Other Articles - September - 2021

Still Slipping and Sliding – A Look at Issues with the GM VT40 CVT

A couple of years ago, I introduced you to the GM VT40 CVT (CVT250), used in multiple applications such as the Chevrolet Malibu, some Chevrolet Trailblazers, and the Buick Encore. The VT40 is identified with an RPO of MRG and is the base transmission on some models and an optional transmission for other models.

Like other CVT applications, the VT40 varies pulley diameter to change transmission ratios. Unlike most CVTs, the VT40 utilizes a chain rather than a belt to provide propulsion. The VT40 uses two (2) clutches, one for forward range and one for reverse range. The unit utilizes three (3) speed sensors, an input speed sensor, an output speed sensor, and an intermediate speed sensor. The valve body houses the six (6) VFS pintle-type solenoids and two (2) pressure sensors. An IMS is used to determine the shifter position for operation. Most of the units are equipped for start/stop operation.

The VT40 has seen a lot of use, especially in the rental car fleet market, so we are now getting some experience with the issues you may encounter.

Concern 1: DTCs set. You may experience the following DTCs: P0730, P0746, P0776, P0841, P0846, P0961, P0965. In addition, the customer will likely complain about a lack of power as the unit is not changing ratios.

P0730 will set if: Output speed sensor DTCs are not set. Input and intermediate speed sensors indicate an RPM value of 279 RPM or greater. Engine load at least 4%, but the actual ratio does not match the calculated ratio command for the load and RPM.

P0746 will set if: No speed sensor DTCs are set. Transmission fluid pressure sensor 1 indicates less than 29 psi (201 kpa) while the secondary pulley commanded pressure is greater than 72 psi (499 kpa) for one second.

P0776 will set if: No speed sensor DTCs are set. Transmission fluid pressure sensor 2 indicates less than 29 psi (201 kpa) while the primary pulley commanded pressure is greater than 72 psi (499 kpa) for one second.

P0841 will set if: Primary pulley pressure is not changing as the commanded primary pulley pressure changes.

P0846 will set if: Secondary pulley pressure is not changing as the commanded secondary pulley pressure changes.

P0961 will set if: No speed sensor DTCs are set. Ratio is not changing when solenoid 1 command values change.

P0965 will set if: No speed sensor DTCs are set. Ratio is not changing when solenoid 2 command values change.

As you can see, all of the DTCs could have something to do with system pressure, as it affects the pulley positions. While we have seen a few issues with the solenoids and/or pressure sensors, the vast majority of the time, the issues are related to actual hydraulic pressure problems.

The VT40 does have a line pressure tap as well as scan tool readings from both of the pulley pressure sensors. Like other transmissions you may have dealt with, the VT40 scan data makes diagnosis much easier. Likely you will find the commanded pulley pressure is locked around 580 psi (4000 kpa) as the TCM is attempting to prevent damage to the pulleys and chain from the ratio issues. This is also the cause of the customer complaint regarding poor acceleration as the pulleys are locked at a fixed ratio (Figure 1).

With this application, you may want to take a look at the pressure regulator, which is housed in the valve body. A sticking pressure regulator is the most common cause for the conditions listed above and can also account for why this issue is commonly intermittent and hard to duplicate (Figure 2).

Concern 2: P2797 may be set on start-stop applications. P2797 will set if: The primary or secondary pulley commanded pressure is greater than 58 psi (400 kpa) while the pressure sensors indicate the actual pulley pressure is less than 44 psi (300 kpa) for more than one second.

If you’re faced with this issue, check the start-stop pump circuits as well as the power and ground circuits for issues. You will likely find the issue related to the pump. GM has released an updated pump and harness to address the concern, part numbers 24299204 and 24298763.

Concern 3: Reduced launch response when accelerating from a stop after an auto stop event. In addition, you may find some DTCs such as P0746 and/ or P0776. This is due to a delay in pump prime during acceleration, causing a delayed forward clutch application. An updated software program has been released to address this issue.

So, there you have it. The VT40 is a simple transmission, and most of the issues you will see can be easily addressed. Until next time remember, “It’s hard to beat a person that never gives up.”