In this issue of Keep Those Trannys Rolling, we are going to take a look at a 2024 Infinity Q50 that was referred to us from another transmission shop, which was experiencing erratic shift concerns. According to the referring shop manager, the vehicle had erratic upshift and downshift concerns before and after diagnosing and installing a factory replacement transmission assembly. The shop manager said that the vehicle had been referred to them by their local Infinity dealership.
Initial Inspection:
After obtaining the vehicle, vehicle owner, dealership contact, and referring shop information, we started by performing our initial vehicle inspection. This Infinity Q50 was two-wheel drive and was equipped with a 3.0L engine and a RE7R01A transmission. The vehicle had less than 35,000 miles on it and was in perfect condition, which immediately raised the concern that this vehicle should still be under factory warranty. We performed an under-hood inspection, checking hoses, belts, harnesses, and fluid levels. Everything looked good under the hood. We raised the vehicle on the rack to inspect the underside of the vehicle. We checked the engine, transmission, differential, and suspension. Again, everything looked good there. With this vehicle still being under factory warranty, we decided to contact the vehicle’s owner directly and get the dealership service department involved before we proceeded any further.
Contact Made:
We contacted the vehicle owner to obtain permission to inspect the vehicle and discuss his concerns. According to the owner, the vehicle was having erratic upshifts and downshifts at times. He said he had taken his vehicle to the local dealership several times, but they were unable to duplicate his concerns. According to the customer, the dealership referred him to a local transmission shop for inspection. After talking to the owner and obtaining permission to inspect his vehicle, we contacted our local Infinity Dealership Service Department manager to discuss their customer’s concerns.
The Story Unfolds:
According to the service manager, he was aware of this customer’s concerns. He had referred the customer to a local transmission shop for inspection, as he did not have a transmission technician available at the time. According to the service department manager, the transmission shop diagnosed the erratic shifting concern as an internal transmission problem and requested a transmission assembly. The service manager said they did everything to expedite the transmission order and delivered the transmission assembly to the transmission shop within just a few days. The service manager said the transmission shop had contacted the dealership parts department a few days later and requested another transmission assembly, saying the new transmission assembly had the same problem as the original transmission. The service manager said that he had contacted the transmission shop manager to discuss the erratic transmission operation after installing a new assembly. The service manager requested that the transmission shop manager take another look at the vehicle before ordering another transmission assembly. After obtaining permission from the service department manager to inspect the vehicle, we contacted the referring shop manager to discuss the diagnostic procedures taken to diagnose the transmission’s erratic shift concerns.
What Diagnostic Procedures?
After a brief conversation with the vehicle owner and the dealership service manager, we contacted the referring shop’s service manager to discuss the diagnostic procedures used to diagnose the transmission’s erratic shift concerns. According to the transmission shop manager, there were no pressure taps on the transmission, and there were no codes in the Transmission Control Module (TCM). With the complete story from the vehicle owner, the dealership service manager, and the referring shop, it was time to head out on our initial test drive to see if we could duplicate the problem.
Duplicate to Diagnose:
In preparation for our initial test drive, we connected our scan tool to the vehicle Data Link Connector (DLC) and checked each module for codes. There were no codes stored in the TCM; however, there was code U1001 (CAN Comm) stored in the Powertrain Control Module (PCM) and code C1707 (rear LH tire pressure low) stored in the Tire Pressure Monitoring System (TPMS). We checked the tire pressures and found the left rear tire pressure was just under 26 psi, while the other three tires were 32 psi. We inflated all tires to 35 psi, as recommended by the manufacturer. We documented the codes and cleared them from the PCM and TPMS. With all computer systems cleared of codes, we headed out for our initial test drive.
Initial Test Drive:
With the scan tool connected to the DLC and codes cleared from all computer systems, we headed out for our initial test drive. During the test drive, we drove through residential areas and city streets. We drove in light, heavy, and stop-and-go traffic with no problems at all. We drove the vehicle on the freeway at speeds of up to 60-70 mph, and the vehicle seemed to work perfectly. We continued to drive the vehicle several miles with no problems at all. After completing our initial test drive with no problems at all, it was time to head back to the shop.
