A triple transmission threat

Feb. 28, 2014
So what is a triple-threat-trans? It can oppose you on any one of three fronts, or a combination thereof: mechanically, hydraulically and/or electronically.

In the late 19th century, gridiron (grid – iron) football emerged here in North America, a deviation of both rugby and association (soccer) football. Nowadays, we simply call it football. But back in the day, in addition to playing defense, a player in the gridiron game could be referred to as a “triple-threat-man” if he excelled at kicking, running and passing. He was the team’s utility man and a big threat to the opposing team.

What does this have to do with transmissions? Well, it’s just my own personal viewpoint of how I observe a problematic transmission. I call it a “triple-threat-trans,” for it can oppose you on any one of three fronts, or a combination thereof: mechanically, hydraulically and/or electronically.

I find by breaking the transmission down into these three basic categories contributes to the diagnostic process. The computer calls the plays; the electrical solenoids receive that command, which in turn, directs hydraulic pressure to operate mechanical parts to complete the play. Each play begins with a command based on computer programming ending with a controlled gear transition. Besides the initial command and the achieved shift, there are many things that can go wrong in between. With some causes being less thought of than others, to mention a few of these overlooked points might be helpful.   

Starting with programming (Figure 1), there can be challenges in determining a problem in this area affecting the direction one would take in the diagnostic process. At times, manufacturers might not develop the cleanest software affecting the quality of a shift and/or shift scheduling. Premature failure of the transmission occurs during the warranty period prompting reprogramming repairs. As a result, these types of failures do not regularly show up in aftermarket repair facilities.

But when it does, a transmission comes in with just 15,000 miles on it for example, yet one or two clutch assemblies are totally destroyed with no visible signs as to why. The fluid isn’t burnt, no bushing failure, sealing ring failure or rubber seal failure can be found anywhere. In fact, you stack up the assembly with new steel and friction plates and it pressure checks perfectly. Usually, at this point, the valve body and/or solenoids become suspect when all along the route cause is really a programming issue.

The opposite also is known to occur. For fuel economy purposes, a perfectly good program is written for what is commonly referred to as a “neutral idle stop.” The computer commands off the driving clutch during “in gear” idle stops, such as stopping at a red traffic light. As soon as the brake pedal is released, the computer engages the clutch before the accelerator pedal can be depressed. In time, valves in the valve body wear into the bore hydraulically compromising the task resulting in a flare-bang engagement.

Rather than replacing the valve body, reprogramming the computer is done to remove the original program. The vehicle now remains in gear during any stop-idle conditions. However, had a new valve body with solenoids been purchased and installed, the vehicle would have worked as originally designed. The choice is between an $800 to $1,000 valve body and a $50 to $75 programming charge. Which do you think occurs most? I am sure you guessed correctly. Although the re-flash cured an immediate problem, the transmission is headed for failure as the valve body is not going to get better with time.

In comparison, another neutral idle control problem with an entirely different manufacturer is the result of a programming problem. In this case, when the clutch is released, the program also entails lowering engine idle. The problem is getting the engine idle back up while engaging the clutch all before acceleration after a brake release. This results in a similar complaint of a flare bang engagement.  A slight change in the programming via a re-flash and the problem is resolved.

These are just basic examples with which to reveal the challenges a tech faces with this aspect to the triple-threat-trans, particularly since most manufacturers’s programming information remains proprietary. Of course the electrical facet to this triple threat also consists of input information to the computer, output commands and hardware (connectors, wiring sensors, switches and solenoids). With this one threat alone consisting of such a variety of components, it can be a respectable opponent.

The major link between computer commands and the actual shift are the solenoids and valve body (Figure 2). The solenoids receive the electrical commands, which in turn operate their respective hydraulic circuits. Most operate valves within the valve body to direct pressure for a specific task. Others receive full line pressure which the solenoids then regulates to directly ramp on and ramp off their respective clutch assembly.

This aspect of the transmission is not as near involved as the electrical. There are several straight forward areas prone to problems and failures that typically can be identified rather quickly, if one knows where and how to look. The exception to this would be related to mechanically failed solenoids, especially if they fail intermittently. 

