5 replies to this topic

[jakejakejake1]

Hi Everyone,

A short guide to electrical theory in order to increase peoples understanding of electrics and therefore hopefully enable them to problem shoot thier own cars.
Firstly there are three main things to be concerned about Voltage, Current and Resistance.

Electrical circuits can also be likened to many other things, for example a fluid circuit

Voltage: This is the driving force behind electrics. The bigger the voltage, the more force there is to overcome resistances This is also comparible to pressure in a fluid circuit.

Current: This is the 'flow rate' of electrics. This is comparible to flor rate in a fluid circuit.

Resistance: This is as it sounds, it resists the 'flow' of electricity.

Now for a simple equation, V = IR where V is voltage, I is current, R is resistance - this is the most important equation, but to be fair, when trouble shooting your car you probably won't use it!

[lrostoke]

we were always taught remember Ivy Watts :)

I * V = W

amps x volts = watts

[jakejakejake1]

But now the stuff you really want to know: Why isnt X component working?

Firstly if you know that other similar components are working you can narrow it down to being a problem with a specific item rather than a flat battery etc.

Next you protruce your trusty multimeter (you can get these for less than £10-15, and will help you solve the majority of those pesky electrical problems), and set it to the voltage setting (20V range or so should be good) and then find the power source of X component and poke this with the red (live) prong of the multi meter, and then place the black (ground) prong on either the ground for the component, or any other ground point you can find.

Now hopefully you will have a number somewhere between 0 and 12 (for a 12V component). If you have 12V or thereabouts you are happy for now.
If you dont, the issue is somewhere between the battery to the point you poked with the red prong, you can follow the wiring back and do the same test with the live prong and when you reach a point where the voltage is higher you know you have reached a 'good' point.
Example: The heater fan isnt working. Check the voltage at the switch/ input wire and to ground but get 0V, which means the fan isnt getting any power which explains why it isn't working. Next I check one side of the fuse box where the correctly coloured wire attaches, again I get zero volts.
I then check the other side of the fuse box, magically I now find 12V.
This means the problem must be in the fuse box, or the fuse.
I check the fuse and it looks fine, but I'm not convinced - So i set my multimeter to measure resistance and put one prong on either end of the multimeter. Now if the fuse is ok this will be fairly low number, if the fuse is broken there is no way electricity can get through so the resistance will be infinite.
I find that the fuse is ok, so I put it back. Then I check at one terminal where the fuse sits for voltage again, but get zero volts. This means somewhere between where the wires attach and where the fuse sits is where the problem is. Normally by this point you should hopefully be able to tell whats causing the issue. Normally with minis cleaning all the crap off the connections and ensuring they are connecting correctly will solve the issue, sometimes replacing the offending component is the only option.

[jakejakejake1]

A few safety tips and general information regarding using a multimeter and dealing with electrics:

When measuring voltage and resistance, no electricty flows through the multimeter, but when measuring current it does flow through the multi meter.
This is reflected in how you use the multimeter to take these mesurements.

For voltage and resistance you 'bridge' over components and measure the difference between the two points. And because 'ground' is effectively 0 volts this allows you to see the voltage at a point.

For current you have to use the multimeter 'in line' with the electric circuit, but be very careful as it is possible to cook a multimeter by allowing too much current through. So to measure you would disconnect some wires and put one prong on either side of the disconnected joint.
To be fair though, you normally won't have to measure current in order to problem shoot your mini.

[jakejakejake1]

we were always taught remember Ivy Watts :)

I * V = W

amps x volts = watts

Yeah there are many ways of remembering the equastions and that's quite a good one, but I have left out references to power as generally for trouble shooting you don't need to worry about it.

[KernowCooper]

Or The V I R Tree of Ohm's Law

V= Volts
I= Current
R=Resistance

V = I × R I = V / R R = V / I

The formulas of Ohm's Law

Ohm's Law can be rewritten in three ways for calculating current, resistance, and voltage.
If a current I should flow through a resistor R, the voltage V can be calculated.
V = I × R

If there is a voltage V across a resistor R, a current I flows through it. I can be calculated.
I = V / R

If a current I flows through a resistor, and there is a voltage V across the resistor R can be calculated.
R = V / I

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