Volvo Penta 120SD saildrive electrical isolation

[Ammonite]

I have a MD2040C fitted with a 120SD saildrive that is supposed to be isolated from the engine, however having tested this using the continuity function on my multimeter this doesn't appear to be the case. The boat is out of the water and I have disconnected the gearbox cable. There is an isolating gasket between the engine and the gearbox and isolating bushes under the bolts securing the two. The saildrive intake hose does have steel reinforcement but its connected to the metal strainer using a trudesign hosetail and there's no continuity between the two. There are no other connections to the gearbox. Last season I had to engage reverse rather quickly on one occasion and there was a resounding clunk from the transmission (although this could have been the central rubber bush in the autoprop bottoming out). Is there any way I could have damaged the isolating vibration damper on the flywheel without losing drive? Also, is there anything else I have missed before I start taking things apart? The saildrive anode has started wearing at an alarming rate so something isn't right. Previously it used to hardly wear at all. And if I do have to replace the damper can this be done by sliding the engine forward 100mm (this all the clearance I have) or does the saildrive have to come out? Thanks

 

[vyv_cox]

Is the rubber gasket clean, salt free and dry? I was once investigating crankcase explosions in a reciprocating compressor and discovered that the isolation was being breached by a trail of dripping cooling (sea)water and oil.

 

[Ammonite]

It's a plastic gasket and as far as I know it's dry / clean but I will add it to the list - thanks. I have also just learnt that one of the bolts holding the flywheel housing to the engine has a metal insert, rather than the plastic insert used for the others, and that this can cause problems if not precisely aligned. Apparently a bit of insulation tape should resolve it so that's something else to check. There is also another plastic gasket under the bell housing that can cause problems if not aligned correctly.

 

[VicS]

I have a MD2040C fitted with a 120SD saildrive that is supposed to be isolated from the engine, however having tested this using the continuity function on my multimeter this doesn't appear to be the case. The boat is out of the water and I have disconnected the gearbox cable. There is an isolating gasket between the engine and the gearbox and isolating bushes under the bolts securing the two. The saildrive intake hose does have steel reinforcement but its connected to the metal strainer using a trudesign hosetail and there's no continuity between the two. There are no other connections to the gearbox. Last season I had to engage reverse rather quickly on one occasion and there was a resounding clunk from the transmission (although this could have been the central rubber bush in the autoprop bottoming out). Is there any way I could have damaged the isolating vibration damper on the flywheel without losing drive? Also, is there anything else I have missed before I start taking things apart? The saildrive anode has started wearing at an alarming rate so something isn't right. Previously it used to hardly wear at all. And if I do have to replace the damper can this be done by sliding the engine forward 100mm (this all the clearance I have) or does the saildrive have to come out? Thanks

It's a plastic gasket and as far as I know it's dry / clean but I will add it to the list - thanks. I have also just learnt that one of the bolts holding the flywheel housing to the engine has a metal insert, rather than the plastic insert used for the others, and that this can cause problems if not precisely aligned. Apparently a bit of insulation tape should resolve it so that's something else to check. There is also another plastic gasket under the bell housing that can cause problems if not aligned correctly.

ITYWF that the isolation is achieved with an "insulating plate" between the two parts which make up the flywheel housing. The 8 bolts between them have insulating bushes, insulating washers and plain washers ( if the washers were swapped over it would probably bridge the isolation)

See the parts diagram here:- MD2020-C Cylinder Block and Flywheel Housing - Marine Diesel Engines

AFAIK there is no isolation between the engine block and the front part of the flywheel housing.

Cannot comment on any damage to the vibration damper / flexible coupling. .

 

[Ammonite]

Thanks Vics. Yes that's my understanding too, although I could have worded it better. The bell housing is in two parts as you say but the result is the same in that it isolates the engine from the gearbox. The guide ring in the schematic is also part of the isolation apparently. I haven't had a chance to check but my local marine engineer said that one of the insulated bolts was slightly different from the others and that this could bridge the isolation. It was only a phone discussion so I may have misunderstood and will need to investigate further and I wonder if he was referring to some form of locating dowel? I cant see anything obvious in the schematic. Btw - the new marine parts website is horrible compared with old one...at least when using a mobile!

