GM 6T40 transmission and TCM problems: Cruze, Trax

If you have an automatic Chevrolet Cruze, Sonic, Trax, Equinox or Malibu, or a Buick Encore, it most likely has a transmission from GM's 6-speed 6T40 family. We explain which models have it in Mexico, how to tell your generation, what to check before replacing the well-known TCM module, why reverse stops working, which codes it sets, what fluid it takes and which TSS parts are replaced.

What is the 6T40?

It's GM's Hydra-Matic 6-speed front-drive automatic. It's really a family: GM groups the 6T30, 6T35, 6T40, 6T45 and 6T50 together in its bulletins, and its newer documents call the 6T40 the GF6. The 6T30 is the lightest, the 6T45 and 6T50 the strongest, and the 6T40 sits in the middle; the 2016 Cruze's 6T35 uses the same ratios as the 6T30. Per Sonnax, the 6T40 debuted in the 2008 Malibu.

It's not a CVT: even though people search for the "Sonic CVT transmission", they're all planetary automatics with a torque converter. And although it's a cousin of Ford's 6F35, because GM and Ford developed their 6-speeds together, Transmission Digest says almost nothing interchanges between them: GM built the transmission computer into the valve body with the solenoids, and Ford kept it outside.

Which Chevrolet models have it in Mexico?

Chevrolet Mexico doesn't publish the unit name: its spec sheets say "6-speed automatic with manual mode", and the Equinox's says "Hydra-Matic". We inferred the name from GM's US documents for the same engine and years; on the Cruze, the spec sheet's final drive confirms it. For the Sonic 1.6 and Trax 1.8 we found no document saying whether they're 6T30s or 6T40s. And we found no evidence that the Chevrolet Orlando listed in parts catalogs was sold in Mexico.

First, second or third generation?

The generation matters when buying parts, because the valve body, module and pump don't interchange between generations, per specialists such as Sonnax. Gen 1 (from 2008) has four pressure switches in the module; Gen 2, which started with the 2012 models, has none; and Gen 3, from around 2015, has a redesigned pump. They overlapped: in 2012, GM said the US Cruze 1.8 and Sonic 1.8 used Gen 1 controls, and the Cruze 1.4 turbo, Equinox, Malibu and Captiva Sport 2.4 used Gen 2. Shops also call Gen 3 units the 6T31, 6T41, 6T46 or 6T51.

• RPO code: it's on the service parts label, in the glovebox or the trunk depending on the model. MH8, MHB, MNH and MNK are 6T40s; MH9, the 6T30; MNU, the 6T35; MH7 and MHC, the 6T45.
• Module: per TransGo, Sonnax and TSS, if the eighth digit of the module's code is a number, it's Gen 1; if it's a letter, Gen 2 or Gen 3.
• Pump: the Gen 3 pump is a chain-driven vane pump, off the transmission's axis, per Sonnax.
• 6T30 or 6T40: per Transmission Digest, the bell-to-case gasket has 16 holes on the 6T30 and 17 on the 6T40.

The TCM module: check this before replacing it

What parts counters call the 6T40 "TCM module" is the TEHCM: the transmission computer built together with the solenoids on the valve body, inside the case, under the side cover. GM sells it as an assembly, with different part numbers for Gen 1 (24256524) and Gen 2 (24265367), and the replacement is programmed with a scan tool: TSS states that its rebuilt modules require programming with the VIN.

Before replacing it, GM's US bulletins ask for three things to be ruled out:

• With code P0776 (slipping or no reverse, 3rd or 5th), inspect the 3-5-R piston and clutch: GM says not to replace the module before inspecting them.
• Measure line pressure: a damaged pump causes similar symptoms and codes, and GM asks for that test before replacing the module or the valve body.
• With code U0101 (lost communication with the module), check the connector pins, the fuse block, the module ground and that the harness isn't rubbing on top of the transmission: per GM, the fault is often there and not in the module.

On Gen 1 units, specialist shops report that the module's pressure switches can fail and set codes such as P0752, P0872, P0877 or P0989, sometimes with limp mode, and there are kits to repair them without replacing the whole module. Sun Transmissions explains code U0101, lost communication with the TCM.

No reverse, 3rd or 5th: the 3-5-R piston

It's the 6T40 failure GM documents most. In a 2015 US bulletin, GM explains that code P0776 with slipping, or no reverse or third, can come from a broken 3-5-R apply (waved) plate on units built through January 15, 2011, and recommends a new 3-5-R piston and waved plate any time the transmission is disassembled; the piston's notches were removed during the 2011 model year. In another 2012 bulletin, GM warns that air testing through the case frequently leads to a wrong diagnosis.

A caution about what you read online: GM's "wave plate" special coverage (14404) is for the larger 6T70 and 6T75, not the 6T40.

Missing gears: one solenoid per group

Each 6T40 clutch works in certain gears, so the symptoms tell you which circuit is failing. In a 2016 US bulletin, GM describes 2011 units built before April 1, 2011 with a non-functioning pressure control solenoid inside the module:

In the same bulletin, GM notes that the fluid in those units tends to turn dark from rust inhibitors, and that neither the transmission nor the fluid should be replaced based on its color alone.

Bump when stopping or shudder at a stop in Drive

It's often the neutral idle feature, which partly releases a clutch when you're stopped in Drive. On 2012 and 2013 Cruze and Sonic 1.4 turbo models and the 2013 Encore built before 2013, GM tied the bump or shudder at a stop in Drive to that feature and to variation in the 1-2-3-4 clutch plates, and replaced them. The feature only works with fluid above 37 °C (99 °F), and manual mode disables it, which makes a useful test. On 2017 to 2022 Trax and Encore models, GM says a single bump when stopping or releasing the brake is normal and should not be repaired.

