Stroke Choke - Transmission Digest

Stroke Choke

The forward-clutch control valve in the AX4N valve body (see Figure 1) plays a large role in the apply and release of the forward clutch (see Figure 2). The valve must be stroked or opened to allow pressure into the forward-clutch circuit to the shift valves and closed or unstroked to block pressure from the circuit.

Technically Speaking

  • Author: Wayne Colonna, Technical Editor

Sticking forward-clutch control valve causes shift problems in AX4N

The forward-clutch control valve in the AX4N valve body (see Figure 1) plays a large role in the apply and release of the forward clutch (see Figure 2). The valve must be stroked or opened to allow pressure into the forward-clutch circuit to the shift valves and closed or unstroked to block pressure from the circuit.

Since the forward clutch is on for reverse, first and second gears, if the valve gets stuck in an unstroked or closed position, a condition of no movement either forward or reverse would occur (see Figure 3). The most-common way this valve “chokes on the stroke” is after a shift into 3rd or 4th has taken place. The valve is closed, blocking pressure to the forward clutch. When the vehicle comes to a stop, the valve sticks and stays closed temporarily, causing the transmission to neutralize at a stop. With solenoid 3 pressure working on the end of the valve, the valve suddenly breaks free and strokes, engaging the forward clutch and causing the common complaint of a neutral bang engagement at a stop.

A simple test to help you determine whether the forward-clutch control valve is causing the transaxle to neutral at a stop is to drive the vehicle with the shift selector in the D range, or press the OD-cancel switch so that it indicates OD OFF (vehicles without an OD-cancel switch have a shift quadrant of OD, D and L; with the switch, the quadrant has D, 2 and 1). If you’re road-testing the vehicle this way and the neutral condition at a stop goes away, it is very likely that the forward-clutch control valve is sticking. The reason that this is helpful is that when OD is not selected, the PCM does not energize shift solenoid 3 during the upshift to 3rd gear. This will leave the forward-clutch control valve in the upshifted position, keeping the forward clutch applied (see Figure 4).

The other way this valve can choke on the stroke is that it gets stuck in the stroked or upshifted position, where it will allow the forward-clutch apply circuit to be charged with pressure at all times. Should this occur, the transmission no longer will have fourth gear. This no upshift to fourth gear is not a 1-2-3-neutral complaint. If it neutralized going into 4th, then the shift indeed was made but the OD band didn’t apply. That is not the case here. Once you reach third you stay in third, regardless of increased speed.

Figure 5 shows what occurs hydraulically after the transmission has shifted to 3rd gear with the forward-clutch control valve stuck in the upshifted position (the selector lever is in the OD position). Solenoid 3 pressure is not present, but with the valve being stuck in the upshifted position it continues to charge the forward-clutch circuit past the valve. This allows mainline pressure to be present at two different locations on the 3-4 shift valve, preventing this valve from shifting, which keeps the forward clutch applied and prevents the 3-4 shift.

Figure 6 shows what takes place when the forward-clutch control valve operates normally after the transmission shifts to 3rd gear. You will notice that the valve has moved to the left, allowing the forward-clutch pressure to exhaust and leaving the 3-4 shift valve free to stroke into an upshifted position when the PCM commands the shift to fourth gear.

If you find that the forward-clutch control valve is sticking, cleaning up any debris and checking the valve and the valve bore for any damage are recommended. If the valve retainer is bent or broken, it can be replaced with Ford part number F8DZ-7F194-AA. If the forward-clutch control valve shows any signs of damage on either the valve nose or valve spools, you should replace it. There are aftermarket companies that provide a replacement forward-clutch control valve.

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