4L60E Converter Valves:
Operation, Test Procedures and the Effects of Wear
by Bob Warnke, Sonnax

The 4L60E was introduced in 1993. Since then, many changes have taken place, both in terms of mechanical components, hydraulic components, and electronic components. One of the most important changes to the 4L60E was the introduction of the PWM converter clutch system. Let's look at how both the non-PWM and PWM systems work. We'll also take a look at the newest change in converter clutch apply-the 1997 and later EC³ system.

Valve and Application Identification
1993 and '94 non-PWM (or on/off) valve bodies contain a converter clutch signal valve. You can identify this transmission by its 12-pin case connector, and it only has a 3-2 control solenoid on the front of the valve body. The three- spooled, steel, one-piece converter clutch signal valve installs in front of the EPC force motor.

1995 and later PWM (Pulse Width Modulated) valve bodies contain the regulated apply valve in the same bore as the early signal valve. The isolator valve and spring work with the regulated-apply valve. The valve body casting is the same, but a TCC PWM solenoid has been added, so it now has two solenoids at the front of the valve body. The case connector on this transmission has 13 pins instead of the 12. The separator plate for the PWM design has two holes (large enough for a 19/64" drill) above each solenoid. The non-PWM with a single 3-2 control solenoid has only one hole over the solenoid.

TCC PWM refers to the TCC pulse width modulated solenoid, used to control the level of flow. The TCC PWM solenoid is N.O. (normally open), which means no actuator feed limit oil goes to the isolator valve when the solenoid is off.

The converter clutch control valve is located in the stator support of both units, and is controlled by the TCC on/off solenoid. The PWM and non-PWM valves are not interchangeable; but the non-PWM will interchange with the valve in the earlier 700R4.

Important: Whenever oil passes through an orifice or control valve, that oil usually changes names. This is true for actuator feed limit, converter clutch signal, torque signal, converter feed and regulated apply oil. These circuits are affected by the valves we've already described; use the oil circuits provided to follow along with the explanations.

Do the early non-PWM valve bodies wear out?
When the bore of the converter clutch signal valve wears out, second clutch signal oil leaks to exhaust (see non-PWM valve design illustration, figure 1). When this leakage occurs, the TCC solenoid will not receive sufficient converter clutch signal oil. The result is insufficient pressure for complete stroke of the converter clutch valve in the pump, which prevents the converter clutch from applying.

The computer may display TCC-enable, but the valve has not actually stroked. The valve may then stall in its bore and cut off converter flow, which could overheat the converter and result in superheated fluid venting out the fill tube.

Is wear on the converter clutch signal valve detectable at the bench?
A wet air test (with fluid) will identify leaks at either the second gear/PWM boost oil or the TCC signal/converter apply lands on the valves. The leak will allow air to exhaust from the filter side of the valve body If you remove the valve, you'll see the bore is severely worn, and may have a depression worn into it. If the wear isn't severe, you may only see a polished or shiny surface.

Can you block the non-PWM converter clutch signal valve?
Blocking the valve toward the end plug won't repair a second gear/servo or TCC signal oil loss. If you block the valve toward the end plug, the TCC solenoid will receive signal oil in all gears, so if a solenoid becomes restricted or the converter clutch valve bore wears, the unit could develop premature lockup. If you block the valve inward, away from the end plug, no TCC signal oil will reach the TCC solenoid and lockup valve, so the converter clutch won't apply.

How does the PWM unit's regulated apply valve affect the converter clutch?

When the regulated apply valve is blocked into the maximum flow position, regulated apply pressure could be equal to line pressure (rather than actuator feed limit pressure), but leaks from bore wear will reduce the pressure.

The best way to understand this is by examining an oil circuit diagram. Don't use the 1992 printing of the GM Technicians Guide, as these circuits only apply to non-PWM control circuits. It's easy to identify the correct oil circuits: They have the converter feed going to the TCC solenoid signal at the pump (figure 2).

There are three critical oil circuits to focus on: line pressure, converter clutch signal taken from actuator feed limit oil, and the resulting regulated converter apply oil.

Regulated converter apply is the pressure that holds the piston to the converter cover under engine load. This is the key to understanding why insufficient apply oil pressure will allow the converter clutch to slip and overheat, similar to common clutch overheating.

This valve is a simple regulator that balances actuator feed limit/converter clutch signal oil against apply oil. The OEM isolator and regulator valves are the same diameter, so equal oil pressures are acting on both ends to balance the valves (see figure 1). Line pressure is the source of pressure for the apply circuit.

Where do the TCC and regulated apply valve bores leak?
The regulated apply valve bore wears out close to the exhaust ports, and reduces boost, actuator feed limit and apply pressure. This wear may appear as a polished area or deep grooves. You can check for leaks with the wet air test (figure 1).

The converter clutch valve is another area of valve bore wear on both non-PWM and PWM units (figure 3). Wear occurs on the spool closest to the TCC solenoid. Wear causes converter feed oil to cross leak, which allows the converter to overheat. The TCC may apply in second gear, causing a complaint of no power, or the valve will partially stoke and cut off converter oil flow. This often results in overheating, which blows fluid out the fill tube.

Figure 4

Bore wear occurs on many electronically-controlled units, particularly at duty-cycled valves. In the case of poor solenoid control, inspect the actuator feed limit valve bore next to the exhaust ports (figure 2).

