Diagnosing electronic automatic transaxles is not always simple. Join us as we take the 'diagnostic fear' out of getting a third generation Camry into gear.

The first electronic transmission (electrically driven with no gear shift) was developed by E.M. Frazer in 1927. The first vehicle to feature electronic shifting (via a remote control gearshift, nick-named 'the electric hand') was a 1935 Hudson. Electronic transaxles/ transmissions, as we know them today, have been with us for almost three decades. However, manufacturers didn't bring them into production until the early '80s. One of the first manufacturers to equip its transmissions with electronic controls was Toyota, in 1983. Although Toyota's use of electronic controls has evolved in the past 15 years, their transaxle is still a simple mechanical machine.

A machine, by definition, is any device that allows you to do work (transmit energy). We know that a conventional automatic transaxle's operation is based on hydraulics. A hydraulic machine is something that is operated by a liquid under pressure. In this case, the liquid is automatic transaxle fluid (ATF). The ATF fills many roles inside a transaxle. It cools, lubricates, moves valves and pistons, and transmits torque from the engine to the transaxle via the torque converter. The transaxles covered in this article all use Dexron® II ATF.

The third generation (1992-'96) Camry can be equipped with one of three different 4-spd. automatic transaxles. Camry models equipped with 4-cyl. engines use the A140E transaxle and those equipped with the 6-cyl. engine use the A540E (1992-'93) or A541E (1994-'96) transaxles. The 1992-'93 models are equipped with the 3VZ-FE V6 engine and the 1994-'96 models are equipped with the 1MZ-FE V6 engine. The A541E transaxle is an enhanced version of the A540E model and it includes an enhanced electronic control system. The electronic control system on the A541E evolved from OBD I to OBD II. The hydraulic control systems, including the gear train and torque converter clutch, were also revised in order to be compatible with the new electronic system.

Transaxle basics

In an electronic transaxle, the shift and torque converter lockup timing are controlled by a computer in order to maximize driving comfort and performance as well as fuel economy. In addition to providing self-diagnostics and reducing gear shift shock, the electronic control also reduces vehicle 'squat,' which occurs when the vehicle starts out. All of this is achieved through the use of various sensors, switches, and actuators. In addition, a fail-safe mode is built into the control module programming so the transaxle will still operate when a malfunction occurs. The third generation Camry's transaxle and engine are both controlled by the powertrain control module (PCM).

Although this sounds complicated, the only difference between a conventional and an electronic transaxle is the way the hydraulics are controlled. For example, a governor was used to control shift timing in a conventional transaxle. The governor was used to sense road speed by being mounted on or driven by the transaxle output shaft. It used weights and springs to control an opening in the body of the governor. As the output shaft rotated, centrifugal force would carry the weights outward. As the weights spun outward, a pressure port in the governor shaft would allow fluid (line pressure) to reach the shift valves. The pressure (now called governor pressure) would build against the shift valve until it was great enough to overcome the combined throttle and spring pressure on the other side of the valve and the transaxle would upshift. Consequently, as pressure decreased, a spring, located on the throttle pressure side, would push the valve back and cause the transaxle to downshift.

The electronic transaxles use a vehicle speed sensor (VSS) in place of a governor. The VSS senses road speed and sends an electronic signal in the form of on and off pulses (the number of pulses per revolution varies according to the sensor's role) to the PCM. The PCM then sends a signal to a shift solenoid which controls the fluid pressure on the shift valves.

There are three solenoid valves and a few speed sensors in these transaxles. The No. 1 & No. 2 solenoid valves are located in the valve body and control the shifting functions. The third solenoid or SL solenoid is located in the valve body and controls the operation of the lockup torque converter. The A541E transaxle uses a fourth solenoid called the SLN solenoid. The SLN solenoid, also located in the valve body, is used to control the hydraulic pressure acting on the accumulator control valve when gears are shifted, thus providing smooth gear shifting and less shift shock when the transaxle is shifted into gear from park or neutral.

The A140E used two vehicle speed sensors until 1994. The No. 2 VSS is a reed switch located on the differential drive pinion and it replaces the governor valve. This is the primary input to the PCM for vehicle road speed. The No. 1 VSS is located in the transaxle case (post-1991) and is gear-driven off the differential ring gear. The pre-'92 or speedometer cable-equipped Camrys used a reed switch type No. 1 VSS which was located in the speedometer. Both No. 1 sensors served as a backup VSS for transaxle control. Additionally, the transaxle mounted sensor was also used to provide a signal to the speedometer in the instrument cluster since speedometer cables were not used after 1991. The A540E also used two speed sensors. These sensors mirrored the 1992-'93 A140E in both operation and location. The A140E transaxles from 1994-'96 dropped the No. 2 VSS and used the No. 1 VSS for speedometer operation and the primary road speed input for transaxle control.

