Volvo Penta Saildrive isolation: how?

[nigelmercier]

I keep reading that VP saildrives are sometimes electrically isolated from the gearbox, to reduce galvanic effects.

How is this done? And how can I find out if my 120S should be?

 

[scottie]

There is a nylon gasket between the bell housing and top gearbox along with washers under the bolt heads and the drive plate has a rubber bush a I understand

 

[dje67]

Yup!

There is a nylon gasket between the bell housing and top gearbox along with washers under the bolt heads and the drive plate has a rubber bush a I understand

I took mine apart yesterday to replace the rubber plate, or vibration damper, and it is exactly as described above.

 

[tawhiri]

Saildrive isolation

If you are on the dry, an Ohm meter between the engine and saildrive. Should be open circuilt.
In the water I think it is slightly Ohmic.

 

[nigelmercier]

I took mine apart yesterday to replace the rubber plate, or vibration damper, and it is exactly as described above.

You didn't take pictures by any chance, I'm having trouble visualising it.

 

[dje67]

You didn't take pictures by any chance, I'm having trouble visualising it.

Yup, but they are not really of much use for what you're looking for; they were more for me to put it all back together later (hopefully!).

Link attached.

http://s553.beta.photobucket.com/user/dje67/library

There are 8 bolts holding the flywheel cover onto the engine (the gearbox is bolted onto the flywheel cover). Each bolt screws into a female hole in the engine block. However, the between the flywheel cover and the engine block is a gasket arrangement, so there can be no electrical contact between the cover (and gearbox) and the engine. Each bolt head mates against an insulating washer between the bolt and the cover, so again there's no electrical connection between the cover and the engine. Also, the bolt-hole in the cover is much wider than the diameter of the bolt, so the threaded section of the bolt is insulated from the cover by an air gap or nylon bush. The final bit of the whole setup is that the gearbox shaft doesn;t make physical contact with the engin because of the vibration damper plate. THis has 2 metal plates; one mates with the gearbox splines and the other with the flywheel. The 2 plates are joined by a rubber sandwich which serves 2 purposes - it transmits the drive/torque between the damper plates and it provides an electrical barrier. So between the gaskets, the washers, the air gaps and the rubber there is no electrical path between the engine and gearbox.

My gearbox is a standard shaft variety, but the insulation technique used will be sam as on yours.

I have a separate shaft-mounted anode, which protects the prop and shaft only (as there's no electrical connection through to the engine). I've a separate anode which joins to the battery -ve and engine block.

 

[VicS]

Good explanation.

Any idea why the ordinary gearbox and shaft are insulated? I understand its done with saildrives to help stop corrosion of them.

You mention a hull anode bonded to DC negative and engine block? What is that fitted to protect ?

 

[dje67]

Good explanation.

Any idea why the ordinary gearbox and shaft are insulated? I understand its done with saildrives to help stop corrosion of them.

You mention a hull anode bonded to DC negative and engine block? What is that fitted to protect ?

The -ve is also bonded to the keel (which is epoxy encapsulated...). Other than that, I'm not really convinced that the main anode does very much. There's nothing else bonded to the -ve, other than the mast lightning conductor cables, which doesn't need an anode. That's how Beneteau fitted it all, I believe. THe only change I've made since getting it was to disconnect the mains earth from the 12V -ve, as I had no galvanic isolator initially (I do now, but haven't bothered re-connecting the link).

THe anode doesn't show wear at all each year, which backs-up my thinking that it doesn't do much of use, really...!


I've no idea why an ordinary shaft would need to be isolated. I guess it makes it a standard setup for all gearbox types. Maybe there's some benefit in ensuring that the engine and its electrics are not allowed a path to the sea through the prop and shaft in case faults occur elsewhere on the boat's mains system?

 

[nigelmercier]

There are 8 bolts holding the flywheel cover onto the engine ... between the flywheel cover and the engine block is a gasket arrangement ... Each bolt head mates against an insulating washer between the bolt and the cover ...

Thank you, do you mean ...

I thought the insulation was later in the chain, and I was worried I had breached it when replacing the large rubber seal (AKA "Elephant's Ars'ole")

 

[dje67]

Thank you, do you mean ...

I thought the insulation was later in the chain, and I was worried I had breached it when replacing the large rubber seal (AKA "Elephant's Ars'ole")

That's the ones. In that photo, 7 of them have been removed already (obviously the gearbox is away as well...!).

 

[scottie]

These engines were always shipped without the transmission being fitted so makes sense to have the same set up whatever transmission ws later fitted.
The installing dealer would get three boxes engine transmission and electrical panel and they would put them together.

This means that the insulation was standard and fitted to all engines of his type

 

[nigelmercier]

That's the ones. In that photo, 7 of them have been removed already (obviously the gearbox is away as well...!).

