Keep Those Trannys Rolling - May - 2021

LCT1000- No Move. Duplicate to Diagnose

In this issue of “Keep Those Trannys Rolling,” we are going to take a look at a 2016 Chevrolet Silverado K3500 that was having an intermittent “no move at times” concern.

History Review:

This concern had been going on for some time. The customer had taken the vehicle to multiple dealerships. The first shop found the transmission fluid was discolored and smelled bad, so they recommended replacing the transmission as an assembly. The second shop removed the TCM-to-transmission harness for inspection and found the insulation on the wires had become brittle and was flaking off. So, they replaced the harness.

According to the dealership repair order, the dealership was unable to duplicate the customer’s concern, so they performed pinpoint tests according to the symptoms. The pinpoint test did not identify any faults and recommended replacing the TCM with a known good TCM. With the customer’s authorization, the dealership replaced the TCM and programmed it with the latest calibration. Unfortunately, replacing the TCM did not resolve the customer’s concern.

Initial Inspection:

The 2016 Chevrolet K3500 was well maintained and was equipped with a 6.6D and an Allison LCT1000 transmission. During the initial inspection, we checked under the hood to see if we couldn’t see any obvious problems. We checked the transmission fluid level and condition. The fluid level was correct, and the fluid was red and clean. We checked the TCM-to-transmission wiring harness to verify proper installation and routing. Everything looked good there. We inspected the TCM for proper installation. We then accessed the TCM with our scanner and verified the TCM had the latest calibration installed. We lifted the vehicle on a rack to inspect the underside. Everything looked good. With the initial inspection completed, it was time to check the vehicle operation to see if we could duplicate the “no move at times” concern.

Duplicate to Diagnose:

After completing the initial inspection, we connected our scanner to the DLC connector to monitor the transmission operation. We started the vehicle and monitored the PRNDL display and pressure switches operation. Everything looked good. We headed out for our initial test drive to see how the vehicle operated. During the test drive, we monitored transmission PIDS, such as transmission temperature, transmission range switch, output and input speed sensors, pressure switches, and solenoid operation. The transmission seemed to work correctly, and the transmission PIDS were responding properly. We returned to the shop and checked the TCM for codes. No codes were present. We checked all other computer systems for codes. There were no codes present in any of the modules. Unfortunately, we were unable to duplicate the concern during our initial test drive. We continued to drive the vehicle several times a day for the next week. According to the customer, the “no move at times” concern happens frequently, usually after starting the vehicle and then engaging the transmission into gear. The customer indicated that the PRNDL display would go blank when the “no move at times” concern occurs.

We brought the vehicle back to the shop and then crawled underneath the vehicle. We pulled and tugged on the transmission harness (wiggle test) while monitoring the transmission range switch PIDS to see if we could see any changes in the PIDS. There were no changes. The transmission range switch circuit seemed to be working properly. We again contacted the customer.

Concern Duplicated:

We asked the customer to come to the shop to see if he could duplicate the problem. We connected the scanner to the DLC connector to monitor the transmission range switch input. Part of diagnosing is listening and duplicating the customers driving habits. The customer got into the truck and slid the seat up, commenting that he was short and needed to be closer to the steering wheel. The vehicle started up and seemed to run perfectly, and the PRNDL display was illuminated. The customer then tilted the steering wheel down, again commenting that he needed to be close to the steering wheel. As soon as the customer tilted the steering wheel down, the PRNDL display went blank, and the transmission would not engage into gear. While monitoring the transmission range switch, the scanner momentarily lost the transmission range switch input, and then it reappeared. This was an indication of a TCM power or ground loss. This was good news. We had duplicated the “no move at times” concern, and we had an idea of where to go to correct this concern.

TCM Power and Grounds:

The first step to our diagnostic routine would be to monitor the power and grounds going to the TCM while moving the tilt wheel up and down. With our DVOM back probed into TCM connector pin #66 (Figure 1) (batt +), we moved the tilt wheel up and down to see if there was a change in the battery voltage going to the TCM. There was no change. We then back probed into TCM pin #35 (run/crank/ignition 1, figure 2) and moved the tilt wheel up and down. During the tilt wheel movement, we noticed that the voltage going to TCM pin #35 would drop off intermittently, causing the TCM to go offline. Problem Found and Repaired: After verifying the power drop on TCM pin #35, we disassembled the steering column and found the ignition switch was starting to come apart. We replaced the ignition switch (Figure 3) and reassembled the column. We rechecked the transmission operation while moving the tilt wheel through its ranges. The transmission worked perfectly. We continued to check the transmission operation throughout the day, and each time the transmission worked perfectly during tilt wheel operation. We returned the vehicle to the customer, and he has been driving around for the last month with no problems.

Well, there you have it. We fixed another one. With a little bit of patience and the ability to duplicate and diagnose the concern, you should have no problem “Keeping Those Trannys Rolling” down the road.