 

[VicS]

Thanks Vics. Yes that's my understanding too, although I could have worded it better. The bell housing is in two parts as you say but the result is the same in that it isolates the engine from the gearbox. The guide ring in the schematic is also part of the isolation apparently. I haven't had a chance to check but my local marine engineer said that one of the insulated bolts was slightly different from the others and that this could bridge the isolation. It was only a phone discussion so I may have misunderstood and will need to investigate further and I wonder if he was referring to some form of locating dowel? I cant see anything obvious in the schematic. Btw - the new marine parts website is horrible compared with old one...at least when using a mobile!

All 8 bolts are the same part number likewise the 8 insulating washers and 8 plain washers.
The only odd thing is there is apparently only one insulating bush . I thought that was a mistake in the parts list but it is confirmed by the workshop manual. VOLVO MD2010 WORKSHOP MANUAL Pdf Download
I think I might consider wrapping the other 7 bolts in insulating tape
I guess the guide ring is important to line up the two halves correctly then only one insulating bush is needed.

 

[Ammonite]

Thanks VicS. I probably misunderstood what he was saying but it sounds like there is an insulating tape solution in there somewhere I'd assumed there were 7 insulating bushes but as you said it looks like there is only 1.
In general though he wasn't too concerned about the isolation on the basis that most of the other manufacturers (Beta etc) don't bother, providing the saildrive anode is lasting reasonable well and that VPs solution wasn't very well designed any way. He also said the new spec aluminium anodes would help whereas I was (wrongly) under the impression this was just an eco thing. In any event I'm planning on adding an extra hull anode attached to the gearbox just in case although Id like to restore the isolation if its not too big a job.

 

[VicS]

Thanks VicS. I probably misunderstood what he was saying but it sounds like there is an insulating tape solution in there somewhere I'd assumed there were 7 insulating bushes but as you said it looks like there is only 1.
In general though he wasn't too concerned about the isolation on the basis that most of the other manufacturers (Beta etc) don't bother, providing the saildrive anode is lasting reasonable well and that VPs solution wasn't very well designed any way. He also said the new spec aluminium anodes would help whereas I was (wrongly) under the impression this was just an eco thing. In any event I'm planning on adding an extra hull anode attached to the gearbox just in case although Id like to restore the isolation if its not too big a job.

Quite.
The loss of the saildrive isolation alone should not lead to your saildrive anode eroding "at an alarming rate" although it might have prevented it.

I think I would also be looking for an electrical defect. ........ you don't want electrolysis to destroy the sail drive leg.
It happened to Nigel Mercier (RIP) a few years ago .
MD22 engine, which is not isolated from the sail drive but has an isolated electrical system. An electrical fault combined with the fact that the DC system was earthed to the keel resulted in the loss of the whole drive leg due to electrolysis.

 

[Ammonite]

Quite.
The loss of the saildrive isolation alone should not lead to your saildrive anode eroding "at an alarming rate" although it might have prevented it.

I think I would also be looking for an electrical defect. ........ you don't want electrolysis to destroy the sail drive leg.
It happened to Nigel Mercier (RIP) a few years ago .
MD22 engine, which is not isolated from the sail drive but has an isolated electrical system. An electrical fault combined with the fact that the DC system was earthed to the keel resulted in the loss of the whole drive leg due to electrolysis.

It's a late 1990s Moody so the dc and ac circuits are bonded to the keel and there's a hull anode connected to the fuel filler, fuel tank, fuel filter, keel cooler, engine and rudder, which links the whole lot together, which given the various threads on here I was planning to revisit. I don't have shore power available on my mooring but when testing the continuity of the saildrive while on the hard I found that there was continuity between the engine/gearbox (no isolation currently as discussed) and the metal strainer, but only when the mains charger was on. The charger is mounted on an aluminium faced plywood board with the (metal) strainer someway below this but on the same aluminium faced board. The charger is an old sterling 3 output 20amp with a metal case so I was planning to mount this on a rubber mat to resolve this. Do you think this could have caused the excess saildrive anode depletion given that we only use the shore power when in marinas, so probably not more than 2 or 3 weeks a season. I deliberately hadn't mentioned this before as I wanted to tackle the gearbox isolation issue first. I'm also fitting an additional saildrive anode as a belt and braces solution having read a number of saildrive horror stories and in case I have to leave the boat in the water for an extended period but I also want to address any underlying fault there might be.