Keep Those Trannys Rolling - March - 2020

Conquering the Ghosts of an AS68RC

In this issue of “Keep Those Trannys Rolling”, we’re going to take a look at a 2010 Ram 5500 Cab and Chassis truck that was having erratic transmission operation and turning on the malfunction indicator light (MIL) at times.

This 2010 Ram 5500 Cab and Chassis truck is equipped with a 6.7D and an AS68RC transmission. The Ram 5500 (Figure 1) was towed into our shop due to erratic transmission operation. The customer indicated that he had been experiencing erratic transmission operation, such as wrong gear starts, harsh or erratic shifts, no shifting at times and the “check engine” (MIL) light would illuminate during the erratic transmission operation. The customer also indicated that “cycling the ignition key” would turn off the “check engine” light and allow the transmission to go back to operating properly.

figure 1: vehicle towed in for transmission repairInitial Inspection

On the initial inspection, both battery connections were checked and the under hood harnesses for signs of chafing or damage, none was found. The transmission fluid was dark, smelled burnt and had clutch material floating around in the fluid, possibly indicating an internal transmission problem. Next, the vehicle was checked for codes. There were no codes in the powertrain control module (PCM), but there were numerous codes stored in the transmission control module (TCM). The shop documented and cleared all of the codes from the TCM. The scanner was left connected to the DLC connector, so they could monitor the transmission operation during the initial test drive, and see if they could duplicate the customer’s concerns.

figure 2: insulation missing from wires

Duplicate to Diagnose

While driving the vehicle, they monitored the engine load inputs, such as accelerator position sensor (APPS), manifold absolute pressure sensor (MAP) and mass air flow sensor (MAF). They monitored transmission input and output speed sensors (ISS/OSS), solenoid operation, transmission range switch (TRS) and transmission fluid temperature (TFT) operation. They had started the test drive through city streets and then headed for the freeway. During the drive through city streets, the transmission seemed to be working properly with no concerns. While accelerating to get onto the freeway, they noticed the transmission was starting to slip. As the transmission temperature neared normal operation, the transmission started slipping during hard acceleration.

Back at the Shop

After duplicating a transmission slip concern, they checked the computer system for codes. There were no codes set in the PCM or the TCM. They had installed a pressure gauge on the line pressure tap (located on left side of transmission) and checked base pressure at an idle and pressure rise at 1500 rpm. Base pressure and pressure rise were both within specifications. They removed the transmission pan to perform a visual inspection. During this inspection, they found the transmission fluid was burnt and there was clutch material floating around in the fluid. They had decided to remove the transmission and inspect it for an internal problem.

Transmission Inspection and Repair

With the transmission apart on the bench, they air checked each component with 35 psi of regulated air pressure. Each component air checked and seemed to hold pressure. The K2 and K3 clutches were slightly burnt. All other components seemed to be okay. They replaced all of the seals and clutches, went through the valve body and replaced the torque converter. After filling the transmission with fluid and performing a visual inspection, it was time to head out for another test drive and recheck the transmission operation.

figure 3: transmission harness removed

Rechecking Transmission Operation

Prior to test driving the vehicle to recheck the transmission operation, they installed the scanner to the DLC connector to monitor transmission operation during the test drive. During the test drive, the transmission seemed to work perfectly with no problems at all. But, as soon as they started heading back to the shop, the transmission did a weird downshift and the “check engine” light came on. After returning to the shop, they checked the computer systems for codes. There were no codes stored in the PCM, but there were several codes stored in the TCM. These codes were P0702 (Transmission Control System Electrical), P0706 (Transmission Range Switch) and P0711 (Transmission Fluid Temperature). They cleared the codes and monitored the transmission range switch & transmission fluid temperature sensor circuits. The PIDs for both seemed to be within range, but, as soon as they started the vehicle, the “check engine” light came back on. They rechecked the system for codes and found P0745 (Line Pressure Solenoid Circuit), P0717 (ISS), P0748 (Pressure Control Solenoid A Circuit) and P0973 (Shift Solenoid A Circuit) stored in the TCM. Cleared the codes again from the TCM, and again, the “check engine” light came back on immediately. They rechecked the TCM for codes and found P0711 (Transmission Fluid Temperature), P02742 (Transmission Temperature Sensor 2 Circuit) and U0001 (CAN C BUS) stored in the TCM. Each time they cleared the TCM of codes, the “check engine” light would come back on immediately and a different set of codes would set. It was as if it had been infected by a ghost. With codes being set almost immediately after starting the vehicle, it was time to take a look at the TCM circuits.

figure 4: wire insulation melted, falking offTCM Circuits

They started checking the circuits by monitoring the transmission PIDs with the scanner. With the key on and engine off, the PIDs seemed to be working properly, but as soon as we started the vehicle, the PIDs would become erratic. They inspected the wiring harness going from the TCM to the transmission. The wiring harness looked okay, with no signs of damage or chaffing. They decided to remove the protective covering from the harness to inspect the individual wires and that’s where we found our ghost.

Ghost Located and Eliminated

After removing the protective covering from the harness, there were several wires with the insulation missing from the wire (Figure 2). They removed the entire TCM to transmission wiring harness to fully inspect the harness. After removing the protective covering from the harness (Figure 3), several wires were missing sections of insulation in different spots on the harness (Figure 4). The insulation on the wires was hard and brittle and would break off in sections while bending the wires around. They contacted their local Chrysler/Dodge dealership and obtained a new TCM to transmission wiring harness for approximately $225. After installing the new harness, they monitored the transmission PIDs while pulling and moving the transmission harness around. The transmission PIDs did not change, so it was time to head out for another test drive and recheck the transmission operation. During the test drive, the transmission worked perfectly and the transmission PIDs were steady. We test drove the vehicle several times with no problems and delivered the vehicle back to the customer.

Well there you have it, conquered a ghost and was able to bring the transmission back to life. With a better understanding of how the electrical system controls the transmission, you should have no problem conquering your own ghosts and keep those trannys rolling down the road.

(Special thanks to Brad Benrud at Allen Automatic Transmissions in La Crosse, Wisconsin for his contribution to the vehicle repair process and story line)