Back on the Bench - December - 2020

Nag 1/722.6 P0741 TCC Slip

A friend of mine recently called me on a 2012 dodge charger with a NAG 1, aka, 722.6 in it. The customer stated the check engine light was on, and he found a P0741 set, no debris found in the pan and clean fluid that didn’t have an odor to it. And lock up seemed to be working fine.

We discussed possible causes that might set the code, and some things to check. The computer commands the solenoid, the solenoid strokes the valve. The valve opens a port for regulated pressure from the lube regulator valve, to the piston in the converter to lock down the converter clutch. So possible causes could be a computer failing to command the solenoid properly, a solenoid mechanical malfunction, valves, or a leak in the circuit from the valve to the converter.

Next, we discussed normal operation of the torque converter clutch and computer commands. The torque converter clutch is always slipping somewhere between 5% to 95% when engaged, which depends on vehicle speed and engine load.. We discussed the information to monitor on the scan tool, there is an actual slip, desired slip, and TCC status. There are four modes of TCC operation in status, open, openslip, slip and slip-open, the computer is only monitoring the actual and desired slip during slip mode. If this slip exceeds sixty RPM, when command is in slip mode, three times in one key cycle, the code P0741 will set.

I recommended we back probe the solenoid command wire, and record data with a scan tool in graph mode. The first thing I want to verify is the duty cycle on the wire matches what we see on the scan tool. Excessive deviation between these could indicate an internal computer malfunction or a power or ground issue to the computer.

We also wanted to monitor line pressure command and engine load percentage. A drop in commanded pressure would also cause slippage in the transmission. Normal load percentage is between ten to twenty percent at a idle, and should come up to ninety to a hundred percent at WOT in gear. And problems here we would want to check mass air flow sensor and throttle position sensor operations for any malfunctions.

And last but not least, we wanted to monitor torque converter clutch slip, actual and desired, during slip mode. From the time the vehicle was cold, until we saw slip greater than sixty RPMs, while in slip mode. Which according to the customer, the problem usually occurred after about thirty minutes of highway driving.

He took the vehicle out on the road, with equipment attached. At first, all the signals matched up, scan tool to meter, actual slip and desired slip was very close, within parameters. After it got warm, he noticed the actual and desired slip was falling out of parameters (figure 1), while the meter and scan tool information was matching, and engine data appeared to be what was expected. The problem is in the transmission.

We previously discussed possible causes, so we decided to drop the valve body, vacuum test the valve body, check for broken or collapsing springs (figures 2A&B), check the solenoid, and air check the converter clutch through the case (figure 3). There are three valves involved in the control of the converter clutch, the TCC damper, which works as an accumulator, the lube pressure regulator, which works as a pressure regulator, and the TCC regulator, which works as on/ off for the converter clutch. Springs are good, bores good, the valve body and solenoid was hot tanked and retested to verify no valves was sticking after it was warm, and the solenoid was still functioning properly.

Air checking the converter didn’t go so well. The Mercedes converter clutch is set up just like a clutch pack in the transmission (figure 4), and should air check just like most clutches fed through sealing rings. This one seemed ok under 35 psi, but with full shop air pressure, it would allow an excessive amount of blow back through the converter feed. So a little deeper into the transmission we go.

This leak could be in the converter, the seals on the K2 drum, or in the bore of the stator where they rode, in the pump, pump to case, or a problem in the case. At this point, I recommend we pull the transmission, remove the K2 drum, and air check the converter clutch on the bench, through the K2 drum (figure 3). If it air checked good, we need to check the other components, if not, replace the converter and inspect the transmission for any damage.

The inside of the transmission still looked like new, and the converter failed the air check. Mike cut the converter open and found the piston had a hairline crack (figure 4). He offered the customer the option of going with a rebuild, for the purpose of a full warranty, or resealing the transmission and replacing the converter, which he stated is all it really needs, and would be a lot less money out of her pocket.

The customer decided since it didn’t really need it, and she would like to have a little more vacation money, that reseal and converter option was the way to go. So Mike resealed the transmission, put in a new converter, filter and some fresh fluid. The vehicle was driven home overnight to verify the fix, the code did not return, and was delivered the following day.

It’s always a great thing when we can save a customer some money. After all, it’s hard to come by, and most people would rather spend it on things they want and enjoy over things they have to spend it on, like car and home repairs. Taking the time to save a person some money, and preventing it from ending back on the bench, can provide us with something worth more than money. And that’s a good reputation through helping our neighbors.