Some basic things to look for with a valve body would be a stuck or sticking valve, worn or broken valve plugs, broken or incorrectly located retainers, loose end plates, broken springs, bore wear, out of flat surfaces, missing, distorted or incorrectly located check balls, limit/check valves missing or out of place or blown out gaskets (Figures 3 through 7).


Worn valve plugs are easy enough to inspect but is often times out of mind. With many solenoids now being pulse width modulated, the problem of valve plugs leaking has increased.

One example of this can be with the Toyota A750 Transmissions. The SLU solenoid pulse width modulates to influence both the Lock-up Control Valve and Relay Valve for flex lock-up control (a controlled slip of the converter clutch as early as second gear).

A careful look at the hydraulics (Figure 8) shows how this solenoid sends pressure to the relay valve between one end of the valve and the valve’s plug retainer (Figure 9). With oil pressure from the solenoid being used to pulse this valve, the plug will rock or resonate in the bore making its perimeter susceptible to wear. Eventually, the plug will be unable to retain pressure. With the plug now leaking solenoid pressure, the relay valve no longer will stroke properly compromising converter clutch apply. This typically causes an SLU solenoid performance code P2757 to be logged due to the loss of proper TCC pressure control.

This is just one simple example. There are many several transmissions that can develop this type of concern. Not only with solenoids related to the converter clutch, but shift and clutch control solenoids as well. It will produce a performance code for the related solenoid and/or its related clutch assembly, whichever the system is equipped with to produce.

Besides doing a visual inspection of the valve body, pouring solvent between the valve and bore plug to see what runs past the plug or captive vacuum testing are additional ways to inspect this area of the valve body. There are aftermarket sources for replacement end plugs.

If in a pinch, slightly flaring the outer perimeter of the plug using a tube cutter will provide enough of an interference fit to seal off the leak. Just be sure you carefully tap the plug in just far enough for the retainer to drop in place. Do not over travel the plug placing it too far in-board compromising the stroke of the valve and making yourself a different problem.

One other point to consider that can cause a similar complaint is using substandard filters. There have been occasions where the filter element itself introduces enough particles into the fluid that it clogs the solenoids. It’s comparable to a rebuilt transmission that left behind excessive rag lint. Once the solenoid screen is clogged, performance codes will set in the same way as a leaking end plug.

The often times overlooked mechanical aspect to the triple-threat-trans is related to bushings and end play. An AW55-50SN/AF23/33-5 transmission is a perfect example of this. The planetary components are quite heavy with inadequate bushing support. In time, the bushings wear causing the gear train to become sloppy. As torque loads up and release, bumps and clunks can be heard and felt. Coast downshift bumps and clunks are typically the initial early stage observation of this problem. Bcause faulty solenoids, valve bodies and shift adapt programming have been known to cause this complaint, they become the first approach to resolving the problem. One reason worn bushings in this transmission are often times overlooked are that they usually look fine.

With many of the Asian transmissions on the road today, one way this could be identified quickly is to check lubrication pressure. When bushings are worn, this pressure can drop to 0 psi at operating temperature (Normal pressure runs around 6 to 10 psi at operating temperature; 220°F).

Fair warning: This could also be due to a worn secondary pressure regulator valve bore. The sure thing is, with zero psi there is a definite problem in the transmission, which can be sorted out quickly once the unit is out and apart.

Bushing wear also will cause sealing rings to dig into the surfaces they seal against. This in turn causes soft or flared shifts and burned clutch assemblies. Excessive endplay in the gear train can present itself much in the same way as worn bushings. Clunks and bumps, loss of lubrication pressure and sealing ring grooves. Overlooking excessive gear box end play does not occur frequently but when it does, it typically remains an obscure possibility in the technicians thinking.

If a unit was taken down to be rebuilt, if all the hard parts are good (case, pump, drums, planetary assemblies, etc.), as well as all the washers, endplay basically will remain the same once rebuilt. For this reason, some rebuilders bypass end play checks. But when a rebuild requires a hard part or parts to be replaced, end play can be affected and overlooked. Especially if special tools are required to properly check end play.

To overcome a triple-threat-trans you no doubt need to be a triple-threat-man by keeping some of these mechanical, hydraulic and electronic issues not typically considered in the diagnostic process.

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