How a PWM-controlled TCC (Torque Converter Clutch) works

[theholycow]

I found this great fully illustrated PDF:
http://www.motor.com/article_pdf_dow...rticle_ID=1039
It's a detailed explanation about how PWM-controlled TCCs work. They use the GM 4L60E as their example, and follow it over the years of changes in TCC solenoid control / clutch material.

They've probably got a bunch of great articles like that. Here's the page for the magazine issue where that article was published:
http://www.motor.com/articles.asp?sh...show_year=2006
From there you can view articles available from that issue or go to pages for other issues.

Here's the text, but it's better in the PDF with illustrations:
Slip-Sliding Away: Deciphering GM Transmission DTC P1870
Colonna, Wayne

The newest transmission technology provides nearly unnoticeable torque converter clutch application. As is often the case, an appreciation of how it works will help you understand why if something goes wrong.

Diagnostic trouble code P1870 (Transmission Component Slippage) is a General Motors-specific code whose set parameters are determined by transmission type and engine size. The PCM monitors and compares engine speed to vehicle speed after the converter clutch has been commanded ON in either high gear or Overdrive. Should the expected rpm ratio exceed predetermined parameters, DTC 1870 sets. And upon doing so, the PCM elevates the transmission main line pressure and stops converter clutch apply.

In some instances, the PCM also will inhibit 4th gear and may or may not illuminate the MIL. This really is an added safety strategy, as the PCM had already monitored the upshifts and determined all was well through those ranges. Once the vehicle has made it to a cruise state, the PCM then begins to monitor the vehicle's cruise ratio. If at some point the transmission begins to slip, P1870 sets and line pressure is elevated in an attempt to stop any further slippage and prevent further damage. This would explain why DTC 1870 Ls often accompanied by a complaint of a hard 1-2 upshift.

When this code sets, it could have any number of causes, ranging from the sump running low of fluid to a malfunctioning torque converter. Take a GM 4L60-E transmission, for example. When it's in Overdrive, the forward clutch and the 3-4 clutches are applied, as well as the 2-4 band. Now add the converter clutch. This is what the PCM monitors at cruise. If the ratio breaks away, the cause could be any of three components, the hydraulics that operate them or the electronics that control them. Fortunately, experience has shown that in most cases, the cause can be traced to some form of converter clutch failure as a result of valve body concerns. With GMs 4L80-E transmission, this code has usually been the resuit of a cracked converter clutch piston.

Strategies for delivering power directly from the crankshaft into an automatic transmission have ranged from a purely mechanical connection via a high clutch drum and shaft transmitted through a damper plate assembly, to an actual clutch apply, all taking place inside the torque converters fluid coupling. The converter clutch apply method has been the strategy of choice among vehicle manufacturers. This strategy has gone through several changes through the years, and the GM 4L60-E transmission is a good example of a transmission that has gone through redesigns to accommodate these changes.

From 1993 to 1994, the 4L60-E utilized a simple ON/OFF solenoid in conjunction with an encapsulated check ball assembly at the tip of the input shaft. The solenoid turned the clutch on and off while the check ball assisted in a controlled apply of the clutch (Fig. 1, above). To enhance converter clutch engagement for improved fuel economy, a pulse width modulated (PWM) torque converter clutch (TCC) solenoid was added to the system in 1995 (Fig. 2). This required a change in the pump assembly and valve body to accommodate the required hydraulics. (The acronym PWM is cast into the pump cover for easy identification.) With these changes, the PCM provides a duty cycle to this PWM solenoid, which in turn regulates the pressure in the TCC hydraulic circuit, allowing the torque converter clutch to apply gradually. The amount of slip that occurs during the apply is proportional to the duty cycle (Figs. 3,4 and 5, page 38).

The construction of the PWM solenoid is such that when the solenoid is completely turned off, feed pressure (AFL) to the solenoid is blocked at the solenoid. When the solenoid is duty-cycled, it opens to a circuit that allows pressure to act on the isolator valve. This increases the spring tension acting on die TCC regulator valve, which then increases regulated TCC apply pressure.

