Brake fluid testing

Brake fluid is hygroscopy so it may be contaminated by water. To test if the brake fluid in a vehicle has been contaminated we use a Brake fluid tester which has three different coloured LEDs in it. If the green light goes on then the fluid is not contaminated, if the yellow light goes on it means it has been contaminated but it is still fine to use, if the red light goes on it means that the fluid is contaminated and will need to be replaced. Brake fluid does not stay uncontaminated and will need to replace about every two or less depending on the climate of the country the vehicle is in. When the brake fluid is contaminated the water brings the boiling point of it down and it could cause components in the system to corrode.

The vehicle that I tested showed a green light which meant that the fluid was fine and it did not need to be replaced. If the brake fluid needs replacing then drain all the the fluid out of the reservoir and put new fluid in then start bleeding the brakes on the wheel furthest away from  the master cylinder. Put a piece of wood under the brake pedal and get someone to pump it and then hold it down once they hold it down open the bleeder nipple and allow the brake fluid to escape into an allocated container then close it again. Repeat this until you can see that the new fluid is coming through then do it on the next wheel furthest away from the master cylinder. Do it until all of the fluid is out of the system and make sure that there is always enough fluid in the reservoir.

Servicing brakes

Disc brakes

If the brake pads need replacing, loosen the sliding bolts and remove the bottom one. Then lift the caliper up  and the pads will be able to slide out. The brake pads can be measured to see if they can be used again. Use a DTI gauge to check if the rotor is warped or not. If the rotor is worn then the inside and outside of it should be measured with a micrometer, the measurements should be compared to specifications. If it is worn and with in the specified thickness then it can be machined if it is not or if it is warped then it will need to be replaced. Once the rotors have been replaced the new brake pads can be replaced. The new brake pads obviously have more lining than the old ones so the slave/caliper pistons need to pushed back this can be done with a G clamp or by clamping the brake hose and opening the nippple so that it can be pushed back by hand. Before the pads are fitted make sure they are correct and remember to replace any shims and anti squeal plates. An anti squeal spray may also be sprayed onto the back of the pads. Once every thing has been put back into place the caliper and sliding bolts should be torqued to the correct setting.

If one of the rotors in the front is warped the driver will feel it in the steering  wheel  if one in the back is warped the whole car will vibrate.

Drum brakes


To remove drum brakes make sure that the cars handbrake is off. Then either hit it with a brass hammer or put two bolts in and turn them until the drum comes off. Once it has been opened check under the dust seals to see if the pistons are leaking then clean it with brake cleaner or water to get rid of the brake dust. Then using a screw driver  to move the pads from side to side to make sure that the pistons that activate the brakes are not jammed. Then measure the thickness of the brake shoes and check the inside of the drum. If the drum is worn inside it will need to be machined. Before getting it machine measure the inside diameter of the drum and check it with the specifications. Once the all the checks have been done and it has been repaired if necessary the brakes need to be adjusted by turning the nut on the spreader until the brakes are in the right position. the last thing to do is to adjust the handbrake cable.

The breaking system

When the break pedal is depressed it pushes the piston in the master cylinder forward. The master cylinder pushes the break fluid through the break lines and into the slave cylinders that push the break shoes against the rotors.

The force applied by driver alone is not enough to stop the vehicle therefor the car uses mechanical systems to multiply the force. The first multiplication is mechanical advantage (leverage) by the pedal. The next multiplying step is the break booster which uses vacuum from the engine to increase the force applied. Then the hydraulic system also multiplies the the force by having the master cylinder smaller than the slave cylinders. If the master cylinder is smaller than the slave the master cylinder moves more than the slave but the slave will have more force as it has a larger surface area.

The friction between the break shoes and the rotors convert kinetic energy into heat energy. The wheels lose kinetic energy therefor the vehicle loses kinetic energy too (slows down).

Jacking up a car (safely)

When jacking up a car it is important to make sure that the area on the ground is level and smooth. If the area on the ground is not level the car may roll back and slip off the jack. If the ground is not smooth the jack's small wheels could get stuck and as the jack goes up the jack will pull away from the jacking point, it could cause the vehicle will slip off.

