irish.david wrote:Galvanic corrosion is only an issue when there are two dissimilar metals that are linked with an electrolyte. It's a big issue for ships with metal hulls as the sea water acts as an electrolyte and and you get electrons flowing from the metal higher up the galvanic series to the one lower down. The solution for ships is to fit sacrificial anodes which are designed to corrode and be replaceable at set intervals, or an impressed DC current system to counteract the flow of electrons that causes corrosion.
The good news for your gearbox, galvanic corrosion isn't an issue as there isn't an electrolyte between the aluminium gearbox and steel body to allow ion migration between the two. The only issues you might have are where there is physical contact where you get the standard reaction between dissimilar metals, but most of these points are fitted with vibration mounts which act as a galvanic barrier and the others (such as the earth cable connections) are easy to take apart and clean from time to time.
Long story short, the corrosion on your gearbox is nothing to do with your earth connections.
Dave
sorry I have to disagree, the constant 12V potential in the presence of salt water film across disimilar metals causes corrosion, with the mag ali alloy case being the sacrificial and the steel plate taking the metal salt. This is aided by the reverse switch being permenantly live so when damp lallows a path for current and the resulting corrosion at the case join with the reverse housing is such a problem that vw intoduced an additional thick oversize paper gasket to reduce the effect in 1989 by which time they had seen lots of rotten cases
It is a major problem with the 5 speed boxes as the mount is only 1mm or so from the case so provides the perfect void for the salt water film and subsequents salts built up and enhance path for conduction, the stand offs on the 4 speed end case are more substantial but you will often seen quite bad local corrosion around the 4 studs and it is not from the studs but from the plate - I have seen dozens of rotten unusable cases becasue of the corrosion and get half a dozen requests for end cases for 5 speeds from other repairers every year they will concur with what I find
the earth strap is only located where it is for ease of assembly of the vehicle on the production line as it shares one of the mounting bolts when fitted to the chassis so meant one less thing to attach and thus saving 30 seconds per vehicle x 2.5 million vehicles equals a few lifetimes of assembly workers, wages and benefits and pensions
personally I'd just drill and clean up the body close to the starter and bolt through and fit a short strap direct from body to starter motor stud - obviously necessitates a 10mm round terminal and there isn't much spare thread, so it can be easier to go with the top bolt at the nut end and use the cable terminal as the washer, just be aware of the water pipes, and live cables and ensure no mechanical interference or uninsulated conductors