Fig. 6 on page 40 provides a 100-millisecond snapshot taken from a dualchannel scope to capture the TCC and PWM solenoids soon after converter clutch apply was commanded. You can see in Channel 1 that the TCC solenoid has been pulled to ground. This is the command that applies the torque converter clutch. Channel 2 is the command to the PWM solenoid that controls the feel of the clutch apply. The PCM operates this solenoid with a negative duty cycle at a fixed frequency of 32Hz. As Fig. 6 shows, we're in the vicinity of a 30% ON-time duty cycle. Looks similar to an injector pulse waveform, doesn't it?

This TCC activity can be easily observed through a scanner by way of three PIDs:

*TCC Enabled - Yes/No. This command is provided to the TCC solenoid to turn the clutch on or off.

*PWM solenoid duty cycle percentage, commanded by the PCM for shift feel.

*Converter slip rpm (Fig. 7).

It's important to note that the duty cycle percentage for the PWM solenoid on the 4L60-E transmission is usually presented in a scan tool as "ON Time." In this way, the technician can follow the percentage of ON time in relation to the regulated apply pressure. As the duty cycle decreases, the regulated apply pressure decreases. As the duty cycle increases, so does the regulated apply pressure.

Starting with partial coverage in 1997 and continuing with 1998 and later vehicles, the TCC strategy changed again. Similar to what we've just covered, the new strategy allows for the converter clutch to begin slipping as early as 2nd gear and to continue slipping until a cruising speed of approximately 54 mph has been reached. For this type of strategy to be successful, the type of material used for the clutch had to change. It should be noted that the original ON/OFF strategy utilized a cellulose-based converter clutch, otherwise known as a paper-lined clutch. The second strategy utilized a Kevlar-based lining (several designs, depending on 2WD, 4WD, heavy-duty, etc.) and for this third strategy, a carbon-based woven lining is used (Fig. 8). This type of converter clutch apply is referred to as Electronically Controlled Clutch Capacity (ECCC), or EC^sup 3^, for short.

In addition to these changes in material, the tension of the damper plate springs, which are integral to the clutch disc, was also altered. These springs act as shock absorbers, and their tension has progressively lightened from the ON/OFF strategy all the way through to the ECCC strategy. Should anyone modify the PWM or ECCC strategy to function as an ON/OFF apply while using a converter with a damper plate designed for PWM or ECCC, a tail end bump on the apply of the clutch would be felt, possibly resulting in a customer complaint. In some cases, a code for Converter Clutch Stuck On may also occur as a result of this modification to ECCC strategy vehicles.

When watching TCC data stream parameters with a scanner or reviewing a movie of them, you'll notice that the PCM begins a 90% PWM solenoid duty cycle at approximately 8 mph. This duty cycle has no effect on the pressure inside the converter at this time because the pressure it regulates is blocked by the converter clutch valve. But once the TCC solenoid is turned on, it strokes the converter clutch valve, and this pressure becomes the converter clutch apply pressure. When this command occurs, the PWM duty cycle drops to 0%, then increases to about 25%. Afterward, it ramps up to about 40%, holding a constant 20- to 40-rpm slip on the clutch. Only under high torque or high vehicle speeds will the PCM command the converter clutch to be fully locked. Otherwise, it always slips.

Due to some driveability-related issues, a reflash was introduced to alter this strategy. What will be noticed after the reflash is that the PWM duty-cycles at approximately 96% at approximately 8 mph. As soon as the TCC solenoid is commanded ON, the PWM duty cycle drops to approximately 30%, then ramps up to about 50%, bringing TCC slip down below 10 rpm. These are approximate values and will vary slightly with engine size and load.

We've looked at three different strategies used to apply a clutch within the torque converter of a 4L60-E transmission. We can now delve into the causes of a DTC P1870. The difference between the second and third design TCC apply lies primarily within the computer strategy. And, of course, the material for the converter clutch must accommodate the strategy. A first design or second design clutch lining will not survive in the third design strategy. The woven carbon fiber or equivalent must be used in all EC^sup 3^ applications. But from a hydraulic standpoint, both the second and third design strategies suffer in a similar way. The isolator valve and TCC regulator valve lineup wears out the bore it lives in.

