Understanding Allison Transmission Fault Codes

Allison Transmissions are known for their durability and performance, so it’s no surprise that they’re a popular pick for commercial trucks, buses, and off-road vehicles. However, like any component in a vehicle, especially complex mechanical systems, they can run into issues from time to time.

To streamline the diagnosis and repair process, these systems come equipped with fault codes that help identify specific issues. Learning to identify and interpret those codes can help you address issues as they arise, ensuring you can keep downtime to a minimum. Here are the basics to get you started.

How to Access Allison Transmission Fault Codes

Before you can remedy the fault codes, you need to figure out what you’re dealing with. Accessing the codes is fairly straightforward in an Allison Transmission, given that most vehicles equipped with these transmissions have a diagnostic mode accessible through the shift selector or dashboard controls.

Here’s how you can retrieve the codes:

1. Turn the ignition key on, but don’t start the engine.

2. Press the “Up” and “Down” arrow buttons on the shift selector at the same time.

3. Watch the display for the fault codes currently present in the system.

Alternatively, you can use a diagnostic scanning tool to get more detailed diagnostic information about the issue. This makes it easier to troubleshoot the issue. If you don’t have one, our experienced technicians at Point Spring & Driveshaft Co. can run diagnostics to pinpoint the issue for you. 

Once you retrieve the fault codes, you can cross-reference them with the appropriate fault code guide, which will give you more information about the specific problem you’re dealing with. 

How to Read Allison Transmission Fault Codes

There are hundreds of possible fault codes you might encounter with an Allison Transmission, but some are more common than others. Each code represents a different problem. Read the code and refer to the fault code guide to determine what you’re dealing with. 

The most frequent fault codes we see include:

1.    P0700 - Transmission Control System Malfunction

This is a general fault code that indicates a problem in the transmission control system. When you see this code, it usually means that another, more specific code is stored in the TCM, which can provide more information about the underlying issue. 

The exact issue can stem from various causes, including faulty wiring, transmission solenoids, or a defective TCM. The steps to fix the problem vary based on the exact cause. 

2.    P0876 - Transmission Fluid Pressure Sensor/Switch “D” Circuit

The P0876 code indicates a problem with fluid pressure in the system. This particular transmission relies on hydraulic pressure to shift gears, so when there’s an issue here, this code may appear. This code lets you know that the pressure is either too high or too low, which can stem from various problems, such as low transmission fluid levels, clogged filters, and a faulty pressure sensor. 

3.    P0720 - Output Speed Sensor Circuit

This fault code is triggered when the transmission’s output speed sensor fails to send proper signals to the TCM. The sensor monitors the output shaft’s speed and helps control shifting. When this code appears, you might be dealing with a faulty speed sensor, damaged wiring, or a poor electrical connection. 

4.    P0731 - Gear 1 Incorrect Ratio

The P0731 code indicates that the transmission isn’t maintaining the correct gear ratio for first gear. This code often appears when the transmission is slipping or not shifting properly, which can happen for multiple reasons, including internal transmission damage, low transmission fluid, or a malfunctioning shift solenoid. 

5.    P0894 - Transmission Component Slipping

This fault code signals that a component within the transmission is slipping. The TCM detects that the expected gear ratio doesn’t match the actual ratio during shifting, causing a fault code. This particular issue is often associated with more serious problems that may require internal transmission repairs, including worn clutch plates, low transmission fluid, or damaged torque converters.

Common Allison Transmission Fault Codes Table

What Causes Allison Transmission Fault Codes?

Allison Transmission fault codes can pop up for various reasons, some requiring minor repairs, others pointing to major mechanical failures. The most common causes include: 

●      Low or dirty transmission fluid: The transmission needs transmission fluid to operate. If that fluid runs low or becomes contaminated, you may encounter issues like overheating or improper shifting, which can trigger fault codes.

●      Faulty sensors or wiring: The transmission relies on multiple sensors to keep an eye on its performance. If any of these sensors fail to function or a wiring issue gets in the way, the TCM may trigger a fault code.

●      Mechanical issues: Various mechanical issues, including worn clutch plates, solenoid malfunctions, and internal transmission damage, can lead to fault codes.

●      Electrical problems: Issues with wiring, voltage issues, poor connections, and shorts are common culprits behind transmission codes.

How to Fix Allison Transmission Fault Codes

The process of fixing a transmission fault code hinges on the problem you’re dealing with, but there are a couple of things you can check that may help, including:

1. Check the transmission fluid: The transmission fluid is critical to the transmission’s function, so if you’re having issues, start there. Check the transmission fluid level and condition, ensuring it’s at the correct level and isn’t contaminated. 

2. Inspect wiring and sensors: Check for obvious signs of wear or damage to the transmission wiring or sensors, replacing any faulty or worn components as needed. 

3. Use a diagnostic tool: If you’re at a loss, try using a professional diagnostic tool to get more detailed information about the fault code. This can help pinpoint the issue and steer you in the right direction. 

4. Consult a technician: If mechanics aren’t your forte, consult with our skilled technicians at Point Spring & Driveshaft Co. We have the specialized diagnostic tools and years of expertise necessary to diagnose and repair the problem.

Prevent Transmission Faults

While every component in a vehicle, including a transmission, is bound to encounter issues from time to time, prevention can help limit these occurrences. Prevention is always better than the cure, especially when it comes to pricey transmission repairs that get in the way of your productivity. 

So, do yourself a favor by:

●      Completing regular maintenance: Fluid changes, filter replacements, and general inspections can go a long way in preventing transmission faults, so pencil in routine maintenance as needed.

●      Monitor fluid levels: Always check your transmission fluid level and condition to ensure it’s adequate and clean before taking your next trip.

●      Address issues early: If you notice signs of transmission trouble, such as slipping, delayed shifting, or unusual noises, have the transmission checked immediately. Little issues can spiral into much bigger ones, especially if you wait too long, so it’s best to check sooner rather than later.

Remedy Allison Transmission Fault Codes With Point Spring

Allison Transmissions, like any other transmissions, are bound to encounter problems from time to time. When your transmission encounters issues and throws a fault code, knowing how to interpret that code can make a big difference in catching the problem before it spirals out of control. This can keep you productive and on the road with a smooth-functioning vehicle, thus bolstering your bottom line.