Wednesday, January 18, 2017 Detailed Auto Topics
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When a modern automatic transmission begins to fail, few signs may let us know. Computers allow them to operate and not show outward signs of a problem that is occurring. When symptoms begin to show, it is normally too late. By learning to watch the shift patterns, we can recognize signs that may help greatly reduce our costs.

How an automatic transmission helps the engine

Our automatic transmission does far more than connect the engine to the drive wheels. Transmissions are necessary because engines only operate efficiently within a narrow range of speed. Fuel mileage suffers and emissions increase when an engine turns too fast or too slowly.

With an automatic transmission, our engine speed is kept at an optimum level, through a wide range of vehicle speeds. Based on the speed we drive, an automatic transmission shifts into the best gear to keep the engine at the proper RPM. Watching these shift points can provide an early warning of automatic transmission failure.

The speedometer, tachometer and automatic transmission

We measure engine speed in revolutions per minute or RPM. The tachometer is a gauge that shows engine RPM. They equip most vehicles with a tachometer, but many drivers fail to understand its importance. Understanding our automatic transmission is easier by getting familiar with the tachometer. This gauge can also help us to pinpoint automatic transmission problems, before failure occurs.

Tachometer and speedometer in fist gear

When not moving, our speedometer reads zero miles per hour and the tachometer shows the engine idle speed. As the vehicle starts to roll, the miles per hour increase and so does engine RPM. In the illustration above, our vehicle is in first gear. If we remain in first gear, the engine races and we do not move very fast.

Tachometer and speedometer after 2nd gear shift

At a programmed shift-point, the automatic transmission shifts to the next gear. The tachometer shows the engine is turning less RPM, in relation to the vehicle speed. For instance, engine RPM is 1,100 at 10 MPH in first gear. Shifting to second, we travel 25 MPH with the same engine speed. Each time the automatic transmission shifts to a higher gear, the engine needs less RPM to travel at a higher MPH. Watching the tachometer tells us when the automatic transmission shifts.

Automatic transmission shift points

To make an automatic transmission shift, the computer applies or releases the clutches or bands inside the unit. With modern transmissions, sensors supply information to the computer, which determines when the shift occurs. The exact point at which an automatic transmission shifts, varies based on several conditions. For instance, if the accelerator pedal position or APP sensor reports heavy acceleration, the computer waits longer to shift. This increases engine RPM and power output, at the expense of fuel mileage.

Many other factors are considered, including engine vacuum, engine temperature, engine load and the previous driving history. Our automatic transmission learns base-shift points, from our driving style. If we accelerate lightly, our automatic transmission learns to shift sooner. This provides better fuel economy. Driving more aggressively will cause the engine to run faster before the computer commands the shift.

Normal transmission shifting patterns

Learning the normal points, at which our automatic transmission shifts, is easy. Paying attention will reveal subtle changes which may show a problem. Because a computer controls the transmission, it can disguise many symptoms. If we are not familiar with the normal shift patterns, we may easily overlook subtle signs, until it is too late.

Early warning signs of automatic transmission problems

The signs of a transmission problem may be subtle

A problem inside our automatic transmission will often interfere with shift timing. An example is, a vehicle that stays in first gear longer than normal and the RPM drops more than normal after the shift. This may mean second gear is not working. The transmission is shifting from first to third gear, rather than second.

Simple things can cause shift timing problems, but left unaddressed can cause our automatic transmission to fail. A broken spring can cause shift timing to change. If we diagnose and replace the broken spring, the cost is small. If we continue to operate the vehicle with a shift timing issue, it may lead to a more extensive repair and often a complete rebuild of the transmission.

What is automatic transmission slipping?

When the components in an automatic transmission do no firmly lock into gear, they may slip. Slipping creates friction and causes the temperature of the fluid to rise. This is extremely damaging to the transmission. We can see when a transmission slips, by carefully watching the tachometer. When engine speed increases, vehicle speed should correspond. A rise in engine speed, without a change in vehicle speed, shows a slip in our automatic transmission.

When the transmission slips, the engine races but we do not increase speed

An example is pressing the accelerator and seeing the tachometer rise. If the vehicle seems to stay at the same speed or slow down, transmission slipping is likely. We must stop driving the vehicle until the condition is corrected. A slipping transmission will very quickly fail.

The most simple cause of a slip in an automatic transmission is low fluid. A leak can cause our fluid level to decrease. Lower fluid may reduce the pressure available to the transmission and slip will result. At this stage, repairing the leak and servicing the transmission will often cure the problem. Driving for even a short while may cause automatic transmission damage. This damage can result in a need to rebuild or replace the transmission.

In our next Detailed Topic, we discuss other early warning signs of automatic transmission failure.


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