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[Marks DTM Calib]

And to confirm a car has a ground but not an earth:

A ground is considered to be a 0V reference (e.g. ground plains etc).

An earth is a reference which is at the same (or very close to) the earths surface potential.

Important to remember as the ground on a car could be many thousands of volts above the potential of the earth, this will become more obvious as electric vehicles are used because under a fault condition the vehicles chassis ground could become live at mains volts.

Are you sure?  I would expect the car to only ever to become live with respect to the battery negative, never the earth (planet), even in fault conditions.  For example if you connect a multimeter to any battery positive and the other lead to the earth you stand on, there would be no reading (or next to no reading).  Unless somone has bolted the battery negative lead to the earth (planet)!

If refering to the later satement note that this is under a fault condition (as it states).

[Webby the Bear]

ok, while i really appreciate the input you guys have made... i'm going to confess i'm very lost.

to try and get my mind back on track as i really really want to understand it.....i'm going to go back to very basics.....

so we have a component, unless its unique like the crank sensor, it should get a voltage. but unless we know what voltage it should get its pointless looking for it? i suppose as a general rule of thumb we could have a list of certain components that get battery voltage.... cooling fan i believe being an example? so in this instant we could backprobe the loom coming in, touch it to groud and get a reading.... if we dont see 12v or there abouts then we can search for resistance in the wires thus looking for a short (as kev mentioned).

and pretty much from what mark is saying that resistance in a component is pointless because of all the things he mentioned (dont really understand it but thats cos of my lack of understanding; not your explanation  )

i'm still getting to grips with this but i will get there! 

[Kevin Wood]

I think the issue is that you are taking a diagnostic approach that's fine for simple circuits (lighting and motor circuits, perhaps - indicators, wipers, etc) and trying to apply it to circuits associated with the ECU, some of which are far from simple.

Some of the circuits defy normal diagnosis with a multimeter completely (the crank sensor included) beyond verifying that the wires in the loom are making the connection they should be. Some components send digital data to/from the ECU, for example. This scenario is why ECUs report live data, and it's probably as important, on a modern car, to master diagnosis by what live data and stored fault codes tells you as it is using a multimeter IMHO.

Many cars of post-Omega vintage are now doing away with even the simplest circuits and incorporating CAN controlled light clusters, etc. The only way to diagnose these will be using a code reader. That said, their adoption should make problems much less common in the first place. Should. 

[dbug]

I think the issue is that you are taking a diagnostic approach that's fine for simple circuits (lighting and motor circuits, perhaps - indicators, wipers, etc) and trying to apply it to circuits associated with the ECU, some of which are far from simple.

Some of the circuits defy normal diagnosis with a multimeter completely (the crank sensor included) beyond verifying that the wires in the loom are making the connection they should be. Some components send digital data to/from the ECU, for example. This scenario is why ECUs report live data, and it's probably as important, on a modern car, to master diagnosis by what live data and stored fault codes tells you as it is using a multimeter IMHO.

Many cars of post-Omega vintage are now doing away with even the simplest circuits and incorporating CAN controlled light clusters, etc. The only way to diagnose these will be using a code reader. That said, their adoption should make problems much less common in the first place. Should. 

My last company recently bought a new expensive 650tonne injection moulding machine incorporating on board diagnostics, and remote diagnostics (direct to the machine manufacturers).  This m/c, in production mode, just kept throwing many and various fault codes, to the point it was totally unacceptable.  Problem eventually traced to faulty CAN bus!!

[Webby the Bear]

I think the issue is that you are taking a diagnostic approach that's fine for simple circuits (lighting and motor circuits, perhaps - indicators, wipers, etc) and trying to apply it to circuits associated with the ECU, some of which are far from simple.

Some of the circuits defy normal diagnosis with a multimeter completely (the crank sensor included) beyond verifying that the wires in the loom are making the connection they should be. Some components send digital data to/from the ECU, for example. This scenario is why ECUs report live data, and it's probably as important, on a modern car, to master diagnosis by what live data and stored fault codes tells you as it is using a multimeter IMHO.

Many cars of post-Omega vintage are now doing away with even the simplest circuits and incorporating CAN controlled light clusters, etc. The only way to diagnose these will be using a code reader. That said, their adoption should make problems much less common in the first place. Should. 

I think you've hit the nail on the head there mate.

in that case perhaps its worth making myself a guide as to what a multimeter can look for and diagnose..... off the top of my head battery, alternator (dynamic test), starter motor etc.

anything else that's of the simple variety that I can add to the list.

and in fact thinking about it... if I take the approach of finding out what components can be tested via MM then it'll be easier to search for individual specific MM tests. that make sense?