I'll leave you in peace.

[TimmyT]

Tony, I have bought a 70 amp relay and socket, amidi fuse holder and 40 amp fuse, using 6mm2 wire 50 amp. I'll give it a try following your wiring advice.   Thanks again TT

[Tony Brooks]

11 minutes ago, TimmyT said:

Tony, I have bought a 70 amp relay and socket, amidi fuse holder and 40 amp fuse, using 6mm2 wire 50 amp. I'll give it a try following your wiring advice.   Thanks again TT

Good, that will add a bit to the ignition switch life.