Back at The Shop:
With the vehicle back at the shop, we checked each of the computer systems for codes. With no codes set in any module and unable to duplicate the owner’s concerns, we knew we would not be able to diagnose the erratic shifting concern. According to the customer, the vehicle started shifting erratically a couple of months ago. He said he took the vehicle to his local dealership several times, but they were unable to duplicate the problem. The owner requested that we keep his vehicle for a couple of days, saying, “Use it like it was yours,” to see if we could duplicate the problem. We agreed to keep the vehicle for a couple of days.
Days Go By:
With the owner’s permission, we continued to drive the vehicle. Using the vehicle to shuttle customers back and forth to their homes and to work. We even made parts runs with it. Each time, the vehicle worked perfectly. One, two, three days go by with the vehicle working perfectly, no problems whatsoever. At the end of the fourth day, after taking our last customer home and returning to the shop, our driver said he noticed a harsh downshift clunk while coming to a stop. It was late, so we pulled the vehicle into the shop, closed for the evening, hoping the morning would shine a new light on the problem.
The Next Morning:
The next morning came bright and early. As usual, we came in early to open the shop and get ready for the day.
As our driver was pulling the vehicle out of the shop, he noticed that there was a warning light illuminated on the dashboard. After parking the vehicle in front of the bay, we connected our scan tool to the DLC to check for codes. We performed a complete vehicle scan and found no codes stored. We scanned each module and found C1707 (rear LH tire pressure low) set in the TPMS, and the “low tire pressure” warning light illuminated in the Instrument Panel Cluster (IPC) (Figure 1).
We checked all of the tire pressures and found the rear LH (left/rear) tire pressure was around 25 psi, while the other three tires had 35 psi, indicating that the rear left tire had a slow leak. We contacted the vehicle’s owner to get permission to have his tire repaired. During the conversation, the owner indicated that the low tire warning light had been coming on for the last month or so, but he did not have time to fix it. He said he would add air to the tire when the dashboard light came on. I asked him if the erratic shifting was happening at the same time the low tire warning light was illuminated. He said it was possible, but he did not know for sure. Rechecking Vehicle Operation:
Usually, we would not test drive a vehicle with a low tire; we would inflate it and then go for the test drive, but we wanted to see if the problem occurred with the warning light illuminated. As before, we drove through residential areas and city streets. We drove it in light, heavy, and stop-and-go traffic. This time, the transmission shifting was erratic, with intermittent harsh upshifts and coasting downshifts. There was no need to drive the vehicle on the freeway because the erratic shifting was obvious. This was the concern that the vehicle owner was describing. With the erratic shifting concern duplicated, it was time to head back to the shop to get a better understanding of how the TPMS can interfere with transmission operation.
Understanding Infinity’s/ Nissan’s TPMS:
The 2024 Infinity Q50 has a very unique TPMS compared to other manufacturers. Infinity/Nissan incorporates its TPMS into the Remote Keyless Entry System (RKES), while other manufacturers use a stand-alone TPMS module. The Infinity/Nissan TPMS uses the RKES to monitor individual tire pressures and then transmits that information to the vehicle’s Body Control Module (BCM) over the data line system (Figure 2). The BCM then sends information to companion modules over the data line system. The IPC uses the information to illuminate the low tire pressure warning light. The PCM uses that information for engine load and torque management. The PCM then shares the engine load and torque demands with the TCM, which adjusts transmission shift timing and feel to correspond to engine load and torque demands.
Repair in Sight:
With a better understanding of how the Infinity/Nissan TPMS can interfere with transmission operation, we were confident that we had found the cause of the problem. We removed the rear left tire and sent it to a tire repair shop for repair. We received the repaired tire later that afternoon and installed it onto the vehicle. We cleared the TPMS and drove the vehicle extensively before delivering it back to the vehicle’s owner. We spoke with the vehicle’s owner several times over the next few weeks, and the transmission was working perfectly.
Well, there you have it. With a better understanding of TPMS operation, you may be able to repair those erratic shifting transmissions and keep those trannys rolling down the road.
