Volvo 290 outdrive corrosion problem

[jimmy_the_builder]

Evening all

I've got an odd corrosion problem on one of my outdrives: the port outdrive casing is corroding quite badly; the starboard casing is fine. Both transom shields are fine.

The boat is out of the water at the moment and so the engineer went over both drives with a multimeter. One marked difference is that the resistance between the yoke and the helmet on the corroded drive is about 40 ohms; on the good drive it's close to zero.

Anyone got any idea on why this should be? My feeling is that resolving the electrical differences between the drives should in turn help us solve the corrosion problem.

Thanks
Jimmy

 

[eddieperkins]

I would fit a Galvanic Isolator in any case and check your earthing wires between engine and leg. Even though they may look ok the crimp eye on the end may be loose or corroded.

 

[jimmy_the_builder]

Thanks for this. Galvanic isolator is on the list; we've been checking continuity all over the place today, and the electrical continuity up from the leg through the transom shield and back to the engine block fails at the steering helmet. I just need to understand a bit better how the electrical continuity is supposed to work through the helmet and yoke, then I can try and figure out what's gone wrong.

Cheers
Jimmy

 

[Col]

On the top of the steering yoke on the inside of the transom shield (where the steering arm and trim sender are located) should be an earthing wire. I suspect this is broken.

The steering fork itself runs in nylon bushes, so is virtually insulated.

 

[jimmy_the_builder]

Excellent - thanks for this - gives me something to go on.

Cheers
Jimmy

 

[oldgit]

On my 280 legs the earth to the engine block was connected via a ring terminal to the pinch bolt holding the steering arm to the steering fork.
Shortly after buying boat,quick visual check showed both mine had become detached and by the look of them many years before.

 

[Callisto]

The increased resistance will undoubtedly be having an effect here as the drive with the high resistance is not effectively "bonded" to the other metallic bits around it.

For this reason, a "potential difference" will occur between the points. Any potential difference can cause an electric current to flow. What you seem to have here is an electrolytic action which is eroding the Port Outdrive.

get the bonding right so that you have a close to zero ohms between all the metal bits and you should find the problem goes away or is significantly reduced. I assume that your zinc anodes disappear fast too.

Galvanic isolator is a good idea too.

Cheers
JH