The filter inside the transmission filters the fluid down to 60 microns. The clearance between the valve and the bore is about .001 in., which is less than 60 microns. It's said that a dirt particle 20 to 40 microns in size is large enough to hang a valve. Anything smaller becomes a wearing abrasive. Valves are typically side-loaded, and when you're dealing with a regulated valve along with an abrasive fluid, wear is bound to occur. GM put ring grooves around these valves to relieve the side-loading factor. This helped, but the bore still eventually wears. Several designs of this valve lineup have been installed over the years, but the wear continues. And this wear causes a drop in converter clutch apply pressure, producing the infamous DTC P1870.

The isolator and regulator valve line-up in the valve body is not the only valve location that is susceptible to bore wear. The actuator feed limit valve is another problem area. This is the valve that feeds the PWM solenoid. So if this pressure is low, the PWM solenoid will not influence the isolator valve sufficiently to raise converter clutch apply pressure. The TCC ON/OFF solenoid and the converter clutch valve have also had problems. Failure of any of these components may prevent a full stroking of the valve, and will also produce DTC P1870.

Several aftermarket companies offer a means to repair a bore wear problem. For minimal bore wear, Superior Transmission Parts (850-575-0788) produces a PWM powershift valve to replace the converter clutch valve that the TCC ON/OFF solenoid operates. This valve is designed to overcome minimal system leaks and is only for EC^sup 3^ applications. If only the actuator feed limit valve has bore wear problems, Transgo (626-443-7456) offers a dedicated reamer and valve kit. Sonnax Industries (800-843-2600) provides a replacement converter clutch valve with a Teflon ring in one of the lands, which eliminates bore cross-leaks there. They also cany reamers for the pressure regulator valve, the actuator feed limit valve and the TCC regulator valve, with valve replacements (Fig. 9).

After repairs are completed, and for preventive maintenance, it may be a good idea to add an inline filter to the transmission cooling lines. Filters from Magnefine (866-746-3455) or SPX Filtran (847-635-3810) will filter the fluid to approximately 20 to 25 microns.

Diagnosing driveability concerns that may or may not be related to converter clutch apply was relatively easy in the past. A general rule of thumb in diagnosing shudders, chatters, bucking or surges is that if the converter clutch is causing it, the condition usually occurs during the apply, not after it. A shudder may occur when crowding the throttle, and if the converter clutch is causing it, it will usually be followed by a full release. Typically, after the clutch has applied, a cracked plug or plugs, bad wires, a malfunctioning injector or a DIS with a coil pack problem can definitely cause an engine to buck. Using a vacuum gauge will pick up an engine-related problem, as will a quick look at oxygen sensor activity. This, too, may reveal that a misfire is causing the buck.

For the most part, all of these diagnostic strategies remain effective today. The newest technology employed provides nearly unnoticeable converter clutch apply, so we must give additional diagnostic consideration to the strategy that makes it all work. A simple random misfire could prevent the PCM from engaging the converter clutch, and there need not be a DTC set for this to occur. Yet open-loop scenarios, faulty ECT sensors and VSS problems are still the familiar reasons for a loss of TCC. A more sophisticated TCC apply strategy provides additional possible causes for its malfunction.

As stated earlier, DTC P1870 is produced when the PCM sees a breakaway in the slip ratio while at a cruise. If the ratio breaks away, the cause could be compromised trails components, hydraulics or electronics. Fortunately, time has shown that in most cases it can be traced to some form of converter clutch failure-in particular, bore wear. Unfortunately, since bore wear is a frequent cause for the code, when it is not, other reasons are often overlooked.

Remember, the code is set after the vehicle is in high gear with the converter clutch applied. With the 4L60-E transmission, that means the forward clutch and the 3-4 clutches are applied as well as the 2-4 band, and if the 3-4 clutch or the band slips for any reason, DTC P1870 sets.

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Copyright Hearst Business Publishing Jun 2006
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