HowTo Read Electrical Diagrams and Diagnose Problems



Here’s a quick guide on how to read wiring diagrams and diagnose problems, it is not exhaustive and I’m sure it can be improved (be my guest…) but it will get you started

Occams Razor states that among competing hypotheses, the hypothesis with the fewest assumptions should be selected.


Do NOT touch the starter motor leads whilst cranking the engine, very high current

Exercise caution when examining coil packs or working with HT leads, high voltage, low current but it hurts when you get a belt

Do NOT touch HID leads - 23,000v - whilst the headlamps are lit

Do NOT short circuit the battery - Doh!

Lethal voltage during my time in the RAF was defined as 30v AC or 50v DC, less during conditions of high humidity so for the vast majority of your electrical investigations +12v will not hurt you

Where possible – which will be almost all circumstances – you will not require the engine to be running


Most car wiring switches either +12v or Earth, it depends upon the circuit under examination

All circuits feed from the battery as the source via fuses, relays, modules and components to earth

Fault diagnosis works best starting from the component that has failed and working back to the battery using the “split-half” technique. Essentially halving the distance between known good behaviour and the point of failure, it is this technique I will demonstrate

Intermittent problems are the hardest to solve as you never know whether you have fixed the problem or not until some time afterwards

Sometimes disconnecting the battery and touching the leads together will clear problems, it seems the General Electronic Module (GEM) can get its knickers in a twist

Batteries do fail and low voltage produces unrelated problems so if you are experiencing several unrelated issues, a knackered battery may be the cause

Always replace fuses for one of identical value unless instructed otherwise by a competent authority

Don’t leave leads connected unless absolutely necessary, check then check again to confirm your diagnosis. Print out the relevant wiring schematic and write down your results

You will need a multimeter

Diagrams and Symbols

There are 4 flavours of schematic diagrams found here: - /xtype/Electrical/ Download the one(s) appropriate to your model for three reasons – Adobe provides a search facility (Control F), it is faster and Sods law states that your Internet connection will be down when you need it most.

Note there are differences according to fuel type and model. If your car is a crossover year then the fuse panel layout may give you a clue as to the correct schematic to use

Schematics give the layout and interconnections between modules, they are not circuit diagrams so sometimes we have to infer behaviour based upon what makes sense at the time

Examples taken from 2007 wiring schematics

This symbol I call Box n (where n is a number) is a permanent live feed from the battery and may itself be connected to another Box, in this example Box 71 is fed by Box 2 and via fuse F46 7.5A feeds Page 10.1

This symbol I call Circle n I (where n is a number) is switched live from the battery which is only live at key position I ie no lights lit on dashboard, engine NOT running, it is also referred to as ACC1 – Accessory position 1, an example would be playing the radio with the engine off.

This fragment shows Box 76 feeding the Accessory Relay (which is fed +12v from the Ignition Key) which then feeds +12v via fuse F4 20A to Circle 9 I and Circle 10 I:

The Ignition Key would be as shown:

This symbol I call Circle n II (where n is a number) is switched live from the battery which is only live at key position 2 ie lights lit on dashboard, engine NOT running, it is also referred to as ACC2 – Accessory position 2, an example would be opening and closing windows with the engine off

This fragment shows fuse F12 7.5A feeding +12v via Circle 24 II to Fig 03.6 and F13 5A feeding +12v via Circle 25 II to Figs 05.1, 05.2 and 05.3:

The Ignition Key would be as shown:

Check your ignition key position AND the schematic to ensure you have the correct key position for voltage to appear

There are very few electrical problems which require the engine to be running, ignition problems excepted

Sample Fault I – (thanks funkkydrummer)

On turning the key to the second position:-

A host of lights appear including the red temperature warning light, engine management light and the red cog with "!" light.

Cooling fan fires up to full speed.

On trying to start the engine:-

Nothing. Completely dead. No starter motor, no clicks nothing.

First things first – how SHOULD the ignition circuit work?

Inserting the ignition key into the barrel sends a signal to the GEM which outputs Ready To Start to the Passive Anti Theft System inside Instrument Cluster - note the flap inside the barrel is known to fail and sometimes a squirt of WD40 can free it up

Instrument Cluster takes 2 x +12v feeds from Box 61 (Check fuse F71) and Circle 15 II (Check fuse F84) and feeds +12v to Passive Anti Theft System (PATS) - we can't see the internal circuitry but one of the two fuses will supply power to it

Assuming Instrument Cluster and PATS are happy an OK To Start signal is sent to the Engine Control Module

When the ignition key is turned to Crank, +12v is fed into Engine Control Module which then outputs an Earth to R10

Now we know the order in which the ignition circuit works we can fault find.

Find the appropriate wiring schematic for year and engine – model is irrelevant in this case

Referring to this schematic: Guide.pdf

Fig 01.1

Check fuse F28 15A in the Power Distribution Fuse Box (the one in the engine bay) as this powers the ignition switch via Box 4 which supplies several places

Fig 02.1

On the right hand side of the diagram is the starter relay R10 located in the Power Distribution Fuse Box - Page 18 identifies this as being the first relay on the right as you look at the Power Distribution Fuse Box from the front of the car

Get someone to turn the key to the RUN position and see if this relay clicks

If R10 clicks, check fuse F29 30A as this is supplied from Box 28 on Fig 01.2 and supplies power to the starter motor. If R10 doesn't click there are several options:

1) Relay R10 is knackered – swap with R9 as they are identical
2) Despite F28 being OK there is a miniscule chance of a break / high resistance between fuse holder and relay
3) Bad earth to the other side of the relay coil - also unlikely
4) Ignition barrel flap intermittent connection
5) GEM got its knickers in a twist, disconnect battery and short leads together
6) Anti theft playing up

Now apply Occams Razor – the simplest solution is the most likely - which are GEM or Ignition Barrel Flap, a known failure point

Sample Fault 2

Indicators don’t work

First things first – How should the indicators work?

When the Hazard button is pressed and the ignition key removed, all four indicators should flash

The GEM needs an earth to operate the indicators

Referring to this schematic: 2007/09 - Exterior Lighting.pdf

Fig 08.1

Hazard switch supplies an earth to GEM

Power is supplied to GEM via Box 64

Do we have power to Box 64? – where is it supplied from? For this information we need to look at 2007/02 - Main Power Distribution.pdf which is the battery distribution schematic. Scroll through or search for ‘64’

Fig 01.3

Fuse F35 15A feeds Box 64 to Fig 08.1 and several others – on checking the fuse we find it has blown – by the way, this happened to me recently…


It is worth trying to solve a problem before asking for help, you will learn something and hopefully reap satisfaction from fixing a problem yourself

The forum is a great resource for advice and help

If in doubt ASK! The only dumb question is an un-asked question


Feb 19, 2012
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A valuable source of information OC, thanks! I hope I never have to use it but I am reassured by having it nonetheless. :)


New member
Jun 1, 2013
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Mate, this is a SUPERB post !

It`s people like you that keep these forums alive and kicking out there, and whilst this is way above my meagre talents, there are some out there who will benefit greatly from this thread.

Thanks for taking the (considerable) time and trouble to prepare and pen all this down for the rest of us.

Well done sir ! :cool: