An understanding of hydraulics is essential for properly diagnosing and repairing transmission problems. This is part one of a two-part article on how an understanding of converter clutch control aids in transmission diagnostics. Part one describes fluid path and torque converter terminology. Part two addresses the valves that control those circuits and the clutch control.


As rebuilders, we often divide responsibility between sub-assemblies. Whether we build torque converters, valve bodies or transmissions, we place an expectation on the condition of the pump, gear train, valve body or converter that will be paired to our build. However, during diagnosis of any problem, we need to be able to trace and diagnose the entire affected circuit, not just the part of the circuit we are responsible for. Whether we build transmissions or converters, we must be familiar with the circuitry involved in each sub-assembly to ensure successful diagnosis. In modern vehicles, the converter clutch is very active and has become critical to drivability.

Most converter failures are related to the TC clutch and overheating conditions. To sort out these failures, we will divide the types and paths of various designs. Torque converter circuits include:

• Charge
• Release
• Apply (CA): Converter Apply
• CBY: Converter Bypass
• CI: Impeller In
• CT: Turbine Out

These definitions may vary between manufacturers. 

Defining One-, Two- & Three-Path Converters

Because of advancements in converter technology, fluid control has changed significantly over time. We start with the simple “fluid coupling,” defined as a stator-less, open converter. Early open (non-lockup) converters had one path: in/out. The inlet was charged by way of the pump stator support and fed by the line pressure regulator valve (Figures 1 & 2). 

Figure 1 – 4L80 Converter
Figure 2 – Honda Main Regulator Valve

Charge pressure purges air from the converter and ensures fluid force between the impeller and turbine. Low converter pressure shows up as higher engine stall, low power, poor mileage, turbine blade damage and noise. 

Before we proceed, we should understand that pressure and flow are not the same.  Pressure is created by and ahead of a restriction as flow is forced to it. A converter assembly can be charged, or build up internal pressure, because of a designed restriction on the outlet. Flow exiting a converter will be sporadic if charge pressure is low. A poor charge with traditional ATF can starve the lube circuit for several miles after an initial start. Synthetic ATF improves flow in extreme cold, but the charge relies on proper valve and pump operation. 

Internal pressure and the movement of fluid perform the work. Work creates heat, so circulation or flow is required to reduce heat. A non-lockup converter generates heat whenever the turbine RPM is not matched to the cover RPM. During deceleration, the wheels drive the turbine shaft faster than engine RPM, which also generates heat. Coupling speed (lockup in a TCC) is when the cover of the converter is turning the same speed as the turbine shaft.   
The two-path lockup converter has a release oil circuit that flows through the center of the turbine shaft and exits the shaft between the TCC piston and cover.  This pressure releases the piston and friction material from contact with the cover. The second path is apply pressure: the clamping force loading the piston onto the cover. As the release oil is exhausted, apply pressure is increased.

Two-path clutch control typically requires three to four valves to regulate the slip rate and turbine shaft RPM. The main regulator valve has a circuit leading to the converter. In the circuit is a TC control valve, which acts as the gateway for apply and release oil. Once the control valve is stroked by TC solenoid output, an apply regulator starts the job of regulating clutch slip. Excess apply pressure can distort the TC piston and overload the damper or clutch material.

Many current TCC pistons are preloaded toward the cover. The GM 6L80 illustrated is one of those, a ZF6HP or Ford 6R60/80 are other examples.  “Preloaded” means they are always applied with the engine off so the turbine shaft will be driven upon start until the clutch releases. The transmission pump and valve control must be adequate to release the clutch from the cover. With insufficient pump volume at idle in Drive or Reverse, a rough idle or engine stall occurs.

As release pressure is reduced, apply pressure is increased, which affects ATF flow through the converter. Let me repeat that: flow through the converter, not flow to the cooler. When the clutch is not applied, the clutch has moved away from the cover for oil to pass. This clutch release clearance is a restriction. Upon lockup, pressure clamps the clutch, but flow from the TC control valve goes to the cooler instead of the converter. Typically, flow has a significant increase at full lockup, as apply pressure is now “dead-headed.” Pressure is holding the clutch, but flow is no longer through the converter (Figure 3).

