MD2020B starter motor & alternator connections

[Jokani]

I am in the final stages of swapping an MD2B for a newer MD2020B, but I am unsure of the correct cable connections for the alternator and starter motor.

The questions I have are:

1. The photo of the engine, the starter solenoid stud, there are two red wires currently connected, is it fair to assume that these are the 2 R2.5 connections to the starter relay and the glow relay? Therefore this stud also needs an R10 connection to the B+ of the alternator, and an R30 to the main switch?
   
   
2. The wiring diagram indicates that the starter motor requires a negative connection? If so, what size should the cable be, and where on the starter of solenoid should it connect to?

 

[PaulRainbow]

You will indeed need connections on the solenoid terminal from the alternator and the isolator switch.

The starter does not need a separate negative connection, this is made through the body of the starter motor to the engine block. The engine itself will need a negative cable, same size as the positive cable, this should already be there, from the old engine to the battery negative.

 

[Jokani]

Thanks for your advice Paul.

The engine itself will need a negative cable, same size as the positive cable, this should already be there, from the old engine to the battery negative.

Are you able to say where on the engine the negative connection should be made?

The starter does not need a separate negative connection, this is made through the body of the starter motor to the engine block

The alternator does require a negative connection, that is not made through the alternator body in the same way as the starter?

 

[VicS]

I am in the final stages of swapping an MD2B for a newer MD2020B, but I am unsure of the correct cable connections for the alternator and starter motor.

The wiring diagram you have posted is for a an engine with an Isolated negative return, Ie not earthed via the engine block.
The alternator is isolated from the engine block and is connected to battery negative by the solid black wire on the B- terminal.
The warning light sensors are two wire type and the return is also via the black wiring to battery negative.

The starter motor and the glow plugs are however not isolated from the engine block and it is is used as the negative return for them. The block is connected to the battery negative by the earthing relay , #4, during the preheating a cranking stages .
Most of wire sizes are shown on the diagram after the colour code. The earthing link ,#5, should be the same cross sectional area as the battery cable as it carries the starter motor current

Is your engine one with an isolated negative return like this?
If so the starter motor will be "earthed" via its mountings but the alternator will not be earthed to the engine block.

IIRC there were several changes to the arrangement during the life of the 20n0 engines so you need to be sure you are referring to the correct wiring diagram for your engine.


EDIT The isolated return wiring applies to earlier engines but not to later (C and D series) I am not sure if it applies to B series or not.

 

[Jokani]

The earthing link ,#5, should be the same cross sectional area as the battery cable as it carries the starter motor current

Is your engine one with an isolated negative return like this?

Vic, I am not sure, I must have an earthing link, but I am not sure what it would look like or where it would be located. I can see in the manual that the A version has an earthing relay and you can see the position of this under the starter motor, but that is just the A version, the manual does bot show where the earthing link is for any version as far as I can see? Can I just pick any bolt and connect to that?

 

[VicS]

Vic, I am not sure, I must have an earthing link, but I am not sure what it would look like or where it would be located. I can see in the manual that the A version has an earthing relay and you can see the position of this under the starter motor, but that is just the A version, the manual does bot show where the earthing link is for any version as far as I can see? Can I just pick any bolt and connect to that?

Having looked into this I conclude that a B series engine does not have an isolated negative and that the wiring diagram you posted is in fact only applicable to the A series engines

The loose connection, with two red wires, on the solenoid is where the battery positive and alternator output should be connected.

The starter motor will be earthed through ts mountings to the engine block.

the battery negative should be connected directly to a convenient bolt on the engine block and there will be no earthing solenoid.


However one thing to bear in mind is that the engine and gear box will be electrically isolated from each other by a system of insulating bushes and washers . It would be important to preserve this isolation if fitted to a sail drive .

 

[Jokani]

Having looked into this I conclude that a B series engine does not have an isolated negative and that the wiring diagram you posted is in fact only applicable to the A series engines

Really appreciate you taking the time to help, thank you.

 

[Jokani]

After a day at the boat yesterday, I think I am almost there.

Just a couple of things I am not sure of:

1) The old MD2B had a black wire connecting the main switch to the starter motor - It would have made more sense to me if that had beeen red?

2) The old MD2B had a wire from the negative distribution post to a bolt connection on the engine block. The same bolt connection also had a wire connecting it to earth. Does the MD2020B also require the connection to earth?

3) The alternator D+ is used for the initial excitation, am I right in thinking this will only be live if the ignition is on, and I can connect a busbar to us to power other items that only want to be live when theignition is on, exhaust alarm etc?

 

[VicS]

After a day at the boat yesterday, I think I am almost there.

Just a couple of things I am not sure of:

1) The old MD2B had a black wire connecting the main switch to the starter motor - It would have made more sense to me if that had beeen red?

2) The old MD2B had a wire from the negative distribution post to a bolt connection on the engine block. The same bolt connection also had a wire connecting it to earth. Does the MD2020B also require the connection to earth?

3) The alternator D+ is used for the initial excitation, am I right in thinking this will only be live if the ignition is on, and I can connect a busbar to us to power other items that only want to be live when theignition is on, exhaust alarm etc?

1) No idea why black has been used for the battery + connection. No colour is specified on the VP diagram. It is what Westerly used I guess. Change it to red if it makes you happier.

2) You need a heavy battery negative connection to the engine block. Yes I believe it can /( should?) also connected to your earthing point. But take note of what the manual for the B series engine says. Ignore any wiring instructions, and the diagram you posted, that refer to the A series.

3) D+ (also sometimes labelled 61) is the connection for the ignition warning light. If you want to power stuff when the ignition is on do so from the switched terminal of the key switch. 15b has the electronic module and tacho etc connected. I beleive, but check, that 15a can be used for the addition equipment you wish to fit

 

[Jokani]

Thanks Vic, yet another beer I owe you!