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Carrying out automotive diagnosis
Symptoms, faults and causes

Activity 1

Tick the correct answer/s. There may be more than one correct answer.

Question 1

The term ‘diagnosis’ means:

Using a scan tool to extract codes
Using a logical process to solve problems
Repairing faulty components
Replacing faulty components.

Question 2

When a vehicle component or system is working the way it should be, we say that it is in:

A normal state
An abnormal state
A paranormal state
A sub-normal state.

Question 3

An automatic transmission, on up-shift, always makes a 2nd to 4th change and never selects 3rd. Is this:

A normal event?
An abnormal event?
A paranormal event?
A sub-normal event?

Question 4

Personal Protective Equipment (PPE) for technicians working in an automotive workshop should include:

Eye protection- glasses or face mask
Gloves to protect hands from burns and chemicals
Skin protection- overalls or suitable protective clothing
Over-lapping steel abdominal protection when balancing wheels
Safety footwear.

Question 5

Working around any of the vehicle systems listed below can be dangerous. Describe (write down) some possible dangers associated with each system:

Fuel system

Ignition system

Batteries

Running engines

Ask your trainer to check your results before proceeding.

Trainer Signature: .............................................................. Date: .....................

Carrying out automotive diagnosis

The importance of diagnosis

Diagnosis is an extremely important task in today’s workshop.
Fast, accurate diagnosis can save money for both the customer and the workshop.

The table below shows some of the benefits of good diagnosis for the three stakeholders in automotive repair.

Who?	Benefits
The customer	Time and money has not been wasted on inefficient diagnosis methods Their vehicle is off the road for the minimum amount of time No re-work required through wrong diagnosis
The technician	Skilled and efficient employees are valued and rewarded by their employer They have the satisfaction of doing a good job
The workshop	The job is completed in the shortest possible time, and productivity (time spent per job) is improved so more jobs can be completed in a day Warranty costs are down because the work is high-quality

Bad or careless diagnosis can have just the opposite effect.

Figure 2: The result of bad diagnosis

Which technician would you choose?
I bought this van second hand not long ago– it’s a rear wheel drive. When I accelerate, there’s a loud grinding and groaning noise, and also a bit of vibration.

What could it be?

Technician 1 - Guessing	Technician 2 - Logical diagnostic procedure
Joe Goodenough took the van for a quick test run. The noise seems to be coming from the manual transmission. It’s an old vehicle, so the transmission bearings are probably faulty. I think the transmission needs rebuilding. Joe didn’t do any more checks.	Jack Getitryte carried out a few more tests and collected more information, including looking in the workshop manual. The manual says that one reason for noise and vibration on acceleration can be incorrect driveline angles. I’ll check to see if that could be the problem. Jack checked and found that the rear axle shims or wedges (which set the drive line angle) were missing, left out after a previous repair.
A rebuilt transmission was fitted and road tested. The noise was exactly the same as before!	The shims were replaced and the problem was cured!

Symptoms, faults and causes

Understanding the language

When we diagnose a problem, we look at the symptoms of the fault and try to find the cause of them. What do these words mean?

Symptom

A symptom is a clue that something is wrong.
It is a noticeable change - we might see, hear, smell or feel something different.
The engine cuts out!

The clutch feels funny...

These are both symptoms of a problem. The owner or driver recognises them as abnormal. They won’t know what the fault is, but will know that something about the car is different.

A problem or fault in a vehicle is usually first noticed by the regular driver of the vehicle, because they are most familiar with the way it normally operates, and spend a lot of time behind the wheel.

It is the technician’s job to look at the symptom and work out what fault has caused it.

In fifth gear, the engine revs hard but the car won’t accelerate normally.

The engine stopped suddenly, and it won’t re-start.

The front tyres have worn out very quickly.

These symptoms are the abnormal things that the driver has noticed about the performance of her/his vehicle. They all point to some problem which has produced the symptom.

Fault

A fault is an abnormal condition in a system or component. Something has gone wrong which we need to identify and repair.

Faults may be:

Hardware faults - Physical faults that we can observe or measure, such as parts which are broken, worn, out-of-specification, damaged, incorrectly adjusted or assembled.

Software faults - May not be directly observable, such as faulty, incorrect or corrupted programs in electronic modules.
The fault may be one or more of the following things:

Type of fault	Example
Not working at all	A blown headlight globe A failed open circuit fuel injector
Working, but not the way it was designed to work	Working headlights which are badly aimed A fuel injector with a faulty spray pattern
Doing something it is not supposed to do	A headlight that won’t turn off A fuel injector with a constant leak
Making something undesirable happen	Causing the headlight fuse to blow Generating excessive exhaust emissions.

Here are some examples of symptoms reported and the faults which were found to match them:

Symptom	Fault
In fifth gear, engine revs hard but car won’t accelerate	Slipping clutch
Excessive tyre wear	Incorrect wheel alignment
Engine stopped suddenly, won’t re-start	Broken cam belt, bent valves

These faults can be repaired by replacement of parts, or by adjustment.

But what caused those faults?

There are many reasons why a clutch might slip, but if we simply fit a new clutch kit without discovering the underlying cause of the slippage, that vehicle might return to the workshop on the back of a tow-truck!

Cause

A symptom might indicate a fault in a customer’s vehicle, and a faulty (worn, broken) component might be found, but what produced that fault?
Something must have caused that change.

Finding the cause of the faults will allow the technician to choose the right repair to correct the fault, and prevent any re-work under warranty (‘come backs’).

Example:

Figure 3: the importance of finding the cause

Remember those symptoms and faults shown in the table above?

Here are the some possible causes of those faults:

Symptom	Fault	Cause
In fifth gear, engine revs hard but car won’t accelerate	Slipping clutch	Clutch driven plate worn out after 200,000km of service. Normal wear.
Excessive tyre wear	Incorrect wheel alignment	Bent steering arm - impact with road gutter
Engine stopped suddenly, won’t re-start	Broken cam belt, bent valves	Belt overdue for replacement. Leaking camshaft oil seal.

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