In this issue of Keep Those Trannys Rolling, we will look at one of the most common calls coming into the ATRA Hotline regarding the 6L series transmission, “repeat repairs.” Repeat repairs can be a big hassle for the shop, the technician, and most of all, the customer. There’s nothing worse than delivering a vehicle to a customer on Friday night, just to have it returned on Monday morning. Repeat repairs cost shops, technicians, and customers time and money, and in the transmission repair business, “Time is Money.” Now, let’s take a moment to explain what a repeat repair is.
REPEAT REPAIR
Unlike a comeback, a repeat repair has to be repeated over and over again, a continuing problem. Most repeat repairs are due to improper diagnosis or no diagnosis at all. In most cases, the transmission is removed and disassembled to locate a problem. Usually, there are signs of failure, fluid burnt, burnt or worn clutches, or metal in the pan. These are all indications of an impending failure. With the transmission disassembled on the bench, you may find several clutches showing signs of wear or even burnt. As you repaired the transmission, replacing parts as needed, you air checked each component at low pressure prior to assembling them into the transmission case. With the components assembled in the case and the pump installed, you air-check each component again at low pressure to confirm each component is capable of applying. Once the case is assembled, you would inspect the valve body for wear. You may find worn valves and bores in the valve body. After repairing the valve body as needed, you install the TECHM onto the valve body and install the TECHM/valve body assembly onto the case. After installing the pan, the transmission assembly is ready to install into the vehicle. Once the transmission is installed into the vehicle, you fill it with fluid, perform the relearn procedure, and head out for a test drive. If everything goes well during the test drive, you deliver the vehicle back to the customer. Unfortunately, this is not the end of the story.
REMOVE, REPAIR, REPEAT
Within a couple of days, maybe a week or two, the vehicle is back on your doorstep with the same problem: fluid burnt, signs of clutch wear, and metal in the pan. You remove the transmission for disassembly and inspection. During the disassembly and inspection, you find the same clutches worn or burnt again. With no apparent cause for the worn clutches, you go ahead and repair the transmission as needed, rechecking components, valve body, and TECHM. You then install the transmission assembly back into the vehicle, refill it with fluid, and head out for another test drive. Again, everything goes well on the test drive so you return the vehicle back to the customer. Unfortunately, the vehicle returns within a couple of weeks with the same problem. This time, you will need to take a closer look at what may be causing this repeat repair.
TAKING A CLOSER LOOK & WHAT SHOULD HAVE HAPPENED
With the vehicle returning for the third time for the same repair, it was obvious you were missing something here. It is time to take a closer look at the transmission operation. You installed a pressure gauge onto the transmission and connected our scan tool to the Data Link Connector (DLC), so you could monitor transmission operation during our test drive. During the test drive, you monitored line pressure and pressure rise. While driving the vehicle, you will notice that the line pressure and pressure rise were okay as long as you were accelerating. Once you moved from an accelerating mode to a cruising mode, you noticed that the line pressure and pressure rise would become erratic, sometimes dropping to as low as 60 psi. With the scan tool connected to the DLC, we could monitor commanded pressure and pressure rise. According to the scan tool, the erratic pressure was being commanded by the TECHM. With an erratic pressure problem, it was time to head back to the shop for further diagnosis.
BACK AT THE SHOP
With the vehicle back at the shop, you tried to use the scan tool to manipulate the pressure control solenoid to see if you could control the pressure and pressure rise. You could control the pressure control solenoid and ramp the line pressure up and down as you requested. This was an indication that the TECHM was capable of controlling line pressure and line rise as needed. With the TECHM capable of controlling pressure and pressure rise, it was obvious that the TECHM was not causing the pressure to drop while driving. With the TECHM capable of producing the needed line pressure and line rise, it was time to get back on the road again for another test drive to see if you could determine the cause of the line pressure drop while driving.
ON THE ROAD AGAIN
With the scan tool and pressure gauge connected, you headed back out on the road to see if you could determine the cause of the commanded line pressure drop while driving. You previously monitored the pressure control solenoid operation and verified that the pressure drop was being commanded. You monitored the engine load percentage when the transmission line pressure dropped. According to the scan tool, the engine load percentage would drop to around 25% during the pressure drop. This was an indication that you may be having an engine load problem. With the commanded pressure drop confirmed, you headed back to the shop to continue our diagnosis.
CONTINUING DIAGNOSIS
With the vehicle back at the shop, you started your diagnostic routine by checking for codes in the computer system. There were no codes in any of the modules. You did a visual inspection of the load sensors, mass air flow (MAF), throttle position sensor (TPS), manifold absolute pressure (MAP), and accelerator position sensor (APPS). All the load sensors seemed to be okay. You monitored each sensor individually and found that the MAF sensor signal was erratic at times. 
You checked the MAF sensor wiring and connections. Everything looked good there. During our MAF diagnostic routine, you raised the engine RPM up and down and monitored the MAF operation. You noticed that the MAF sensor signal would become erratic during steady throttle. After further inspection of the MAF system, you found the MAF hose cracked (Figure 1), allowing air to bypass the MAF sensor causing the engine load percentage to drop while driving. You replaced the MAF hose and rechecked the MAF sensor operation. With the MAF hose replaced, the engine load percentage would remain steady during acceleration and deceleration. It was time to head out for another test drive.
RESOLUTION CONFIRMED
During the test drive, you monitored the transmission line pressure, pressure rise, and engine load percentage. Line pressure and pressure rise were steady throughout the test drive. The engine load percentage would remain steady during acceleration and while cruising. Everything seems to be working properly. With the line pressure and rise working properly, it was time to repair the transmission. You removed the transmission, disassembled and repaired the burnt clutches as needed. You reassembled and installed the transmission assembly and headed out for (hopefully) our final test drive.
FINAL TEST DRIVE
With line pressure and pressure rise working properly, you headed out for your final test drive. During the test drive, you monitored line pressure, pressure rise, and engine load %. Line pressure and pressure rise were working properly. You monitored the engine load percentage and confirmed it was operating properly. With line pressure rise and engine load working properly, it was time to head back to the shop for the final vehicle inspection.
BACK AT SHOP
With the vehicle back at the shop, you rechecked the computer for codes; there were none. You recheck the transmission fluid level and check for leaks. The fluid level was good, and there were no leaks. You had the vehicle washed and contacted the customer to let them know their vehicle ready to be picked up. You did a good job.
With a little patience and a complete diagnosis before removing the transmission for disassembly and inspection, you too can beat the “repeat repair” blues and keep those trannys rolling down the road.
