Demystifying automobile electrical diagnostics. (Part One)
This write up is born out of the fact that in most of the forums I belong, I always see automobile workshop owners/operators put up avert that they need an electrical automobile technician. My response to such advert has always being “Train One”.
In my quest to end this advert I decided to do a detailed research on automobile electrical diagnosis.
Automobile electrical/electronics diagnosis appears to be simple but can be very complicated if you are not patient, perseverant or find it very difficult to follow manufacturers laid down instructions.
It starts with the understanding of ohm’s law, ohm’s law is a relationship between Voltage, Current and Resistance, they are measured in Volts(V), Amp(I), ohm’s respectively.
Ohm’s law states that the current through a conductor between two point is directly proportional to the voltage across the two points, by introducing resistance as the constant of proportionality, a mathematical expression is arrived at:
I=V/RWhere resistance (R) is the constant of proportionality.
Bearing in mind that most electrical components function by the introduction of current, voltage simply depicts the movement of electrons between two point while resistance is the measure of difficulty to pass electric current through a conductor.
A very pictorial example is water running through the tap, the cork on the tap acts as the resistance to the flow of water, when pressure is applied on the cork, it reduces the amount of water that flows through the tap.
In other words the resistance is used to determine the amount of current a component requires to function.
In todays vehicles three things must happen for a component to fail or malfunction, namely;
Open circuit
Short circuit
A dead load
An open circuit is a complete isolation along the path of conduction
Short circuit implies when two parallel conductors meet at a poorly insulated area while a dead load implies that the component is damaged
Our major focus here will be short circuit, because the other two are easy to determine.
Short circuits comes in different forms, namely:
Short to ground
Short to power
Short circuits have different effect on a system/component, depending on the source of control. Automobile systems/components function within thresholds valves, the level of malfunction is most times determined by the type of short circuit that occurred within the circuit.
This brings us to the understanding of electrical controls in automobiles, bearing in mind that the systems/components in automobile has two major sources of power, namely:
Power from the Battery
Power from the ECM
The battery provides 12volts while the ECM provide 5volts, though the original power and ground to the ECM is sourced from the battery and chassis respectively.
The ECM is the brain behind todays automobile, it is built with capacity to control every component that requires controls in the automobile, this control(switching) is achieved by either providing power or ground to the components(sensor & actuators) and both for highly intelligent systems.
Amongst other functions of the ECM, it’s also embedded with a rectifying circuitry that removes the unwanted noise in the electrical feed signal before delivering reference voltage(Ref V).
Talking about control(switching), there are the Power side switches and the Ground side switches.
Power side switches are switches that have their controls on the positive side of the circuit while ground side switches are those the have there controls on the negative side of the circuit as show in diagram 1a & b respectively.
Systems/components can be stuck ON or OFF, depending on the action that was taking place before the short circuit occurred, system circuits can either be shorted to ground or shorted to power.
A short to ground on a ground side switched circuit will cause the system/component circuit to be STUCK ON or OFF, in this case the ECM provides its ground that controls the functionality of the system/component circuit while a short to power will cause total damage or malfunction of the component or even damage the ECM if its not protected by a fuse.
Just a reminder a ground side switched circuit implies that the ECM provides the ground while the battery provides the 12volts, while in a power side switched circuit the ECM provides the 5volts power and the ground is provided by the chassis.
On a power side switched circuit(impling the ECM provides the power while the ground is provided by the chassis), a short to ground will pull down the whole critical system/component circuit causing a crank no start.
There are systems/component circuit that the ECM provides both power and ground, these circuits require discrete power, ground and control.
It’s very important to know the source of power, ground and control while working on electrical system/component circuits, this will aid in proper use of you multimeter to avoid any damage to the ECM.
This diagnosis applies to all electrical circuits and does not differ amongst automobile manufacturers.
Case Study.
Sometime ago a client walked in to our workshop requesting that we come to his house to check his vehicle, the customers complain was that the car cranks but will not start. The vehicle was a 2008 Jeep Patriots, on getting to his house after thorough visual inspections before running a scan, we asked the client a few questions, it turned out that the vehicle was towed from the Eastern part of Nigeria to Lagos. The vehicle was hesitating, stalling and shutting down, most probably due to emission system clog as a result of bad gasoline or a malfunctioning MAF, MAP e.c.t.
According to him, he called a mechanic who started loosen without a detailed diagnosis or asking himself the questions that will bring him out of the box(What would have caused it, Why did it happen, When did it happen).
Like we have always advised that if you don’t have all the answers to the questions in the bracket, don’t pick up your spanners.
To cut the long story short. The vehicle was fixed in less than 30mins after I had extracted, analyzed and interpreted all the fault codes. Detailed explanation of the repair will be done in Part two.
Automobile electrical diagnosis requires critical analytics reasoning, this can only be made possible by your Thought Process or Pattern.
See the solution before you think the charge….
Be a fixer not a part changer!!!
