If you’re a Duramax diesel enthusiast or truck owner, you’re likely familiar with the Allison 1000 transmission. Revered for its durability, smooth shifting, and ability to handle the power of diesel engines like the GM Duramax, it’s a popular choice in heavy-duty trucks. However, even the best transmissions aren’t immune to problems. In this detailed guide, we’ll combine insights on diagnosing common issues, troubleshooting tips, and upgrading your Allison transmission to ensure your truck keeps running strong under any condition.
The P0700 code is a generic diagnostic trouble code (DTC) that signals a transmission-related issue. This code is triggered when the Transmission Control Module (TCM) detects a fault and requests the Engine Control Module (ECM) to light up the check engine light (CEL). However, P0700 is a general indicator and doesn’t specify the exact nature of the problem. Further diagnostics are necessary to pinpoint the exact issue with your transmission.
When the P0700 code is triggered, the following symptoms might appear:
Slipping Clutches: A common culprit behind P0700 codes, especially in trucks tuned for more power or used for heavy towing. The C3 and C4 clutches are engaged most of the time and can wear out, especially under higher stress loads, triggering transmission errors.
Electrical Failures: Damaged wiring harnesses, malfunctioning sensors, or a faulty park/neutral switch can all cause transmission issues without immediate mechanical damage. These problems can disrupt the normal transmission operation and trigger the P0700 code.
Overworked Transmission: Trucks that frequently tow heavy loads or are heavily modified are more prone to transmission failures. Overloading can lead to overheating and premature wear, which in turn can trigger the P0700.
Torque Converter Issues: A failing or slipping torque converter is often a major cause of transmission issues. Though it may not always trigger a fault code, symptoms like shaking or vibrating under load are common signs.
P2 Planetary Gear Failure (High-Power Trucks): For trucks with over 750 horsepower, the P2 planetary gear becomes a weak point. Under extreme loads, it can twist or break, especially during instances of wheel hop (when the rear tires momentarily lose and regain traction). Wheel hop acts like a hammer on the driveline, causing potential damage to the planetary gears.
Diagnosing transmission issues isn’t just about reading a trouble code. As Phil Grandinetti, a seasoned transmission expert, explains, you’ll need a high-quality scanner capable of reading the Transmission Control Module (TCM) directly to get a clear picture. Regular OBDII scanners may only show the P0700 code but miss specific issues related to sensors, clutches, or other internal parts.
Key diagnostic steps include:
Scan for Additional Codes: A higher-end diagnostic tool can help you identify other codes that may indicate issues with input/output sensors, shift solenoids, or clutch pressures.
Fluid Inspection: Healthy transmission fluid should be a bright red color. If the fluid is dark or burnt, it’s a sign of internal wear. Leaking fluid can often point to a simple issue like a trans line failure, but always investigate thoroughly.
Visual Inspection: Check for external issues like worn-out parts or obvious damage. Though sometimes you’re stuck with only a visual check, never clear the codes and keep driving if you feel something’s wrong.
C3 and C4 Clutch Slippage: These clutches are the most frequently engaged components in the Allison transmission. High-power builds or frequent towing can wear them out more quickly, causing slippage and ultimately triggering the P0700 code.
Torque Converter Slippage: Stock Allison transmissions often feature a single-clutch torque converter, which can slip under heavy load or power increases. A slipping converter can cause additional strain on the transmission, leading to a variety of mechanical issues.
P2 Planetary Gear Failure: Trucks with significant power gains, especially those pushing 750+ horsepower, are particularly vulnerable to damage in the P2 planetary gear. The stress of heavy acceleration and wheel hop can break these parts, leading to costly repairs.
If you’re pushing your truck to higher power levels or regularly towing, certain upgrades are essential to avoid premature failure and ensure your Allison transmission performs at its best:
Upgraded Clutches: Aftermarket clutches with more friction material are essential for handling higher power and torque. These clutches are built with better materials than stock components and significantly improve durability.
Billet Triple-Disk Torque Converter: Stock torque converters may struggle under heavy load or increased power. A triple-disk billet converter provides better lock-up and reduces slippage, which is crucial for high-performance trucks.
Improved Electronics: It’s advisable to use new electronics when rebuilding a transmission to avoid future failures. Reusing old electronics may seem cost-effective, but they often fail soon after rebuilds, causing frustration and added costs.
Shift Kits and Pressure Modifications: Enhancing the pressure applied to clutches with shift kits improves their clamping force and holding power, making the transmission more capable of handling heavy-duty workloads.
As our editor Paul Wilson from the diesel performance podcast emphasizes, proper tuning is key. Adding power to your truck without upgrading the transmission to handle it is a recipe for disaster. For trucks with stock or moderate modifications, it’s essential to match tuning to the transmission’s capabilities. High-performance tunes should only be used when the transmission is built to handle the increased power. TCM tuning is also a must have for any modern truck because it can electronically increase your line pressure, providing better clutch holding capacity and strength.
Regular Fluid Changes: Always change the transmission fluid according to manufacturer recommendations, particularly if you’re towing or working your truck hard. Fluid changes help avoid overheating and wear.
Smart Driving: Avoid wheel hop and excessive shifting. Treat your truck gently, especially if it’s heavily modified. Install traction bars if you’re concerned about wheel hop.
Use Quality Parts: When upgrading or rebuilding your Allison transmission, always choose high-quality aftermarket parts designed for higher power applications.
Understand Your Truck’s Limits: Recognize when your truck is being pushed beyond its limits. If you notice signs of strain like harsh shifting or slipping, address them immediately to prevent more serious damage.
Allison transmissions are known for their rugged performance, but like all mechanical systems, they have their limits. Understanding the P0700 code, diagnosing common issues, and performing essential upgrades can help you avoid major failures and ensure your truck stays reliable under demanding conditions. Whether you're towing heavy loads, adding more power, or just enjoying the ride, a well-maintained and upgraded Allison transmission will continue to perform at its best.
For more advice on Allison transmission maintenance, repairs, or upgrades give us a call at (815)-568-7920!