One of the things I’ve always enjoyed about rebuilding transmissions is the required attention to detail. All the mistakes that can be made and all the little things that can be missed during a rebuild that can lead to significant headaches keep it all interesting. Even after successfully building over a few thousand transmissions, I feel a sense of relief and achievement each time the technician returns from the final road test and tells me the transmission worked like a new one. Back in my early days, when I got that one that didn’t work after a rebuild, it would drive me crazy! It would overshadow the last ninety-nine transmissions that did work, sometimes causing me to create more problems on my current transmissions, wondering and worrying about that last one. But I’ve often heard, if you are not having problems, you are not working. Throughout the years, I’ve come to understand that more and more.
A few weeks ago, I had a 2007 Ford Taurus with an AX4N used for mail delivery come into the shop with a no move and a second gear ratio code. The fluid level was high with no forward or reverse engagement, so they pulled the transmission out and put it on my bench. I opened it and didn’t find anything but normal wear and a stripped-out converter. During the rebuild, I did all my routine procedures and installed new molded pistons, a new sprag, and new friction material. I also replaced the boost valve and sleeve due to wear in the sleeve. The rest of the valve body looked good and vacuumed tested within specifications. I flat sanded and checked the valve body with a straight edge; all good, so I assembled the transmission. Thinking this was an easy slam dunk, I gave it to the installer to put it back in and deliver it.
The technician came back and asked me if I wanted the good news or the bad news first, then proceeded to tell me it worked great for about the first ten miles. But after that, it had an intermittent delay on the one-two shift and set a code P0732, 2nd gear ratio error, that might occur on one out of twenty shifts. In my younger days, this is about the time I would go into panic mode, lose my mind, and start wondering what I did wrong or left out. These days, it’s more of a matter of thinking of where to begin in the diagnostics routine. What clutches, bands, and sprags are holding, what valves are operating them, what solenoids are stroking the valves, and the computer command for the solenoids.
We first verified the complaint and took a movie on the scan tool to see the computer commands and monitor our sensors for any glitches. It took us some driving to catch the problem happening and record data to check for anomalies. I looked for abnormal activity from the input and output speed sensors and the shift solenoid number two command. I found no glitches on the sensors, and shift solenoid number two command was on time when the shift was late. Even though the one-two shift was late, it was solid when it happened, but the two-three shift was almost on top of it. So we verified that the two-three shift command was on time.
What could be the source of the problem? Any hydraulic leaks in the circuit would have caused a slip or a slide bump type shift, and it should be more consistent. So, this led me to believe it was a command issue, either command to the solenoid, a possible computer issue, or a sticking valve.
Our next step was to monitor solenoid command at the transmission to determine if it was inside or outside the unit. As you can see in figure 1, SS2 shuts off for the one-two shift. We connected a wire to pin six in the transmission case connector, figure 2, to our scope to monitor solenoid command from the computer. We found that it was commanding the solenoid at the connector the same as it was showing on the scan tool when the one-two shift was late; this confirmed the problem was inside the transmission. So, we narrowed it down to solenoid or the valve body.
Like most on/off type solenoids, these solenoids don’t usually have intermittent issues, maybe hot or cold problems, so my thought was a sticky valve. We removed the valve body and gave it a thorough inspection. According to the hydraulic schematics, when shift solenoid two shuts off, pressure is applied to the one-two shift valve, opening a port to the intermediate clutches. The low sprag overruns once the clutches are applied and the shift occurs. The one-two shift valve, figure 3, was free enough to fall back in the bore when dropped in.
At this point, I could not put my finger on the problem. However, I had to judge based on what information I had. Typically, circuit leaks will cause a slip depending on how severe the leak is and usually worsen with increased temperatures. Solenoids typically react to temperature, sticking when hot or cold, but seldom an intermittent issue. So, this left me with solenoid command or valve body. Since we checked the solenoid command and verified it was correct, that left us with a valve body issue.
Valves that start to stick consistently after warming can be verified by hot tanking the valve body and checking valves. While this process would most likely confirm our suspicions, we figured using a different valve body would do the same. At this point, time is money!
So, we got a good used valve body, cleaned it up, checked everything, and put it in the car. After installing the valve body, the tech drove it for over thirty miles without one late shift. Then, we delivered it to the customer and waited for them to return for the two-week check-up to confirm the fix, which was nothing but good news!
Temperature-related malfunctions can be problematic to locate. Add to the equation an intermittent problem, and the difficulty is multiplied by ten. Typically, intermittent problems will be electrical or valve body-related. By verifying what you know, you can isolate what is not crystal clear. Checking the solenoid command verifies where the problem lies. If it is correct, you can bet the problem is in the valve body. And with most valve body problems, we can fix it in the car, so it doesn’t have to land back on the bench!