The A541E transaxle still continued to use two vehicle speed sensors, but in a different manner. The No. 1 VSS was still located on the transaxle case and was gear-driven by the differential ring gear. It also continued to function as both, a speedometer signal and a primary road speed input for transaxle control.

The No. 2 VSS was renamed the direct clutch speed sensor. It was changed from a reed switch to a permanent magnet type sensor, similar to an ABS wheel speed sensor, and is mounted in the transaxle case. The direct clutch speed sensor was used to detect the input shaft speed, via the direct clutch drum, from first to third gear. By comparing the input shaft speed and the No. 1 VSS signal, the PCM detects the shift timing of the gears and appropriately controls engine torque and hydraulic pressure in response to various conditions via the torque converter lockup (SL) solenoid.

The line pressure on these transaxles is not electronically controlled. Therefore, it is supplied via the pump and mechanically controlled via the pressure relief valve (PRV). Line pressure is used to control clutch apply pistons and servos in the transaxle. The line pressure must also vary proportionately with engine speed in order to provide adequate clamping or holding forces for bands and clutches.

Line pressure is coordinated with engine load via a throttle valve (TV). After the line pressure passes the TV, it is known as throttle or TV pressure. The TV pressure on a conventional transaxle is regulated mechanically either by a throttle cable or engine vacuum via the vacuum modulator. The throttle cable is mechanically connected to the TV and moves it proportionately to throttle position. Whereas the engine vacuum method moves the TV proportionately to engine load via a vacuum diaphragm in the vacuum modulator. The TV pressure in a conventional transaxle performs two functions: it moves shift valves in the valve body and it helps adjust line pressure by backing up the spring on the PRV.

These electronic transaxles still use a throttle cable to actuate the TV, but the TV is only used to help boost line pressure. The TV pressure is determined electronically via the throttle position sensor (TPS). The TPS operates in the same manner as with fuel delivery, except that the PCM also uses the information for transaxle control. It does this by measuring the amount of the throttle opening and combining it with the VSS signal to determine the proper shift timing.

Other important inputs to the computer regarding transaxle control are the engine coolant temperature (ECT) sensor, brake switch, and the pattern select switch. The ECT sensor has direct control over the transaxle operation by limiting overdrive on a cold engine (allows engine to warm up quicker to minimize emissions) and controlling torque converter lockup. The brake switch also affects torque converter lockup by disconnecting it whenever the brake pedal is depressed. The final influence over transaxle control is the pattern select switch. It is usually located near the shift lever on the console and allows the driver to select one of three programmed shift patterns (normal, power, and economy) which covers most of the anticipated types of driving that the vehicle will experience.

Diagnosing transaxle problems

Troubles with electronically controlled automatic transaxles can occur in one of three possible areas: the engine, the control module or its circuits, or the transaxle itself. The key to a successful diagnosis is following a logical approach. The first step is to determine which of the three possible areas is the source of the problem and then start testing the simplest operation. When checking the engine for signs of trouble, remember to check the cruise control system on vehicles that are equipped with a separate cruise control electronic control unit (ECU). The cruise control ECU can interfere with transaxle control by taking priority under light load conditions, such as downshifting on hills.

Once the problem has been isolated to a specific area, such as the transaxle, you must know whether the problem is electrical or mechanical. This can be done by performing the following steps. First, perform a preliminary inspection. This inspection includes checking the transaxle and differential fluid level and condition. If the fluid is low, fill it to capacity and if the fluid is black or smells burnt, replace it. A preliminary inspection also includes verifying that the throttle cable, shift control cable, engine idle speed, and the park/neutral or inhibitor switch are adjusted properly.

Next, check the vehicle for any trouble codes. Codes can be retrieved manually (flashing) on OBD I systems via the O/D OFF lamp which is located in the instrument cluster or automatically by using a scan tool. If using the manual method, the O/D lamp can be checked for proper operation by cycling the O/D switch, located on the shift lever, on and off. Codes on the A541E and 1996 A140E transaxles must be retrieved by a scan tool because these vehicles are equipped with OBD II systems. Any stored trouble codes should be diagnosed and repaired accordingly. If there are any engine trouble codes stored, they should be diagnosed and repaired first, since engine performance directly affects transaxle operation.

