A4BF2/A4AF3 Problems Before SPEAKER and After Rebuild by Mike Souza It s Monday morning: A customer arrives at your shop in a 2000 Elantra and a harsh 4-3 coastdown and kickdown shift complaint. It uses an A4BF2 and was used in 2000 and 2002 Elantras and Tiburons. The 2000 and later Accent also has a version of this unit; the A4AF3. Both the A4BF2 and A4AF3 are equipped with six solenoids (figure 1) instead of four like the earlier A4AF1 and 2, and the A4BF1. Dennis Madden wrote three articles (2006 August, September and October issues of GEARS) that go into great detail on how this unit functions with the addition of pressure control solenoid B and shift solenoid C. The August article, along with the hydraulic diagrams (figures 2 4), explains in great detail how pressure control solenoid B controls the forward (rear) clutch when shifting from fourth gear back down to third in three phases. As Dennis mentions in his article, we ve heard no complaints on the HotLine of any downshift issues on the earlier, four-solenoid version, in which the forward clutch was controlled by the line pressure control solenoid. Harsh Downshift In the last couple years, quite a few of these harsh downshift complaints have surfaced on the new, sixsolenoid version. A harsh coastdown or forced downshift into third with no codes could indicate a mechanical problem with pressure control solenoid B, without causing any noticeable upshift issues. Since these solenoids are hard-wired to the connector, the repair requires replacing all six solenoids. A more practical solution would be to cut and splice in another solenoid. Pressure control solenoid A, which has the black wire, controls shift feel (line pressure). Along side of pressure control solenoid A is pressure control solenoid B, which has the blue wire (figure 1). These two solenoids can be interchanged as a direct replacement. No Forward in Drive; Manual Low and Reverse Okay Other problems after a rebuild may include no forward in drive while manual low and reverse are working fine. With the selector in drive, 1st gear, shift solenoids A and B are energized, moving the shift control valve to the 1st gear position. Pressure control solenoid B is cycled by the computer, controlling the apply rate of the forward (rear) clutch via pressure control valve B. Once the forward clutch is fully applied, pressure control solenoid B remains off until the clutch releases and then reapplies. 12 GEARS July 2009
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A4BF2/A4AF3 Problems Before and After Rebuild Shift solenoid A is turned off, moving the Shift Control Valve to the 3rd Gear position. This drains the oil holding the Rear Clutch Exhaust Valve and 2-3/4-3 Shift Valve. But because of the difference in valve diameters and the forces working to move these valves (spring and hydraulic), the 2-3/4-3 Shift Valve strokes first. This applies the Front Clutch and releases the Servo. As with the other shifts, shift feel is controlled by Pressure Control Solenoid A is turned off; the Front Clutch and the Kickdown Servo apply and release receive full line pressure. About this time, pressure on the right side of the Rear Clutch Exhaust Valve drops enough to allow the valve to move. Recall that Pressure Control Solenoid B controls the apply rate of the Rear Clutch, and the Rear clutch reapplies during the 4-3 downshift. In 4th Gear, Pressure Control Solenoid B is off, providing full line pressure between the two parts of the Rear Clutch Exhaust Valve. During the 4-3 downshift, Pressure Control Solenoid B reduces the pressure to the Rear clutch Exhaust Valve. If this pressure is above about 45 PSI, the two valves in the Rear Clutch Exhaust Valve train will split apart. Rear clutch apply oil seats the #3 checkball. The apply rate of the Rear Clutch is controlled by the orifice next to the #3 checkball. This is also the sequence used if Pressure Control Solenoid B fails closed (failsafe). 14 GEARS July 2009
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A4BF2/A4AF3 Problems Before and After Rebuild Recall that Pressure Control Solenoid B controls the apply rate of the Rear Clutch, and the Rear Clutch, and the Rear Clutch reapplies during the 4-3 downshift. In 4th Gear, Pressure Control Solenoid B is off, providing full line pressure between the two parts of the Rear Clutch Exhaust Valve. During the 4-3 downshift, Pressure Control Solenoid B reduces the pressure to the Rear Clutch Exhaust Valve. If this pressure is below about 45 PSI, both parts of the Rear Clutch Exhaust Valve train will stroke. Now the apply rate of the Rear Clutch is controlled solely by Pressure Control Valve B and Pressure Control Solenoid B. Shift Solenoids A and B are energized, moving the Shift Control Valve to the 1st Gear position. Pressure Control Solenoid B is cycled by the computer, controlling the apply rate of the Rear Clutch via Pressure control Valve B. Once the Rear Clutch is fully applied, Pressure Control Solenoid B remains off until the Rear Clutch releases and then reapplies. The sole purpose of Pressure Control Solenoid B is to control the apply rate of the Rear Clutch. Pressure Control Solenoid A is on, which keeps the Low/reverse Brake off. Shift Solenoid C is off. 16 GEARS July 2009
The hydraulic diagrams (figures 6 and 7) show why this would cause no forward movement in drive. The sole purpose of pressure control solenoid B is to control the apply rate of the forward clutch. Pressure control solenoid A is on, which keeps the low/reverse brake off. Shift solenoid C is off (figure 5). As mentioned before, pressure control solenoids A and B are identical and located next to each other. The only visible difference is the wire color, so it d be easy to accidentally swap them during a rebuild, without setting any codes. The hydraulic diagrams (figures 6 and 7) show why this would cause no forward movement in drive. When placed in the correct locations, pressure control solenoid A is on and the low/reverse clutch is released, while pressure control solenoid B is off and the forward clutch applied. If installed incorrectly (figure 7), when pressure control solenoid B is off, the low/reverse clutch applies, and while pressure control solenoid A is on, the forward clutch releases. The solution? Switch the solenoids back where they belong, and the transmission should start working properly. This mistake can really have you pulling your hair out, with no codes or obvious failures to guide you. Hopefully this information will help prevent dealing with the problems before and after a rebuild. GEARS July 2009 17