For parents, it can be easy to relate to this article. Think about the last time you asked your kids to clean their room and found them 20 minutes later playing on their phones, and only three socks were picked up off the floor. The “Actual” was NOT what was “Commanded!”
Many different computers are utilized for various tasks in today’s automobiles. A computer controls every action in your vehicle, down to the power point that charges your cell phone, which is controlled by the Body Control Module (BCM).
Computers surrounding a truck. Very simplified, each computer is programmed/calibrated to look at inputs it receives, then uses the programmed logic to determine and send an output to the desired component it is connected to. When there is a malfunction, our job is to determine if the actual is what the computer desires.
Moveras’ focus is the transmission, but if you only focus on the component’s output and disregard the computer and its inputs, you will eventually suffer the fate of unnecessarily replacing an expensive part.
COMMAND
On many occasions, I can think back to a repair job in which the customer was convinced the transmission was faulty, and after the initial road test, I would have agreed with them. Only after diving deeper did we find that the transmission was only doing as it was commanded.
What are some examples? How about a transmission that slammed into first gear randomly while driving 30 mph? It was common in the ‘90s and 2000s Caravan, Voyager, and Town & Country minivans with the 41TE/604 transmission. This was because the wiring pigtails connected to the ISS (Input Speed Sensor) and OSS (Output Speed Sensor) would fail and intermittently lose their signal. Now, when the OSS drops to zero, and you are still doing 30, the computer says, “Hey, we stopped. Let’s downshift into 1st gear now,” proceeded by a loud chirp from the tires and a high negative g force that throws you into the steering wheel/windshield. This would be a case of the wrong command being sent to the transmission.
ACTUAL
How do we determine if the problem is in the transmission? Well, it starts with a good understanding of how the system works. What good is watching a bunch of data on a scan tool if you don’t understand what the data means or trust the source of the data? For example, a menu for parameter identification (PID) is listed in the scan tool. There, you may find more than one PID to indicate what gear the transmission is in. While watching both data, you may notice they differ at times. What you should question is where the data is coming from.
One PID could be a value from the shift solenoid output from the Powertrain Control Module (PCM command), and the other data could be determined by the difference in rpm of the ISS and the OSS, which gives you a transmission gear ratio (Actual). Based on the ratio the computer sees, it can determine which gear the transmission is in. “Transmission ratio” is the most accurate depiction of what “actual output” is.
These two PIDs should align (Actual vs. Desired). When they don’t, you need to determine if the command is wrong or if the transmission cannot do what is commanded. Internal issues like a loss of pressure in a clutch or too much clearance in the clutch plates that cause a slip or lack of apply would be a case of the transmission being unable to work as desired.
THINGS CHANGE
I remember when Snap-on updated their software and changed the speed sensor PID data for Honda vehicles. This change happened in the background without notification and certainly caused some confusion until the data source was understood. Being familiar with the old version meant the new version took some research to understand. After spending some time on the phone with Snap-on tech, I found out that they took the ISS and OSS rpm reading I was used to seeing and converted it into a MPH reading. What changed is when everything is working as expected, the MPH reading of the ISS, OSS, and vehicle speed must match at all times. This is why we must question where the data is coming from when something doesn’t look right.
What if the transmission does what the computer commands but isn’t what the driver commands? We must monitor input signals to see if something would prohibit the desired outcome; however, that could sometimes be a moving target.
A CASE STUDY
Recently a great friend of mine reached out for my input on a truck that was being difficult. He is highly respected as one of my area’s best Ford dealer techs, so I knew this would be a good one. The truck was a Ford Super Duty with a 6.7 diesel and a 10R140 transmission. This truck was a classic case of being a New England rot box; the PCM’s case was corroded and falling apart physically, the wiring was full of green corrosion, and some things needed to be repaired just to get the truck roadworthy.
The complaint was that, intermittently, the transmission would hang up in gear. It wouldn’t slip or bind; it just didn’t want to shift into the next gear as scheduled. However, if commanded on the tap shift button, the transmission would make the shift.
Because it was so intermittent, duplicating the issue and capturing data was difficult. The usual things, like the range sensor, speed signals, TPS, etc. were checked and seemed to work normally. He even verified that the brake and reverse lights were working when they should.
After replacing the visibly failed components (PCM, grounds, and wiring) along with a host of other things unrelated to the shifting concern, another road test was performed, and again, it hung up in gear just the way it had previously. Back in the bay, rechecking codes this time, he noticed a code for the brake switch. He also noticed there were no brake lights this time. Another road test with a scan tool showed the brake switch “enabled” without touching the brake pedal, and when it did this, the transmission would hang in gear. After firing a new brake switch in it, everything seemed to be working great and the truck was able to be delivered to the customer.
Because the circuit in the switch didn’t fail, it provided a feedback signal to the PCM and never set a code initially, so the PCM thought the driver was pressing the brake pedal. Also, keep in mind that calibrations vary from vehicle to vehicle or even in the same vehicle. They may vary depending on the drive mode selected. In this case, remember he could push the upshift button manually and make it shift with the faulty brake switch. The PCM was looking at the brake switch and thought the vehicle was going downhill. The no-shift condition is a grade-retard feature found on many vehicles.
Thank you, Michael Mailloux, for this case study!
FIXING OE PROGRAMMING
Programming can be an effective tool when done correctly. The Manufacturer will occasionally release updates to its factory calibrations to fix an issue with driveability. However, there has been an increase in aftermarket programming on the transmission side due to OE calibrations falling short of customer driveability expectations. There are several articles about custom tuning to fix TC shudders and overall poor shift qualities. This is very popular in the 6L and 8L transmissions found in GM trucks and the Ford version of the 6R.
With aftermarket programming, you are essentially dictating the computer output to give the transmission based on the inputs the computer receives. This can be very helpful when tailoring shift quality and transmission longevity to a certain vehicle and driver habits. Like using valve body kits on older transmissions, we can increase pressure in a clutch or speed up an application to better suit the driver’s demand. One popular thing to improve the drivability of these transmissions is to eliminate the lock-up command until 5th and 6th gears, where it is commanded to lock up in 2nd gear from the factory. The best part about this tuning is that it can all be done without removing a single pan bolt and getting an ATF shower.
I mentioned this here because you may find that the command from the PCM is not what is expected from sources like the OE manual, Alldata, Mitchell, and even Identifix, and the transmission is doing what it was commanded to do.
IN SUMMARY
Knowing how it works and what our Actual vs. Desired results are based on the inputs provided to the computer will allow you to determine if there is an issue with the transmission or not.