If no codes are stored, the TPS signal, brake switch signal, and shift position signal circuits to the PCM on OBD I vehicles can be checked using the following procedure. Otherwise, proceed to the mechanical system checks. The aforementioned signal circuits can be checked with a digital volt/ohmmeter (DVOM). Connect the positive probe of the DVOM to the Tt terminal and the negative probe to the E1 terminal of DLC 2 and turn the ignition on, but don't start the engine.

Check the TPS signal by monitoring the DVOM while moving the throttle gradually from closed to wide open throttle (WOT). The voltage should increase smoothly (no drops) in approximately one volt increments from 0V to 8V. The brake switch signal can be checked while holding the TPS at WOT and depressing the brake pedal. When the brake pedal is depressed, the DVOM should read 0V. When the pedal is released, the DVOM should indicate 8V.

The shift position signal can be checked by road testing the vehicle above six mph with the O/D switch on and the engine coolant temperature above 80°F. The voltage should be as specified in figure 1 (pg. 38) for each gear range. The gear position can be determined by a light shock or engine rpm change when shifting. If the voltage is not as specified for the TPS, brake, or shift position signals, inspect the electronic shift control circuit. If everything tests as specified, continue with the mechanical tests.

Mechanical tests

The easiest and first test to start with is the stall speed test. It should be performed after the ATF has reached normal operating temperature (122-176°F). Never run a stall speed test for longer than five seconds or transaxle overheating/damage may occur.

In order to perform the stall test, block all the wheels and verify that the area is clear both in the front and in the rear of the vehicle. If the vehicle is not equipped with a tachometer, connect one for reference. This test can be facilitated by using another person who both reads the tachometer and watches the drive wheels for slipping. Also, make sure there is good traction. If the wheels are allowed to slip, the test will be invalid. Start the vehicle and apply the parking brake. Apply the brakes fully with your left foot, shift the gear selector into drive, and press the accelerator to the floor. Quickly check the rpm reading on the tachometer. This reading is the stall speed for the vehicle. Repeat the test with the gear selector in reverse. The stall speeds can be evaluated as shown in figure 2 (pg. 38). Perform any necessary repairs before proceeding.

Next, perform an engagement or time lag test. This is performed with the ATF at normal operating temperature, the gear selector in neutral with the engine idling, and the parking brake fully applied. Engine idle speed is critical; if the idle speed is incorrect, the test results will be inaccurate. The time delay from neutral to drive should be less than 1.2 seconds and the delay from neutral to reverse should be less than 1.5 seconds. If lag times are excessive, the line pressure may be low. Other causes could be a worn forward clutch (drive) or direct clutch (reverse), an O/D one-way clutch that is not operating correctly, or a worn first and reverse brake (reverse). Make any necessary repairs before proceeding.

The last mechanical test is to verify that line pressure is correct in both the drive and reverse ranges, both at idle and stall speed. The line pressures can be evaluated as illustrated in figure 3 (pg. 40). Perform any of the necessary repairs. If all of the mechanical and preliminary checks turn up nothing, isolate the transaxle problem by performing a manual shift test.

A manual shift test will isolate the problem in the transaxle and indicate whether it is an internal mechanical one or is located in the electrical circuit. The manual shift test can be performed by disconnecting the solenoid harness connector at the transaxle and going for a road test. The transaxle shift and gear positions should correspond with the readings in figure 4. After the test, reconnect the solenoid harness to the transaxle and clear the related trouble codes. If any abnormalities occur during the manual shift test, the trouble is an internal mechanical problem. If the test reveals no problems, the malfunction is in the electronic control system.

This article is not intended to make you a diagnostic ace when it comes to electronic transaxles, but it should prevent you from shying away from the potential for some big money. Although we focused on Toyota Camry transaxles, much of the theory can be carried over to other manufacturers. The electronic transaxle may seem intimidating at first, but if you can diagnose fuel injection systems, you already possess the skills needed to solve some seemingly complex electronic A/T problems. So the next time a vehicle rolls into your bay with an A/T problem, take a stab at it before farming it out. You may be surprised at the result because more than one electronic A/T has caused some baffling driveability problems!

Motor Age thanks Terry Ogilvie at Snap-on Diagnostics, Balco Div. for sharing his Toyota field experience with us. Also, thanks to H.G. Motorcar, Corp. for supplying us with a vehicle on which to work.

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Last Update: Wed Jan 3 18:10:28 GST 1999

Alex Sherbakov (alex@americamail.com)