Keep Those Trannys Rolling - May - 2023

My Silverado Runs Hot

In this issue of Keep Those Trannys Rolling, we will look at a 2020 Chevrolet Silverado K1500 truck, equipped with a 5.3L engine and an 8L90 transmission, that was experiencing a transmission “running hot” concern.

The vehicle first came to our attention during a conversation with a fleet manager who came by the shop to pick up one of their fleet vehicles after servicing. The fleet manager indicated that his company had purchased a used 2020 Silverado several months ago. The dash-mounted transmission temperature gauge said the transmission was “running hot.” He indicated that they had taken the vehicle back to his G.M. Dealership for inspection. According to the repair order, they replaced the torque converter, flushed the transmission system with new fluid, and updated the computer system with the latest calibration update. Unfortunately, the “running hot” condition continued. After a brief discussion with the fleet manager, we scheduled an appointment for the following morning.


The next morning, we arrived at the shop and found the truck sitting in our yard. We contacted the fleet manager to get the whole story and discuss his concerns. According to the fleet manager, the transmission temperature gauge would slowly rise until it reached approximately 260 degrees after a thirty to forty-minute drive. At this time, the “ck engine” light would illuminate. He said the driver would have to pull over to the side of the road and wait for the transmission temperature to cool down before he could continue on. After getting the whole story from the fleet manager, it was time to see if we could duplicate the fleet manager’s “running hot” concern.


We connected our scan tool to the Data Link Connector (DLC) so we could monitor the transmission temperature during the test drive. We scanned the vehicle’s computer systems. We found codes P0712 (tft low volts-exceeds 260°) and P2808 (pcs 7-tcc perf) stored in the Transmission Control Module (TCM) memory, but no current codes in any module. We cleared the codes from the TCM memory and headed out for our first test drive. We drove the vehicle through city streets at 30-35 mph and out on the highway at 65-70 mph. At first, the transmission temperature gauge and scanner were reading fairly normal temperatures, around 195-200 degrees. As the test drive became longer, we noticed that the transmission fluid temperature was climbing. After twenty minutes of driving, the transmission fluid temperature had climbed to just over 250 degrees. After duplicating the “running hot” concern, it was time to head back to the shop to diagnose this concern.


With the vehicle back at the shop, we pulled out our thermal gun to compare the temperature gauge, scanner, and thermal gun readings. All three were within ten degrees of each other. We used the thermal gun to scan the temperature of the fluid going into and out of the cooler. There was a 25-degree drop across the cooler, which indicated that the cooler was capable of cooling the fluid. We checked the airflow going through the radiator and transmission cooler. We checked the cooling fan operation. There were no airflow restrictions, and the cooling fan was working properly. We rechecked the vehicle computer systems for codes and found P0712 (tft low volts-exceeds 260°) and P2808 (pcs 7-tcc perf) pending in the TCM. With codes P0712 (tft low volts-exceeds 260°) and P2808 (pcs 7-tcc perf) pending in the TCM, it was time to take a closer look at the transmission’s TCC operation.


We headed out for another test drive with the scanner connected to the DLC connector and a pressure gauge attached to the line pressure tap on the transmission. During the test drive, we monitored line pressure, PCS 7 status, and TCC slip. During the test drive, we noticed the PCS 7 status would change from “OFF” to “ON,” and the TCC slip would decrease and increase depending upon PCS 7 status. During the test drive through city streets, we noticed that the TCC slip would remain low, approximately 0-25 RPM. During the test drive on the highway, we noticed that PCS 7 would continue to indicate “OFF” and “ON,” but the TCC slip would increase to over 300 RPM at times when PCS 7 was commanded “ON.” The line pressure was responsive and within specification during the test drive. This was an indication that the TCC was slipping at highway speeds. After confirming the TCC slip concern, we headed back to the shop to continue with our diagnosis.


After returning to the shop, we put the vehicle on a rack and lifted the wheels off the ground. We test-drove the vehicle with the wheels off the ground while monitoring tcc slip RPM. With the wheels off the ground, the tcc slip remained at -10-0 RPM. This was an indication that the TCC apply circuit was possibly leaking TCC apply pressure, causing excessive TCC slip under a load. With the torque converter being replaced by the local G.M. dealership with no change to the running hot concern, we were confident that we were looking at a worn valve body. We removed the valve body for inspection. After a thorough cleaning, we inspected the valve body for wear. We vacuum-checked the upper and lower valve bodies and confirmed that the TCC control valve (figure 1), the converter feed limit valve (figure 2), the pressure regulator/ shuttle valves (figure 3), and several end plugs were leaking. With the valve body valves and end plugs leaking, it was time to replace the valve body with a new or rebuilt valve body.


After confirming that the valve body was worn out, we contacted our local G.M. dealership parts department to order a new valve body. Unfortunately, a new valve body was not available. So, we contacted our local parts supplier to order a rebuilt valve body. According to our local parts supplier, the rebuilt valve body was on backorder and would not arrive for four to six weeks. We contacted the fleet manager to inform him of the parts delay. The fleet manager asked if there was anything we could do to repair the valve body while waiting for the rebuilt valve body to arrive. We informed the fleet manager that aftermarket kits were available to address these particular concerns. With the fleet manager’s approval, we contacted our local parts supplier and were able to obtain the aftermarket TCC repair kit. We disassembled the valve body and replaced the TCC apply valve, the converter feed limit valve, and the pressure regulator/shuttle valves. We replaced the leaking end plugs as needed. We vacuum-checked the TCC control valve, converter feed limit valve & pressure regulator valves after the repair, and each of them passed the vacuum check. We flow-tested PCS 7 to verify proper operation and then reassembled the valve body. We installed the valve body back onto the transmission and refilled the transmission with Dexron LV-HP fluid. We allowed the vehicle to reach a normal operating temperature of 194 degrees before rechecking the fluid level. We ran the vehicle on the rack to check for proper operation. The transmission seemed to work properly during the rack test. We lowered the vehicle to the floor and prepared for our final test drive.


With the scan tool connected to the DLC to monitor the transmission temperature and TCC slip, we headed out for our final test drive. We drove the vehicle through city streets and onto the highway. We drove for over thirty miles at highway speeds while monitoring transmission temperature and TCC slip. The transmission fluid temperature stayed between 195 degrees and 205 degrees, and the TCC slip remained low during the test drive. After a lengthy test drive, we returned to the shop and performed our final inspection. We checked for leaks under the vehicle. There were none. We recheck all fluid levels. We checked for codes in all of the computer systems. There were no pending or current codes anywhere. We cleaned up the vehicle and contacted the fleet manager to let him know that his vehicle was ready to go. The fleet manager has been driving his vehicle for the last month, waiting for his rebuilt valve body to arrive, and says the truck is working better than ever.

Well, there you have it. We repaired a worn-out valve body with an aftermarket kit and got this fleet vehicle back on the road. With the help of our aftermarket friends, you too should have no problem keeping those trannys rolling down the road.