No movement or the engine won't start: the pump

In a 2012 US bulletin (2011 to 2013 Cruze, Equinox, Malibu, Captiva Sport, Sonic and others), GM ties a damaged fluid pump to the vehicle not moving, the engine cranking but not starting or not cranking at all, and harsh shifts, with codes such as P0776, P0796, P2714, P2723 or U codes. That's why it asks for a line pressure test before replacing the module or the valve body.

Fluid leaks

• Between the engine and transmission: the torque converter seal. GM released one made of a more robust material (part number 24266675) for units built before February 2013.
• At the left axle: on 2016 and 2017 Equinox and Terrain 2.4 models, GM found the drive shaft damaging the seal, and replaces the shaft and seal.
• 2014 Malibu with engine start-stop: a leak at the auxiliary pump seal.

Other problems GM documented

These are GM's US bulletins; only one, about wind noise at the shifter on the 2019 and 2020 Equinox, mentions Mexico. We found no PROFECO (Mexico's consumer agency) alerts for these transmissions.

Dashboard messages

Chevrolet Mexico owner's manuals list two transmission messages: "TRANSMISIÓN CALIENTE" (transmission hot), which asks you to stop and let the engine idle without moving so the transmission cools, and "SERVICIO A LA TRANSMISIÓN" (service transmission), which signals a transmission problem and asks you to see the dealer. If the second appears, read the codes with a scan tool that reads the transmission computer, not just the engine computer.

Trouble codes

GM doesn't publish an owner-level code list for the 6T40: these meanings come from its US bulletins and trade sources; the rest are generic.

What fluid does it take?

DEXRON VI only, per Chevrolet Mexico owner's manuals. Owners don't need to check the level: the manuals say the only reason to lose fluid is a leak, and that in that case the vehicle should go to the dealer; the Cavalier, for example, has no dipstick and its level is checked with a special procedure. The filter is inside the case: GM replaces it with the case halves separated. And, per GM, dark fluid alone isn't enough to condemn the transmission.

Are there recalls?

Not for 6T40 internal failures. In the US, GM recalled the 2012 Sonic 1.8, with the 6T30, because the turbine shaft could fracture; the Mexican Sonic is a 1.6, so it likely doesn't apply. Other US recalls covered the shifter and its cable (2011 Cruze, and 2014 Cruze, Malibu and others: the lever might not match the transmission's actual position) and missing bolts on the start-stop accumulator cover, which could cause a leak and a loss of propulsion (2018 and 2019 Cruze, 2018 to 2020 Equinox, 2018 Malibu and 2019 and 2020 Encore). On 2011 and 2012 Cruze 1.8 models, a US program covered a coolant leak from the thermostat housing that damaged the shift cable grommet and could bind the lever.

GM published no Mexico figures for these campaigns, and we found no warranty extension for the 6T40. To check for an open recall, look up your VIN with Chevrolet.

How is it diagnosed and repaired?

• Confirm the transmission and generation with the RPO code and, if you're replacing the module, with the module's code.
• Read the transmission computer's codes, not just the dash message.
• Look for leaks: owners don't check the level, and the only reason it drops is a leak.
• With P0776, inspect the 3-5-R piston and waved plate before replacing the module.
• Measure line pressure before replacing the module or the valve body.
• With U0101, check pins, fuses, ground and harness.
• If you replace the module, it must be the same generation, and it has to be programmed.
• If you take the transmission apart, replace the filter, the solenoid screen and, per GM, the 3-5-R piston and waved plate.
• After the repair, the transmission has to relearn its shifts. Sun Transmissions explains why transmissions need to relearn.

See TSS parts for the 6T40

TSS parts for the 6T40

The TSS catalog has parts for this family:

• Rebuilt TCM module for Gen 1 and for Gen 2 and Gen 3; both require programming with the VIN.
• Gen 1, Gen 2, Gen 3 and 6T30 filters, and the solenoid screen for Gen 1 or Gen 2 and Gen 3.
• Gen 1 solenoid set.
• Input and output speed sensors.
• Torque converter seal and left and right axle seals.
• TSS friction plates for the Gen 1 6T40 and 6T45 and the Gen 2 6T30, and the Dorado overhaul kit for the Gen 3 6T31 and 6T35; we explain the difference in TSS Rojo vs Dorado overhaul kits.
• Oil baffle kit and side cover.

To buy online in the US, Sun Transmissions has the 6T30, 6T40, 6T45 and 6F35 collection.

TSS is an aftermarket brand with OEM-comparable quality and quality control on every product. It's sold through our authorized distributors: in Mexico, the full TSS catalog is at Transmisiones Veinte 07, and in the United States at Sun Transmissions.

TSS products

Buy in

MexicoUSA

• Modulo Control Transmision TCM Reconstruido 24268164 14/UP 6T40 6T45 6T30 6T50 con Solenoides Gen2,3
• Modulo Control de Transmisión Reconstruido TCM 24256523 08/12 6T30 6T40 Compatible solo con 1ra Generación con Solenoides Negros
• Juego de Solenoides 1ra Gen 2008/14 VBS 6T40
• Filtro GM Tercera Generacion 2014/UP 6T40 6T45 6T50
• Filtro 2013/UP 24268086 6T40 6T45 6T35
• Juego de Empaques TSS Dorado 6T31 6T35 Gen3
• Pack Discos Pasta TSS 1ra Gen 2008/UP 6T40 6T45
• RETEN DE BOMBA 6T30, 6T40, 6T45, 6T50, MH9, X23F, MH7

See all