Thank you; so now that it is clear where the isolation occurs. I still need to know if my boat has this, or was it done on all VP engines?

I also have a relay whose purpose appears to be to ground the engine during start/stop procedures. This is the one component that confuses me, as the engine is already grounded by the starter motor.

 

[the_branflake]

I also have a relay whose purpose appears to be to ground the engine during start/stop procedures. This is the one component that confuses me, as the engine is already grounded by the starter motor.

Doesn't that relay connect the negative for the starter motor. I thought it basically had two starter solenoids one for positive (i.e standard setup) then this extra negative one. On my boat someone had bypassed that instead of just giving it a quick clean.

 

[VicS]

Thank you; so now that it is clear where the isolation occurs. I still need to know if my boat has this, or was it done on all VP engines?

I also have a relay whose purpose appears to be to ground the engine during start/stop procedures. This is the one component that confuses me, as the engine is already grounded by the starter motor.

This is one of the Volvo Penta MD 2010/20/30/40 series engines ???

The set up is, I think you will find, that the engine block is not used for the negative connections to the instrument and warning light sensors ( they should be two wire types) or for the negative connection to the alternator which should have an insulated negative connection.

There is no permanent connection from engine block to battery negative

It is however used as the negative connection for the glowplugs and the starter motor.

Therefore during preheating and cranking the engine block must be connected to the battery negative . This is done by the "earthing relay" which is in fact a heavy duty solenoid similar to a starter solenoid.

There are two other ordinary relays. One in the glowplug circuit and the other
intermediate between the keyswitch/starter button and the starter motor solenoid.

You should be able to find a diagram in the owners manual or workshop manual.

Not all are as above. The alternative is a conventional arrangement in which the engine block is connected to battery negative and used as the negative connection for all bits and pieces. Then an earthing relay is not required

 

[nigelmercier]

This is one of the Volvo Penta MD 2010/20/30/40 series engines ? ... The alternative is a conventional arrangement in which the engine block is connected to battery negative and used as the negative connection for all bits and pieces. Then an earthing relay is not required

It's an MD22-P. There is an earth point on one of the engine mounts, with a direct lead to the starter motor. But in the control box there is also an earthing relay controlled from the ignition switch, IIRC. I'll take another look at the schematic and report back.

 

[Playtime]

It's an MD22-P. There is an earth point on one of the engine mounts, with a direct lead to the starter motor. But in the control box there is also an earthing relay controlled from the ignition switch, IIRC. I'll take another look at the schematic and report back.

I used to have an MD22. IIRC the starter motor had separate heavy duty positive and negative connections. These were wired direct to the starter battery, via the starter solenoid for the positive. There should be no direct connection to the engine block. It sounds as though someone has made an ill advised alteration to the starter/engine block wiring on your boat.

The third relay in the engine control box makes the earth connection for the glowplugs and also for the stop solenoid (I think) when the 'ignition' switch is on.

 

[scottie]

The md22 was originally an automotive engine used in the Montego etc and as such it was a standard vehicle so when Perkins / Volvo marinised it they wanted a isolated rerun and rather than convert the fuel pump stop solenoid and starter they added solenoids that connected the insulated circuit back to earth return for the short periods thy were required purely cost driven
Trust this gives some understanding but
has caused a lot of head scratching

 

[VicS]

It's an MD22-P. There is an earth point on one of the engine mounts, with a direct lead to the starter motor. But in the control box there is also an earthing relay controlled from the ignition switch, IIRC. I'll take another look at the schematic and report back.

Sorry to get the engine type wrong! You have no doubt said what it is at sometime or another but I could not remember.

I have had a quick look at the schematic ... but you can read it just as well I I can....... looks pretty much as I described above. The relay with an orange wiring going to it is the glowplug relay , the one with red/yellow wiring is the starter relay The one with all black wiring is the earthing relay.
If your engine has a permanent negative connection to the block maybe a mod, perhaps to bypass a failed and expensive earthing relay!

 

[nigelmercier]

Sorry to get the engine type wrong! ... I have had a quick look at the schematic ...

I haven't mentioned my engine type for a long time, so there is no reason you should know Vic, certainly no need to apologise.

I've got a copy of the schematics on a PC at home, but not on my iPad - which I am using at the moment. I'm back at my PC tomorrow though.

 

[VicS]

I haven't mentioned my engine type for a long time, so there is no reason you should know Vic, certainly no need to apologise.

Are the schematics available online? I've got a copy on a PC at home, but not on my iPad - which I am using at the moment. I'm back at me PC tomorrow though.

They are in the owners manuals ... some of the manuals .. the older ones .. they don't put the diagrams in the newer ones

Owners manuals download from http://www.volvopenta.com/volvopent...lication_search/pages/publication_search.aspx


Looking at it again I dont think its quite the same as I described for the MD2010series