Earth Strap

discool said:

Ok my disco is from the stud u have shown to a thinner stud in a plate behind which is held by two screws.
I replace the strap a few years ago.

Click to expand...

discool said:

And I thought on a D1 it’s just a blanking panel, unless a D2 is different, getting to the upper stud with everything in place was a bit fiddly getting a nut tight, other wise, fortunately I had access to a box of certified bonding straps in the firms stores, all yellow sleeved and printed with manufacturers reference number.

Click to expand...

Stanleysteamer said:

To get rid of gremlins when fitting an old fashioned radio, it was often advised that panels such as bonnets be earthed to the main body, dunno why, not sufficiently electrically minded.
Just saying and probably nothing to do with this issue.

Click to expand...

NPG said:

I'd say the reason for that is because old radios do not have safeguards against EMI (or RFI). But in general, the entire chassis and body should form a common ground and because a vehicle is electrically isolated from the ground by the rubber tyres, the closest that one can get to a large enough reservoir that can absorb EMI, spurious currents, spikes, etc. is the battery. For convenience, the vehicle's frame is also used as the negative bus (it was positive not so long ago) and therefore it is important that all elements of the frame are kept at the same potential, hence why they are electrically connected, either via bolted / welded joints or wiring. That way, you eliminate the potential for electrical instability such as a floating ground or a capacitive discharge when you open the bonnet and support it (i.e. connect it) to the frame.

Click to expand...

Stanleysteamer said:

Well thanks for that clarification.
Don't think I ever owned a car that was positive earth, although I do remember knowing how to convert one to negative earth if it were ever necessary. and I am quite old, owned first car in 1973 and that was a Mini old enough to have a dynamo.
Seem to remember there was a reason that positive earth was thought to be better, something to do with creating the spark at the plug?
Obvs didn't catch on!!!!!

Click to expand...

NPG said:

My grandfather used to own a Ford Anglia which had a dynamo and was also positive earthed ! Besides standardisation, I suspect the impetus towards negative-earth systems came with the emergence of electronics and because in A/C, the neutral is considered as the zero reference, which bears similarity to the negative rail in DC and which until recently, even shared the same wiring colour, i.e. black. Perhaps the concept that Earth is the ideal zero-volt reference could have also played a part in deciding the polarity of the common ground. Whatever the reason, it's good to see convergence in a world where we can't even agree on the most basic things such as a standard electrical plug !

Click to expand...

Stanleysteamer said:

And I have now found out why they changed from positive to negative.
https://mechanics.stackexchange.com/questions/1934/what-are-the-tradeoffs-for-positive-vs-negative-ground#:~:text=Originally, the voltages on the,positive with respect to earth.&text=Then engineers discovered that with,copper is protected from corrosion.
All good fun!

Click to expand...

NPG said:

Thanks for that. I've seen this explanation on some forums a few times but I never came across a definitive answer as to why the ground polarity in vehicles changed from positive to negative. Coming to think about it though, corrosion does makes sense since a negative ground permanently connects the body and chassis to the negative supply (battery + alternator), i.e. the cathode. This will allow the vehicle frame to receive a steady supply of electrons which in turn prevents the metal from bonding with the oxygen atoms found in water and moisture. In other words, it stabilises ionic metal (always positive and therefore attracts negative electrons), which is very reactive and decomposes rather quickly through chemical change (oxidation).

Could this be one of the possible reasons why the rear of the D2 corrodes so much quicker than the front ? I don't want to speculate or anything but the chassis is made of the same metal and is subject to the same road and atmospheric conditions, albeit the rear receives more water spray when the vehicle is moving. Don't really know but I always wondered why, so could it be that this is partially due to the fact that most electrical currents are conducted through the front section of the chassis ?

Click to expand...

NPG said:

Well, the rear chassis is rarely involved in the movement of charged particles since most equipment that is earthed to the metal frame is located at the front. So unless the lights at the back are on, no current will ever flow through the rear section of the chassis. Nonetheless, what I've said above is probably a shot in the dark and the rotting at the rear is more likely due to the reasons you gave in your post. Still I cannot help wondering if there's something else at play because the front and the rear sections of the chassis are like night and day. Mind you, I've seen the same thing on other vehicles with different types of chassis so one needs to ask why is it always the rear that degenerates so badly. Could it be that the front fares better thanks to the heat radiated from the engine or even the lubrication from oil vapor and the gratuitous leaks !

Click to expand...