The past 20 years have given us extensive changes in the world of automatic transmissions. The General Motors (GM) lock-up torque converter clutch (TCC) in the early '80s was the first clue that something was about to happen that would revolutionize our segment of the industry. However, the lock-up torque converter wasn't new. I can remember that Studebaker, Jaguar and several other companies used lock-up torque converters as far back as the '50s. It's possible that someone had them before that. I wasn't repairing transmissions in the '50s, but some of those cars were still on the road when I did start in the shop.

The thing that caused the early '80s stir was that THM 350C valve body with an electrical solenoid attached to it. The Chevrolet and GMC pickup trucks had a vacuum switch mounted close to the brake master cylinder that released the TCC when engine vacuum dropped to a certain point. It was not our first encounter with a GM solenoid. The THM 400 had been using a detent solenoid with a simple on/off switch since 1964 but it was so trouble-free that few people bothered to understand how the system worked.

Repair of transmissions has become easier for many people in the industry while some shop owners are closing their doors. Many employees are seeking other types of employment because of changes during the past 20 years. One of the major problems for people repairing automatic transmissions is that they don't understand how transmissions work. They don't understand how the systems function and come together to make a good, working transmission. The days of taking them apart, washing everything, replacing the visually faulty parts along with clutches, gaskets, seals, etc., and delivering it to the customer, are gone. Transmissions today have to be fixed.

Vacuum modulators have been on transmissions for decades but how many people know how they function to make the transmission perform correctly? They're extremely important because if they're not working properly the transmission will either fail shortly after being rebuilt or will shift so rough the customer won't accept it.

I won't go into the makeup of a vacuum modulator here but will cover the effect their function has on the operation of a transmission mainline pressure system. Figure 1 is a typical vacuum modulator layout. Some modulator valves may have only two spools. No matter how many spools, they all function basically the same. Mainline pressure enters between the first two spools on the valve. If the valve is moved to the right, that pressure can go past the valve and down to the right end of the boost valve. The boost valve pushes against the pressure regulator valve, helping the pressure regulator spring and raises mainline pressure in the transmission.

Engine vacuum connected to the vacuum port in Figure 1 retracts the vacuum modulator diaphragm when vacuum is high and allows the modulator valve to move to the left, blocking mainline pressure. When this happens, boost pressure drops and mainline pressure drops accordingly. At maximum engine vacuum there is no boost pressure pushing on the boost valve. The transmission has mainline pressure equal to the value of the pressure regulator spring.

Why have I gone into the explanation of a vacuum modulator system that most transmissions don't even use anymore and some never did use? Take a look at Figure 2 for the answer. The vacuum modulator may be gone, but something has taken its place and the rest of the pressure boost system functions practically the same. The age of electronics has arrived but many systems perform in much the same way they always have. The electronic pressure control unit may be called a force motor, pressure control solenoid or some other name, but most of them perform the same function as a vacuum modulator. Instead of vacuum, they are controlled by electrical current.

The throttle position sensor (TPS) keeps an accurate account of throttle opening. This information, along with input from other sensors, is used by the onboard computer to decide how much torque is being put against the transmission. The computer then regulates the amount of current being used by the pressure control device in Figure 2. That's how mainline pressure is controlled in most modern automatic transmissions. Those that don't understand the system have trouble realizing what happens when a malfunction occurs.

When a transmission goes into limp-in mode it usually stays in second or third gear; no upshifts or downshifts, just second or third gear. When the customer calls, some shops will tell them to “drive it in so we can take a look.” That could be very expensive advice. Limp-in mode usually puts the transmission at maximum mainline pressure. The pressure regulator valve is pushed to the bottom of its bore, blocking the torque converter and lubrication circuit. Take a look at any circuit chart and you'll see how this happens. With the lubrication blocked, an automatic transmission can destroy many very expensive parts in only a few miles.

Transmissions do not have a pressure sensor that stops the vehicle or even illuminates a warning light if mainline pressure goes too high or too low, either of which can destroy the transmission. Pressure gages are as important today as they ever were. Centralized rebuilding facilities that dyno test their transmissions always connect pressure gages to the transmissions being tested. It's not unusual for a transmission shop to do only a “seat of the pants” road test before delivering the vehicle to their customer. With fresh, sharp friction material and new fluid, most transmissions can pass that type of road test with only minimum, or slightly above minimum, mainline pressure. That transmission will usually come back with burned friction material in less than 5,000 miles. Electronics have brought some changes, but the basics remain the same.

Electronics have replaced governor functions as well. Governor pressure always increased in proportion to vehicle speed until the point where governor pressure equaled, or almost equaled, mainline. The convenient story was that governor pressure increased one pound per square inch for every one-mile-per-hour increase in vehicle speed. That was close for some vehicles, but not for others.

Figure 3 shows how a solenoid has taken the place of a governor. The solenoid is normally open, allowing mainline pressure going through orifice “A” to exhaust back into the sump. A spring is keeping the shift valve pushed to the right (downshifted position) and the servo or clutch is exhausted through the exhaust shown above the shift valve.

Most two-wire solenoids have current to them when the transmission is operating and the circuit is completed when the onboard computer or transmission control unit completes the ground circuit. When the vehicle reaches speed for a shift to occur, the computer completes the ground, closing the solenoid. The solenoid valve in Figure 3 moves to the left, blocking the fluid entering the solenoid. Pressure builds between the solenoid and the shift valve, moving the shift valve to the left, compressing the return spring. Servo or clutch exhaust is blocked but mainline fluid passing through orifice "B" is opened to the servo or clutch apply passage. The transmission upshifts to the next higher gear. Shift valve spring pressure or throttle valve (TV) pressure are not important factors for shift timing in a solenoid-operated transmission.

Control has been handed to solenoids, but they too have difficulties from time to time. Technicians who do not thoroughly and regularly check solenoids are having what they think is more than their share of grief. Figure 4 illustrates that every solenoid should be checked for proper resistance and continuity. It is important that fluid flow through the solenoid is not partially or fully restricted when the solenoid is supposed to be open. A restricted solenoid can keep the transmission in the wrong gear no matter what the computer tells it to do.

A few things have changed inside the transmission but the hydraulic systems still function very much as they have for decades. The biggest changes have been outside the transmission. The computer and sensors keep a close watch on all functions and operate the transmission to achieve maximum efficiency at all times. This can be noted when you compare the operation of a governor/TV transmission to a computer-controlled unit on a cold day. On initial fire-up, when you drop a governor/TV transmission in gear and drive away, the shifts are almost always somewhat delayed because of thick fluid or the valves dragging in their bores. Do the same thing with a computer-controlled transmission and it shifts at basically the same time, hot or cold. Fluid viscosity is not a factor.

Technicians who have kept up with the changes as they appeared find that automatic transmission repair is still a great career.

Ed Anderson, former manager of ASA's Mechanical Division, now resides in Arkansas. He has served the automotive industry for more than 40 years, including a stint as a shop owner.

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