Understanding GM 10L1000 Early & Erratic TCC Engagement

Some ’20–’23 GM 3500HD series trucks with 10L1000 transmissions may experience early TCC application or a condition in which the engine drags when coming to a stop. Figure 1 illustrates normal TCC operation on a 2022 Denali diesel application. Note that TCC applies right near the 4-5 upshift at 32–34 mph. Also, TCC slip is 0–5 RPM when fully engaged and it rises to 7–17 RPM during upshifts from 5^th–10^th Gears. This is a normal TCC clutch application strategy.

Figure 1 – ’22 Diesel Denali Drive Cycle Recording
TCC amperage on the 10L1000 shows up on the scan tool as a pressure. The pressure is displayed in kilopascals (kPa). TCC amperage can vary between 360–400 kPa (52–58 psi) with the TCC fully applied.

Complaints have come in related to abnormal TCC apply and, most commonly, the engine drag down coming to a stop or even a complaint related to diesel applications trying to pull through a stop, with or without TCC-related performance codes. This early TCC complaint can be linked to the upper valve body casting cracking and connecting the TCC apply circuit to the “C” clutch apply (2-3-4-5-7-9-10 clutch) circuit. See Figure 2 for the identification of the two circuits and the wall in the casting that separates them.

Figure 2 – 10L1000 Upper Valve Body Circuit Identification

A cracked casting wall will connect these two circuits so that when either the “C” clutch or TCC is on, the other will be applied at the same time. The “C” clutch is applied in all Forward ranges other than 1^st, 6^th and 8^th Gears (Figure 3).

Figure 3 – 10L1000 Clutch Application
*ON = Applied during high-speed shift to Neutral.

Another interesting fact regarding the 10L1000 shift strategy is that these units do not take off in 1^st Gear under normal driving conditions. They take off in 2^nd, and when coasting to a stop, they downshift to 2^nd Gear. So this would explain the drag down to a stop and even the complaint of pulling through a stop, as the “C” clutch is on in 2^nd Gear.

The 10L1000 valve body has been redesigned numerous times, and in late 2023, the upper valve body casting walls were strengthened to prevent them from cracking. The casting walls on the late-style design are much thicker in the area where the crack previously occurred (Figures 4 & 5). See the Sonnax tech resource titled “GM 10L1000 Casting Identification & Clutch Application” for other concerns with the upper valve body casting.

Figure 4 – Early-style 10L1000 Upper Valve Body Casting

Figure 5 – Late-style 10L1000 Upper Valve Body Casting

This more durable casting will retrofit to earlier applications and can be identified by the different casting number on the upper valve body (Figure 6).

Figure 6 – 10L1000 Upper Valve Body Casting Identification

The early-style casting number is 24294637 and the late-style casting number is 24295833. A word of caution when doing performance upgrades on the early-style casting: we have had complaints of the upper casting cracking under the shop’s warranty after overhaul and installation of a performance upgrade that increases line pressure. It was an expensive warranty repair.

Jim Dial is a Sonnax product line manager. He is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Transmission Company technicians.