Alternate methods used to modify a worn regulated apply valve bore:
Blocking the valves into a high converter charge position (both valves outward) is a common modification, but not necessarily a good one. That's because, under some conditions, converter apply pressure could equal line pressure.

If you modify line pressure by changing the springs and installing a larger boost valve, it will create high internal converter forces. The 4L60/E units don't have a converter regulator valve to control maximum charge pressure. Blocking the valves doesn't repair the actuator feed limit oil or regulated apply loss from a worn valve bore.

Grinding flats on the outer spool of the regulator valve is another common modification that just isn't a good idea. This procedure attempts to equalize line and apply pressure. The problem is that this allows regulated apply pressure to approach line pressure. If the bore is worn badly, line pressure will leak to exhaust. Another problem is that line pressure will balance with a lower actuator feed limit pressure, causing an uncontrolled regulator valve. This procedure also doesn't repair actuator feed limit or apply oil leaks at their bores.

Drilling an orifice into the end plug will exhaust critical converter apply oil; also not a good repair. The orifice in the separator plate will control the amount of flow past the end plug. But once again, this doesn't repair the loss of actuator feed limit or apply oil at the valve body bore. The regulator valve will stay in a full flow position, so apply can match line (reduced by leakage from the drilled end plug).

Other modifications to affect converter function
Grinding a flat on the pressure regulator valve increases charge pressure, based on how much material you remove. Grinding off the second land completely removes the control, and regulated converter feed becomes line pressure. In other words, the more you grind off the valve, the closer charge pressure gets to line pressure.

The pressure regulator in the 4L60E pre-PWM is similar to the 700R4. The second land controls the converter feed circuit that goes to the converter clutch valve. The PWM model uses this pressure to feed the converter clutch signal orifice. Grinding a large flat will increase converter feed when not in lockup, but a higher pressure is forced through the converter clutch signal orifice. This could overcome the solenoid exhaust capacity, causing the converter clutch valve to stroke prematurely.

A converter clutch signal orifice with larger diameter, checkball, and plastic seat (introduced in '97) may be of interest. GM's bulletin (9674L60E-15) says it was implemented to reduce delayed engagement caused by converter drainback. This design change relates to grinding this second land on the PR valve.

Enlarging the converter clutch signal orifice in the pump can flood the TCC solenoid with more oil than it can exhaust. If the TCC solenoid can't exhaust the oil, a partial valve stroke may restrict flow and overheat the converter.

The original orifice of GM #8631146 or #8680475 has a 0.025" to 0.031" hole. The OEM solenoid exhaust hole is about 0.068". The 4L60E spring needs 53 PSI and the 700R4 requires 42 PSI (average requirements) to stroke and maintain an applied position.

Removing one of the converter clutch valve return springs can create premature TCC apply, poor release, or a complaint of no power when the valve strokes too early.

What's different about the '98 ECCC system?
The EC³ (Electronically Controlled Converter Clutch) system (introduced on '96 4T60E w/3.4 DOHC) has a slightly different converter regulator valve appearance. The second spool has a groove running all the way around it, to stabilize and reduce side loading (see figure 1).

The electronic controls for this valve differ from previous applications. EC³ controls regulate slip rate of the converter--they only partially apply the converter clutch intentionally. This design initiates converter PWM control in second gear, which makes it very active as it controls slip rate. The slip is higher in second gear, both during apply and coastdown from third through second gear. In second gear at light throttle and low speeds, the duty cycle signal fluctuates wildly as it controls the slip speed. As the vehicle reaches highway speeds, the slip rate drops to zero.

The previous design PWM control would increase the duty cycle until slip dropped, generally after third gear. Once this design started to apply, duty cycle went to 80­90%.

The main concern of this EC³ design is the controlled slip rate that generates converter lining heat and requires controlled coefficient of friction. If the lining material, fluid, or apply pressure changes, the ability to control slip speed changes.

Comparing the apply oil pressure on OEM and altered converter regulator valves shows interesting results. If you raise apply pressure (by blocking the converter regulator valve), the duty cycle signal gets very active while trying to make the TCC piston slip. Higher apply pressure can also create a coast down trailer-hitching effect.

How do the converter friction linings differ between non-EC3 and EC3?
The on/off, or non-EC³ converters, use a higher coefficient of friction lining. This lining, a cellulose-based material, is more aggressive on apply and won't tolerate high slip rates or heat. An alternative to the cellulose lining was Kevlar^™, which has a lower coefficient of friction (creates more slip before a static stop) but withstands high temperature. Both linings are insulating materials and neither conduct heat easily. This means that heat at the piston friction surface won't transfer into the cover and won't shunt away from the piston.

EC³ systems use a friction material with a high percentage of carbon. There are two styles of this carbon-based friction material: One is similar to the cellulose and Kevlar^™ material, but has a high carbon content. Another, introduced by General Motors in 1998, is a matrix of woven carbon fiber. This material allows slip with minimal effect to the lining, since the carbon weave is highly conductive, allowing heat to be displaced into the cover. The carbon materials have very different friction characteristics. Heat conduction and fluid-flow characteristics and aren't interchangeable, either within a converter or between transmissions.

A low-conductance material with a different coefficient of friction than the carbon material, such as cellulose or Kevlar^™, won't stand up to the controlled slip commanded by EC³.

There are many considerations to make while working on these units. Make sure the parts you use, and the modifications you make are a benefit to the overall workability of the unit. Understanding how the various systems work is the first step to meeting this goal.