"ALLISON 1000 STRENGTHS & WEAKNESSES
The Allison's obvious strength is its robust size, as the 1000 series transmission is designated for medium duty applications. Intuitively, this demonstrates that the transmission is inherently sufficient for class 3 and 4 GMC/Chevrolet pickups. The six speed model also has dual overdrives, with the second overdrive ratio being rather high. This allows for low engine speeds at cruising rpm and ultimately opportunities for significantly reduced fuel consumption in relative terms.
The Allison is not without a few weaknesses, which include insufficient lubrication to the C3 clutch and the off-center engagement of the C1 and C2 clutches. Low fluid flow to the C3 clutch has been known to be a leading cause of premature failure in the Allison, although it's not generally a cause for concern and there exists no phenomenon of widespread failures as a result of this issue. The C1 and C2 clutch pistons engage their respective clutch systems from the side (off-center), which typically causes uneven wear on these clutch packs. Again, the problem is frequently noted during transmission rebuilds, but has not been largely attributed to premature transmission failure.
A common occurrence for Duramax owners who chose to install aftermarket tuners/programmers (for performance improvements) is experiencing the transmission's "limp mode". This is a safety feature built into the Allison's programming which locks the transmission in 3rd gear if rapid or excessive slippage is detected. The concept is to protect the owner from being stranded as the result of serious transmissions problems, as the system identifies the slippage as abnormal and possibly a sign of severe problems. When this occurs, owners will experience a significant decrease in engine performance and reduced vehicle speed (performance and ratio limited)."
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