Issues discussed in the material:
- Signs and causes of automatic transmission malfunctions
- 3 methods for diagnosing automatic gearboxes
- Stall-test automatic transmission
- Ways to Extend the resource of automatic transmission
- What motorists say about automatic transmission diagnostics
Diagnostics of the automatic transmission is necessary even for a new car, and when buying a used version, it is a completely mandatory procedure. If the behaviour of the car has changed markedly, then postponing the inspection of the “machine” is a path to losses.
There are various diagnostic techniques, but they all boil down to 3-4 procedures. In our article, we will tell you how these manipulations occur, talk about the signs and causes of automatic transmission breakdown, and give recommendations on extending the resource of this unit.
Signs and causes of automatic gearbox malfunctions
The most frequent breakdowns that are detected during the diagnosis of the automatic transmission are described in the following table (except for rare malfunctions):
Breakage symptoms
- Problem: The car slips, cannot go forward.
Why? Friction discs, piston clutch cups, clutch rings are worn out. The valve of the valve body may be out of order. - Problem: Reverse, 3rd, 4th gears are not included.
Why? The friction disc, piston cup or clutch ring oil seal are worn out. - Problem: When the driver changes gears, a jolt is felt, and the car does not go forward or backwards.
Why? The torque converter is defective; there is not enough transmission oil because the filter element is broken. - Problem: The car can only go in 3rd gear.
Why? This breakdown can occur because the friction discs, clutches, piston cuffs, clutch rings have become unusable. The valve of the valve body may have stuck. - Problem: The car cannot drive if reverse gear is engaged.
Why? The brake band or its piston cuff is worn out; the piston rod is deformed. - Problem: If the car is cold, the gearbox is pushed during gear shifting.
Why? Solenoids or valve bodies are out of order. - Problem: The car cannot go back and forth; the lever is switched with a push from position P to position N.
Why? The torque converter is faulty; the package piston cuffs have become unusable, there is not enough transmission oil in the system, the drive gear of the oil pump is broken, the solenoid is out of order, the oil seals of the clutch ring are worn out, the filter mesh is clogged. - Problem: The engine hums and vibrates, especially at high revs.
Why? The bearings are worn out. - Problem: The box switches only to reverse, 1st and 2nd gears.
Why? The solenoid (or the valve in the valve body) is dirty. - Problem: The car can only move if the selector is in position N.
Why? The adjustment of the cable or lever is broken, the piston of one of the clutches is jammed, the friction discs are welded with automatic transmission elements. - Problem: At high speeds, the gears shift independently.
Why? Throttle cable broken or incorrectly adjusted. The filter element is clogged.
3 methods for diagnosing automatic transmission
To carry out diagnostics without dismantling the automatic transmission box, it will turn out in the following ways: inspect the mechanism, do a computer check, conduct a stall-test and road tests, measure the hydraulic pressure. All methods have both pros and cons. Experts recommend comprehensive diagnostics to obtain accurate results.
1. We inspect the automatic box.
When using this method of diagnosing the automatic transmission, please pay attention to the transmission fluid level and how dirty it is. In many cases, malfunctions occur since an oil change has not been made for a long time. The fact is that the grease in an automatic transmission performs the function of a working fluid, lubricates the elements, protects them, cools them. If the oil is not enough or is filled more than the norm, the mechanism will not work correctly; the box will fail.
When there is insufficient transmission lubrication, oil starvation occurs, the gearbox overheats, its elements wear out quickly, and the automatic transmission starts to operate noisily. If the liquid is filled in more than it should be, it will foam and lose its original characteristics. As a result, the transmission will overheat. Because the automatic transmission is not lubricated enough, the mechanism will break.
When the grease is black and smells like burning, it means that the automatic transmission is faulty. If you see metal shavings in the oil, this indicates that the transmission components are worn out. To check the condition of the fluid, test it with a white cloth or paper. When the grease is dirty, the cloth will not soak; you will see metal dust on the paper sheet. All these signs indicate that it is necessary to replace the transmission fluid as soon as possible, install a new filter element and flush the sump.
The optimum lubrication level for a dipstick machine is between the “Hot” and “Cold” marks. If your car has a plug, check the oil level differently. To do this, add 300–500 ml of liquid through the filler plug. The grease is standardized according to the level flask. If there is not enough oil, it will not flow out of the drain plug. When there is sufficient lubrication, the gearbox will discharge the filled volume of fluid to the outside.
How else can you inspect the automatic transmission when diagnosing it? Checking the throttle control cable. The valve adjusting cable can be worn out. As a result, the motor and the automatic transmission function out of sync. Gear shifting occurs ahead of time. The car consumes a lot of fuel. To eliminate the malfunction, we lubricate and adjust the cable. If the parts are worn out, install new ones.
