Uncovering the Root Cause of Pressure & Complaints – The Critical Role of Solenoid Modulator & AFL Valves

Common problems among all transmissions are solenoid performance codes, wrong gear starts, harsh or soft shifts, slips and flares, missing gears, delayed engagements, no lockup or TCC slip/cycling and clutch failures. The issue becomes trying to find the root cause of these complaints so that the proper repairs can be made, the vehicle is returned to a grateful customer and you don’t have it fail warranty and come back.

While there can be multiple causes of these drivability complaints or component failures, the valve that feeds the solenoids could be responsible for all of them. Depending upon the manufacturer and transmission, it could be called the solenoid modulator, solenoid regulator or actuator feed limit (AFL) valve. Regardless of name, they function the same and are locations that should always be vacuum tested for wear.

The function of the AFL valve is to provide a consistent fluid pressure to the various solenoids so that they, in turn, can control EPC/torque signal and line pressures and the apply and release of the torque converter and various clutches, bands and brakes. The AFL is fed line pressure, which it regulates to a lower maximum pressure to solenoids, typically in the 80–115 psi range (Figure 1).

Figure 1 – 4L60-E AFL Valve

Whether this feed pressure is going to an on/off, variable bleed or linear-style solenoid, maintaining the specified pressure and flow is critical for proper function of the solenoid. Bore wear at the AFL valve can result in improper solenoid feed pressure and flow, which causes the host of drivability problems mentioned. Depending upon the location of the bore wear, high or low AFL feed will be supplied to the solenoids.

Affect on Solenoids

With a typical low-flow on/off solenoid circuit (Figure 2), the AFL feed to the solenoid is either directed to a shift valve or exhausted.

Figure 2 – Typical On/Off Solenoid Circuit

• When AFL feed pressure/flow is too high, the solenoid can be flooded beyond its ability to exhaust, particularly stroking the shift valve inappropriately.
• If AFL is too low, the shift valves may not stroke fully. Either case results in shift concerns, clutch failures, solenoid codes and TCC apply and release concerns.

Variable Force Pressure Control Solenoids

In a low-flow, variable-force pressure control solenoid, the TCM varies current to the solenoid, which adjusts the amount of feed pressure allowed to enter the control circuit by increasing or decreasing the variable restriction that opens to exhaust (Figure 3).

Figure 3 – Variable Force Solenoid

• In a clutch control solenoid, this control pressure is directed to the clutch control or regulator valves, which allows clutch pressure to be regulated during apply and release. If AFL feed is not corrected as it enters the solenoid, this can negatively impact the shift and clutch integrity if the TCM cannot adapt to the feed variation.
• In a line pressure or TCC pressure control solenoid, incorrect AFL feed results in compromised control pressure being fed to the pressure regulator and TCC regulator valves, resulting in high or low line pressure and TCC apply/release complaints.

Linear Solenoids

Linear solenoids are high-flow applications with AFL feed supplying a solenoid with an internal valve that is commanded by the TCM to regulate the outgoing control pressure to a circuit that feeds the same type of clutch control, pressure regulator and TCC regulator valves as the variable force solenoids (Figure 4). As would be expected, the same drivability complaints occur when AFL feed is too high or low.

Figure 4 – Typical Linear Solenoid

Quickly Find Problems by Vacuum Testing

Regardless of the solenoid type, a worn solenoid modulator or AFL valve can create issues that make a perfectly functioning solenoid appear to have failed and have you chasing the root cause of pressure and shift complaints. These valves are quite active, so they are subject to wear. As they are integral in providing proper pressures to all apply components and help maintain proper line pressure, vacuum testing their integrity during a rebuild is great insurance against a comeback.