By Pete Luban O nce upon a time (most fairy tales begin like this), when Honda/Acura electronic controls were relatively simple, it wasn’t too difficult to diagnose a shift problem of an electri- cal nature. If the com- mands were correct, the chances are it was a shift- solenoid problem. If the commands were not cor- rect, then maybe you had a speed-sensor problem, a console-switch problem or a faulty A/T controller. Maybe once in a while an oddball problem such as a TPS-circuit problem or an engine-temperature prob- lem would pop up, but for the most part it remained uncomplicated. As this ar- ticle progresses, it will be- come obvious that these systems have become con- siderably more complex. These early electronic systems began showing up about 1987-88 in Acura Legends with the L5 transaxle and the Honda Prelude with the K4 transaxle. Remember the K4? That’s the one with the shift solenoids inside the unit and you find out they’re bad after the trans is back in the car. Figure 1 shows the transmission- control system in a 1988 Acura Legend. Figure 2 is the related code chart. As you can see, these systems were capable of only 12 codes. The next major change took place in the 1991 model year, when the Acura Legend was introduced with the in-line MPYA transmission. Figure 3 illustrates the transmission- control system and its related components. As you can see, this system is a little more complex than the previous one. Figure 4 is the related code chart that indicates it is capable of 15 codes because of the addition of another speed sensor and a linear solenoid to control accumulator pressure through the throttle “B” valve. Around the 1994 model year the transmission controllers became more sophisticated with the addition of a Grade Logic Control System that is classified as a “Fuzzy Logic” system (which is what I have after happy hour). It’s a form of artificial intelligence that lets computers respond to changing conditions, which means that the 3-4 and 4-3 shift schedules are adjusted automatically for the steepness of upgrade and downgrade driving as opposed to flat-road driving in order to prevent 3-4/4-3 shift shuttling. The example in Figure 5 is the grade- logic strategies programmed into a 1994 Honda Accord control system. Now we get serious. In the 1997 model year some Honda/Acura models received additional solenoids as well as pressure switches, which created a more- complex control system. In this example we will be looking at the transmission-control system in the 1997 Acura 3.0 CL with the M7ZA transaxle. Figure 6 illustrates the transmission-control system and its related components. Notice there are THREE shift solenoids, TWO pressure-control solenoids, only ONE lockup solenoid and TWO pressure switches. The transaxle- control system has grown Honda/Acura: Simple As Shift Solenoid A, B, C Shift Pointers P  R  N  D  D  2  1 Figure 1 1988 Acura Legend With L5 Transaxle Ignition Switch BAT             IG1 PGM-FI ECU B2 B7 B12 B1 B6 A15 A16 A11 A6 A7 A10 A 12 A1 A2 A3 A4 A5 A13 A8 B10 Speed Pulser A/T Control Unit Throttle Angle Sensor Coolant Temp. Sensor A/T Speed Pulser Ignition Coil Brake-Light Switch BAT Shift- Position Switch S4 Switch Cooling-Fan Control Unit PGM-FI ECU A18 B11 B9 B4 B5 B3 B8 Shift-Control Solenoid Valve A Shift-Control Solenoid Valve B Lockup-Control Solenoid Valve A Lockup-Control Solenoid Valve B S4 Indicator Light (with S4 Switch) S4 Indicator Light S3 Indicator Light Shift-Position Indicator Printed in the July 1998 Transmission Digest