2. We carry out mechanical diagnostics.
Mechanical diagnostics of the automatic transmission is understood as testing the hydraulic pressure. How many contours have to be examined is determined by the machine model. It is necessary to find out the total line pressure in several modes of joint operation of the motor and transmission. The results obtained are compared with the optimal values indicated by the manufacturer of the automatic transmission.
The mechanism can be diagnosed using a pressure gauge and a portable tachometer. At the service station, auto mechanics use special equipment and stands with adapter hoses and adapters. It would help if you did testing with an assistant.
Before starting work, the car should be applied to the handbrake, supported under the wheels, or positioned on a lift. The equipment is inserted into the diagnostic connection. It is located in the automatic transmission case. Then you need to start the engine. After that, check the frequency with which idle rotates. Further:
- press the brake, switch the gearbox selector to position D;
- find out the pressure;
- Press the accelerator pedal all the way, then measure the pressure at the highest speed.
The diagnosis should be interrupted when the rear wheels begin to rotate before the motor reaches maximum power. Then repeat testing in R mode.
The line pressure value is the pressure in the hydraulic circuit. It determines the force with which the friction discs are compressed. When the indicators are less than optimal, it means that it is because of this that the automatic transmission slips during gear changes.
3. Computer testing of the box.
Diagnostics of the automatic transmission using a computer shows what faults are in the electrical control. The control module receives data from sensors about the temperature in the system, the lubricant pressure, the number of revolutions of the input and output shafts, the position of the gear selector, pedals. When the sensor signal is outside the measurable range, the computer recognizes it as an error, encodes it in memory using digital values.
To read the code, you need a diagnostic scanner and a table where the digital code is decoded. With the help of a scanner, you can remove errors and improve the automatic box’s operation. This type of automatic transmission diagnostics is used during chip tuning.
When testing a transmission using this technique, it is not possible to obtain complete information about its condition in all cases. The simplest electrical units are installed in 3- and 5-speed automatic machines. In this case, it will be possible to diagnose the box only after reading the errors.
Stall-test automatic transmission
Such diagnostics of the automatic transmission box will show how well the hydraulic transformer is functioning. This work can only be performed by a professional who is engaged in the repair of transmissions. If you are inexperienced, testing can break.
How stall-test runs:
- to begin with, we drive 4-5 km by car so that the grease in the machine warms up;
- we park the car in such a way that there is a lot of space around it (this is necessary if the brakes are not reliable enough);
- we put the car on the hand brake;
- while driving a car, we sink the brake pedal with our left foot;
- set the gear selector to D mode;
- with the right foot, press the gas to the end, hold the pedal pressed for no more than 5 seconds (less is better).
It is forbidden to perform this testing several times in succession. It should take at least 10-15 minutes before repeating the procedure. During this time, the transmission fluid will cool down.
Based on the results of such a diagnosis of the automatic transmission box, one can judge the operation of the friction discs and the hydraulic transformer. At the end of the procedure, the gear selector should be set to N. The engine should run for 5-7 minutes to cool the lubricant in the machine.
Automatic transmission manufacturers advise Stall-test.
How the car can behave during testing with the parking brake:
- The brakes work, a squeak is heard, and the car moves slowly. This indicates that the braking system is out of order.
- The motor is troit, the car shakes. This indicates a malfunction in the ignition system. If the problem is not eliminated, the gearbox will reduce the power of the car. What to do: We test the operation of spark plugs, high-voltage spark plug wires, plug tips.
- If the automatic machine is working normally, the engine speed rises in 1 second to a value of 2,800, if the engine is gasoline, and up to 2,000 for diesel engines.
- These figures are approximate. This testing should be performed by a professional who understands what parameters will be optimal during the steel test.
Ways to extend the use of your automatic gearbox
If you want the automatic transmission to work properly for a long time, it is important to reduce its load. This means that you cannot move at high speed for a long time. There is no need to skid if the car is stuck on a snowy road or in the mud because this overheats the box and breaks it down.
To keep the transmission in top condition, operate smoothly and gently while driving. There should be no abrupt gear changes; reverse gear can be engaged only when the car has completely stopped. It is also impossible to start abruptly. As a result, the blocking clutches will quickly wear out.
On new automatic transmissions, the N mode is set. Often motorists misuse it. In neutral, it is only allowed to move the machine in tow. You should not turn on this mode if you are coasting due to the automatic transmission may break.
To prolong the life of the automatic transmission, you should regularly change the lubricant. It is allowed to fill only high-quality gear oil recommended by the car manufacturer. You cannot use the cheapest fluids or, to save money, replace them less often than the prescribed period. This behaviour of the driver contributes to the fact that the machine quickly fails. The box overhaul costs are high.
Periodically, it would help if you stopped by at the service station, where specialists diagnose the automatic transmission, carry out repair work. For example, consumables will be replaced: oil seals, clutches, Teflon rings. Having spent a small amount on this work, you will save on expensive repairs to the oil pump and hydraulic unit.