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 phone, and only 3 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 that takes place in your vehicle down to the power point that charges your cell phone which is controlled by the Body Control Module(BCM).

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 too common on the 90’s and 2000’s Caravan, Voyager, and Town & Country minivans with the 41TE/604 transmission, for the wiring pigtails that connected to the ISS(Input Speed Sensor) and OSS(Output Speed Sensor) to fail and intermittently lose signal. Now when the OSS drops to 0 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 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, listed in the scan tool is a menu of Parameter Identification(PID), in there, you may find more than one PID to indicate what gear the transmission is in. While watching the data of both, you may notice the two 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 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 the transmission is not capable of doing 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

Remembering back to 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, 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 that 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 is doing what is commanded by the computer, but isn’t what the driver commanded? We must monitor input signals to see if there is something that would prohibit the desired outcome, however, that could be a moving target at times.

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 the best Ford dealer techs in my area, 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 very intermittent, duplicating the issue and capturing data was difficult. The normal 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 that were 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 was providing a feedback signal to the PCM and never set a code initially, so the PCM thought the brake pedal was being pressed by the driver. 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 was able to push the upshift button in manual mode and make it shift with the faulty brake switch. The PCM was looking at the brake switch to determine if an upshift took place or not until it was commanded manually.

Thank you, Michael Mailloux, for this case study!

Fixing OE programming

Programming can be an effective tool when done correctly. The Manufacturer will on occasion 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 this 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 the use of shift 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 gear, where it is commanded to lock up in 2nd from the factory. The best part about this tuning is this can all be done without removing a single pan bolt, or 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 in the transmission or not.