Diagnosed & Fixed: P0747 Code in General Motors 10-Speed Transmissions

What Causes the P0747 Code?

     As many know, we engineer and manufacture upgrades and fixes for the 10-Speed Allison and General Motors transmissions and can provide some useful information on the common P0747 condition. This specific problem is caused when the computer believes the corresponding shift solenoid is stuck in the open position. Your first thought is likely a bad solenoid - think again.

     We build hundreds of these and have all the equipment needed to remanufacture these solenoids in-house with perfect accuracy, and we can tell you that it’s extremely rare that we find one of these solenoids bad. In fact, it’s almost unheard of. We’ll spare you the complicated tech-jargon, but the simple explanation is that these solenoids are very overbuilt for the way they’re used and the amount of electrical energy they need to process. Imagine a bodybuilder lifting toothpicks. They don’t fail often.



     So, why all the P0747 codes? The most common cause for this code to be set in GM/Allison 10 speeds is the “E” clutch being on when it should be off. This is usually detected when the speed sensor that monitors input shaft speed sees it stopped at idle, implying that it’s synchronized with the output shaft. This obviously shouldn’t be because the output shaft can’t move at a stop or the tires would have to move. The computer sees this condition and will log a pending or active P0747 DTC.

How Can We Fix the P0747 Code?

     So what’s the fix? Unless the clutches are physically damaged (check for excess material in fluid) or the solenoids linear actuator is stuck in a protruded position (very rare), the problem is in the valve body. There are 2 main issues that cause this to happen.

     One is the solenoid regulator valve that’s mobilized by this solenoids pintle not being able to move back to a “closed” position when current to the solenoid stops. This is caused by the balance oil used to close the valve being insufficient to move the valve and usually indicates bore damage around the valve or internal wear to the valve body causing balance oil to become insufficient.

     Two is excessive wear around the feed limit valves that establish a minimum and maximum fluid pressure applied to the solenoid regulator valves. It’s common for them to wear around the end where the spring pedestal is causing a leak to the pan that keeps line pressure from reaching the solenoid regulator valves in a timely fashion. We designed Feed Limit Valved with sealing rings to prevent this issue.

     So what should you do? If the clutches are likely to be damaged, replace or rebuild the transmission. If the fluid has discoloration, this is the best step. If the fluid looks okay and the transmission is likely not “roasted”, replace or remanufacture the valve body using our valve body DIY upgrade kits. We manufacture and design our own 10-Speed Allison/General Motors valve bodies and transmissions here that are specifically meant to prevent the P0747 condition from taking place.

     Unfortunately, GM is going to great lengths to keep this information away from service departments. Most dealerships are not equipped with the information necessary to adequately resolve this issue, and many many trucks like yours are down. If you’re going to keep the truck for a while, it would be worth the money to uparmor this transmission and move on with your life. If you need further help, just give us a call and speak to one of our friendly experts!