Back on the Bench - May - 2020

RE0F11A Diagnosing Performance Codes

Often in life, things are not always what they seem, and with today’s automobiles, we frequently come across codes with the same title but a different definition. Solenoid performance codes are one of those that can sometimes be misleading. And most of the time, when we see a solenoid performance code on the scan tool, the solenoid is not the cause of the problem.

Manufacturers usually program their computers to monitor voltage feedback from the solenoid to verify circuit integrity, to confirm it’s not a circuit issue. Many use pressure sensors to check that a solenoid functions properly. Some use simple on-off switches, while others use sensors to monitor the amount of pressure through voltage change.

The computer operates the solenoid, a valve strokes, which opens or closes a port to a clutch, band or pulley and to a pressure sensor. If the computer does not see a change in the state of the pressure sensor or a specific voltage, it sets a solenoid performance code. This means the solenoid has failed mechanically, a valve is unable to stroke properly, or the pressure sensor or its circuit is faulty.

We’ve seen this code several times, and we sometimes get stuck in a mindset that this type of code is a valve body, solenoid, or sensor related issue. With many CVT transmissions, all those components are in or on the valve body. Most of the time, this leads us to get a reman valve body, installing it, only to find none of those was the cause of the problem.

It’s always best to look up the code and carefully read the description. For some manufacturers, a solenoid performance code is set because the computer does not see the proper gear ratio.

Models, like a 2012 Nissan versa, for example, do not have a primary pressure sensor (Figure 1), so it uses performance values as a measure. Code P0746, for example, is set when the pulley ratio exceeds 2.55 for 200 milliseconds or 3.35 for 100 milliseconds. Essentially, this is a gear ratio error code, which we approach a little differently than most solenoid performance codes. And this brings a few other things into the game. The speed sensors, which the computer uses to monitor ratio, a leak anywhere in the entire hydraulic circuit, the pulleys, the cover, cover to case, and case to the valve body.

With a scan tool attached, we want to look at the data stream in graph mode. This usually catches any sudden glitches in performance that you might not see in text mode, as well as seeing the history of operation while we drive the vehicle. Some scan tools provide a gear ratio PID, which makes it easy to verify if the gear ratio is exceeding the set parameters. With others, you’ll have to take the output speed and divide by input speed to determine the gear ratio.

In some rare cases, you might have the correct gear ratio in both ranges the pulleys operate in, and still set a performance code. This problem could be in the software of the computer, and it is often best to check for software updates. We can verify the current software in the vehicle by checking our global OBDII mode nine calibration. This will give us the current software version, with that and the vehicle identification number, we can check with the dealer or a mobile programmer for software updates.

The Versa we are working on seemed to be working fine cold, but once it warmed up, we could see the transmission start to slip and exceed the parameters set by the manufacturer. Our next step was to check the pressure command on the scan tool, and actual pressure on the primary pulley tap. We found the pressure was good cold, but once it warmed up, we could see the pressure drop and the computer try to command the solenoid to raise pressure to compensate for slippage, but unable to manage it, and set a code.

On rare occasions, computers can fail to operate a solenoid properly due to a failed solenoid driver inside the computer and show the correct readings on the scan tool. So before chasing leaks inside the transmission, it’s best to check the amperage or voltage on the solenoid command wire compared to the scan tool reading. For most scan tools, that reading is simply what the computer wants to do, not what it is doing. In this Versa, the voltage versus scan tool reading matched up.

Next, we dropped the pan, which had only minor debris, and removed the valve body to vacuum check it, and air check the pulleys. Hoping to find something in the valve body to be the hero of the day by saving the customer a bit of money, I disassembled, cleaned, and did my vacuum test (Figure 2). The valve body test appeared to pass with slight wear but within specifications.

Air checking the pulleys (Figure 3), is not like clutch pistons or band servos. They’ll barely be moved with air most time. We are listening for any excessive air leaks from inside the transmission. The primary pulley had an excessive amount of air leaking from inside the transmission, so it was time to give the customer the bad news and request removal for further inspection.

Once we got the transmission on the bench and opened up, we found the piston inside the primary pulley was worn (figure 4). This is a common issue with these transmissions, and fortunately, there are aftermarket kits to repair this. We made the repair and then did a standard build. After install, we checked our gear ratios and trouble code after it warmed up. It worked great!

We returned the car to the customer and waited for them to return for the ten-day check. No return of any codes and one satisfied customer.

It’s a great thing when we can do in car repairs like repairing or replacing a valve body to save the customer from the cost of a rebuild. Without the proper diagnostic routine, we are just replacing parts and hoping this will work. Sometimes it does, sometimes it cost the customer more money than they needed to spend. Taking the time to do it right, will make our customers happy, and keep these transmissions from landing back on the bench.