Figure 3 – GM 6L90, 2011 Truck (with Tow Pack) SonnaFlow® Chart

Caption: Condition: Normal Drive & TCC Control

Notes: Values given are with unit over 125° F. TC clutch may not cancel with brake or on deceleration. The cooler return is the lower line on bell housing. Flow will increase by .2 to .5 gpm at TCC apply. Suggest use of graphing multimeter or oscilloscope set at 500 Hz scale for best accuracy
A scan tool and flow meter can be used to monitor TC solenoid activity and the change in flow. As the clutch applies and releases, the turbine RPM, solenoid amperage and slip speed can be monitored. This data verifies the condition of the clutch and valve control. Testing apply pressure is rarely accessible unless the valve body is tapped for gauges.

The three-path converter circuits require more depth of explanation, as there are two types of three-path converter.

• A turbine-fixed three-path has the TC piston splined to the turbine hub by way of a damper, and the piston travels toward the cover. 
• A cover-fixed three-path utilizes a multi-faced clutch built into the cover, similar to a transmission clutch drum or brake clutch.

The advantage of a three-path, turbine-fixed clutch is heat dissipation. As CBY (bypass, release) oil pressure is decreased, the clutch can maintain a modulated slip RPM. Slippage creates heat, but in this design there is continuous flow through CI (impeller) and the CT (turbine) circuit. Heat dissipation with slip is an advantage, but the division of those circuits is critical. Cross leakage, worn seals or circuit restrictions are detrimental and cause RPM cycling, no lockup or engine stall. Examples of problem areas include the Ford AX4S oil pump shaft and seals (Figure 4), Honda impeller hub O-ring failure or radiator restrictions.

Figure 4 – AX4S Three-Path Stator & Pump Shaft

The Honda converter charge is directly from the main regulator valve. With this design, charge is often half of line pressure (Figure 2). Line pressure is priority oil, so circuit leaks or any drop in pump volume reduces converter release pressure, allowing the clutch to drag on the cover. Low converter charge combined with cross leaks results in TC lining failure. Increased cooler restriction opens the torque converter check valve, which reduces apply pressure. When it sticks open from repeated cycling, converter charge is low and the applied slip rate increases. Cooler and converter restrictions also cause pressure to react on valve spool differentials (reaction area). This can position them in a partial stroke, which limits flow in/out of the converter.

Both conditions create overheated linings or converters. This will be further detailed part two of this article.

In the three-path, cover-fixed design, the clutch applies from pressure fed to the clutch through the turbine shaft (Figure 5).

Figure 5 – ZF8 Mechanical View

All the previous clutches applied to the cover by exhausting release oil from the turbine shaft. In the cover-fixed design, the clutch applies by pressure feeding it. The cover-fixed paths are TC clutch apply, charge and converter out.   

Diagnosing problems within a three-path converter requires knowing whether it is turbine-fixed or cover-fixed. Both types can be tested externally with a flow meter, and the flow data will appear similar. The three-path continually flows ATF, so we do not see the dramatic change in flow as the clutch piston moves off the cover.  Both types of three-path will have a small deflection (.3–.7 gpm) in flow as the valves move to apply the clutch (Figure 6).

Figure 6 – Honda 4- & 5-Speed, Odyssey, MAXA SonnaFlow® Chart

Caption: Concerns of high temperature: TCC lining failure; overheat code; TC 740 Code; TCC shudder. Operating temperature high 240–360° F prior to converter charge regulated pressure regulator valve. After valve body upgrades – 180° F maximum.

This deflection indicates command, solenoid activity and valve movement. The scan tool can monitor slip rate and solenoid amperage as mentioned in the two path section. Verifying the specific clutch design requires an oil circuit or exploded view of the converter.  We will dive into those oil circuits and valve body issues in the next article.