Sometimes, when I’m doing automotive electrical diagnosis, I encounter some issues that are real head scratches and challenge my training. Sometimes, I even get to the point of doubting my meters. Recently, we had just that problem.
A local shop asked us if we could diagnose a 2017 Jeep Wrangler, Sahara, with a 3.6 V6 MPI engine and a 722.6 Automatic Transmission. It had only 42,000 miles and was brought to them by a used car lot dealer who bought it at the auction. My experience has shown me to expect the unusual when a vehicle comes from an auction, especially with this Jeep with such low mileage. The shop had pulled the unit out several times and finally installed an entirely different transmission, but the original problem persisted.
A full code scan was done on the vehicle; the only codes were from the Transmission module. Codes P0730 “Incorrect gear Ratio” and P1705 “Input speed sensor two overspeed.” According to the shop, those were the original codes.
Codes were cleared, and the vehicle was driven. Reverse felt good as we backed out of our shop. Good Drive engagement was felt, but as we took off, it felt like it was in fail-safe mode as the vehicle felt heavy. The shifts were very difficult to define, and when we came to a stop sign, the vehicle would not move forward or Reverse, so we had to cycle the key to return to the shop. We also noticed that if you placed the transmission in reverse immediately after the vehicle was started, there would be no Reverse. It is customary to road test the vehicle while communicating with the TCM to record the data. Here is what we noticed:
The park to reverse and neutral to drive all seemed normal. The 3-4 shift valve solenoid is modulated in park, turns off in reverse, modulated in neutral, and stops modulating in drive 1st gear (Figure 1). That is a normal transmission operation.
Taking off in first gear felt like Fail-Safe mode, but the data showed it to be first gear. The 1-2 shift was not even felt, but as the data shows in Figure 2, the N3 sensor starts to read, which is expected and normal on a 1-2 shift. However, the 2nd gear is not recognized by the TCM as it continues to read 1st gear.
During the 2-3 shift, the 2-3 Shift solenoid is modulated during the transition period, and the N3 speed sensor reflects the rpm drop; these are both normal. But the “Current Gear” PID is uncertain what gear it’s in as it oscillates between 2nd and 3rd (Figure 3).
During the 3-4 shift, the 3-4 shift solenoid is modulated during the transition period, but both the N2 and N3 reflect a 1,000 rpm increase and not a drop, and the Current gear PID oscillates between 4th and 2nd. It was also noticed that after the 3-4 shift attempt, the TCM started various 2-3 shift attempts until it placed the transmission into Fail Safe Mode by cutting off the solenoid supply voltage (Figure 4).
We decided to go directly to the TCM to verify the observed signals. We removed the TCM cover from Connector 2 to access the wires. By disconnecting the C2 connector from the TCM we checked for short-to-ground, short-to-voltage, and open circuits on all the solenoid circuits. We also took this time to check solenoid resistance (Figure 5 & 9).
Pin #38 & 14, 1-2/4-5 Solenoid; #38 & 15, 3-4 Solenoid; #38 & 16, 2-3 Solenoid; #38 & 17, TCC solenoid; #38 & 36, Modulating Pressure Control Solenoid; #38 & 37, Shift Pressure Control Solenoid. All checked between 3-4 ohms, well within specifications. Although the shift solenoids are ON/OFF, they are first controlled with a pulse width of about 80% ON and 20% OFF for about 60 milliseconds (Figure 6). This is known as the “initial-in rush time”. After that, the solenoid is modulated 28% ON and 72% OFF (Figure 7) until it is turned off. The frequency is 1kHz, or 1,000 times per second.
Each solenoid was checked with an Oscilloscope to verify there were no problems with the ground, voltage, or TCM solenoid drivers. Figure 7 shows the 1/2/4-5 Solenoid during a 1-2 shift. A 15.24 Volts on the OFF time when it is being modulated shows a good voltage spike as the solenoid magnetic field collapsed during the OFF period, and 0.12V volts on the ground side during the ON time is an excellent ground (All readings are in reference to battery ground). The 14.1 V on the 3-4 and 2-3 solenoids when OFF shows we have a good voltage source. In short, no electrical problems were evident in any of the solenoid circuits.
Our electrical test showed no problem on all the circuits between the TCM and Transmission. We checked the TCM and Transmission harness, including the only connector between them, I302. All the connectors and harnesses looked undisturbed and original. At this point, I’m wondering what I am missing.
We decided to find out what the ratio on takeoff was. It was here the light bulb came on, and it all made sense. Divide the input shaft rpm by the output shaft rpm (ISS/OSS) to obtain the gear ratios. However, the 722.6 transmission is designed in such a way that there is no rpm reading on the N3 input shaft sensor in reverse, 1st, and 5th gear. To obtain the N3 input shaft rpm in these cases, you simply divide the N2 Reading by 0.6081, a factor given by the manufacturer. Using our example in Figure 8, we have 354/0.6081=582, the calculated input shaft speed sensor rpm in 1st gear. We grab this number 582 and divide it by 410, which is the output shaft rpm, and we get 1.42, which is the 3rd gear ratio. For 2nd through 4th, N2 and N3 readings are the same, so just grab one and divide it by the output speed sensor reading to obtain the ratios. The math revealed that the 1st and 2nd gears had a 3rd gear ratio, 3rd gear had a 4th gear ratio, and 4th gear had a 2nd gear ratio.
At this point, we decided to do a continuity check of the circuits from TCM connector to the transmission connector. It was here that we found the problem (Figure 9). Every solenoid circuit checked out, but TCM pin 15 & 16. The pin 15 continuity check would only work on pin 8, instead of 9, of the transmission connector, and pin 16 of the TCM would work with pin 9, instead of pin 8. If you look closely at Figure 5 you can see that the color identification for pins 15 and 16 are inverted. Someone removed the wires from pins 15 and 16 and inverted them. Since both circuits operate and function the same way, neither our meter nor the oscilloscope can show the difference.
Every time the TCM turned on the 3-4 solenoid, the 2-3 solenoid was energized and vice versa. In neutral, the TCM turns on the 3-4 Solenoid in preparation for a gear selected either for reverse or drive, but in our case, it would energize the 2-3 shift solenoid, making the transmission take off in 3rd gear when drive was selected. When the TCM commanded a 2-3 shift, the transmission would receive a 3-4 shift signal instead.
Truth be told, this finding took many hours of work and research. It is not possible to charge a customer for the number of hours it takes. I have often been asked why I spent the time. I consider my shop to be my Transmission University, I have the best teacher (school of hard Knox).
I consider the time I spent without payment my tuition fee. There are things you will not learn in a book; only life’s experience can teach. Personally, I can say that I have learned more when I have made a mistake than when I do things right.
‘Till next lesson.