It is important to put the car in park and put the handbrake up so that the car will not roll backwards or forwards off the jack. For extra safety something could be placed behind the wheels. Before jacking up the car make sure that the jack is in the right position (90 degrees to the wheels) and that it is under a suitable  jacking point where it will not damage the car or be likely to slip of. Once the car has been jacked up put axle stands in as far away from the center as possible to increase the vehicle's stability. Lower the jack so that the vehicle rests on the axle stands. The jack may be left in place as an extra precaution but the handle should be lifted up as far as possible so that other people do not trip over it. Once the car has been jacked up give the vehicle a "shake" to make sure that it is secured before going under it.

Oxygen Sensor Tester

The Oxygen sensor measures the amount of oxygen in the exhaust gasses and it sends signals to the ECU so that it can monitor if the engine is running rich, lean or at stoicheometry. When the tester is connected to the oxy sensor it displays what the air fuel mixture ratio is that the engine is running on. It will display a red light if it is running rich, a yellow light if it is stoich and a green light if it is running lean. The oxy sensor is back probed and connected to the oxy sensor tester. The tester measure the voltage and it displays whit the fuel mixture is.

Exhaust Gas Analyser

The exhaust gas analyser is used to measure the amount of emissions a car is producing. It can also detect what the engines air/fuel ratio is whether it is running lean, rich or at stoicheometry (14.7 : 1). The emissions that cars produce are harmful to the environment so governments started putting laws up on how much emissions a car is allowed to emit. The exhaust gas analyser can also be used to diagnose problems on the engine.

 An example of exhaust gas measurements

Problems related to higher than normal readings:

• High HC (hydrocarbons) - May be due to a misfire
• High CO (carbon monoxide) - Could be from the engine running to rich
• High O2 (oxygen) - Means the engine is running lean, there is a misfire or there is a hole in the exhaust
• CO2 (carbon dioxide) is an indicator of how efficiently the engine is running

It is important to allow the exhaust analyser to measure the residue in the pipes and allow it to re-calibrate before inserting it in the exhaust pipe. It is also important to have a ventilation pipe securely fitted to the exhaust to prevent inhaling poisonous exhaust gasses. The analyser should be set to catalytic converter if the vehicle has one, if it is not set up properly incorrect measurements may be recorded.

Automatic Transmission

An automatic gearbox has complex gear sets in them. The gear sets consist of sun, planet and ring gears. The gear ratios are controlled by the bands braking the ring gear forcing the planet gears to drive the output shaft or it releases the ring gear and allows it to drive the output shaft. The bands are controlled by pressurised hydraulic fluid that is pump through the gearbox. The same fluid is also used to lubricate the moving parts in the gearbox like the bearings. Older gear boxes would control the gear selection using mechanical shift valves  linked to the kick down switch that is connected to the trottle and the governor. Newer cars use mostly clutch packs instead of the the bands and the fluid is directed to them using solenoids that are controlled by the ECU according the the engine speed, the speed of the vehicle and the load on the engine (the amount of vacuum in the intake manifold.

We took an old rear wheel drive automatic gear box apart, it had two bands that would lock the ring gears according to which gear is selected. The bands were controlled by an actuator piston that would push the two ends of the bands together locking down on the ring gear. The governor was located on the output shaft and the oil pump was located on the input shaft. The oil pump in the gearbox is a crescent type pump.

Before removing and stripping down the gearbox it is good practice to take out the dip stick to check the colour and smell of the transmission fluid. The colour and smell can help a lot with finding the problem. If the fluid smells burnt it means that the clutch pack or the bands are slipping causing them to burn. If the fluid is milky it means it has been contaminated by water. If the engine flares between gear changes it means that the transmission oil is low or the clutch pack is worn down or the bands need to be adjusted, it could also be because the oil pump is worn out and it is not supplying the correct amount of pressure or there is a blockage in the fluid passages.

 Bottom of the gearbox with its sump off

 Park locking mechanism

 Outside of the oil pump

 Governor, the governor controls the oil pressure using centrifugal forces

Axle, gears and the oil pump

This is a diagram to show how the oil pump works

The inside of the oil pump from our gearbox looked like the one in the picture above