Truck won't always downshift

[amelie9112]

Hi, I have an MDrive transmission and am having a problem with my jake brake. When I put it at maximum, the majority of the time it does not downshift. It actually downshifts, but for half a second. We hear it downshift, it displays the lower gear but immediately returns to the gear it was. So I always have to downshift manually. I noticed that he did it especially at high speeds like 70 km/h and above. I was wondering if there is any programming to do to fix this. I got it new and it's always been like this. THANKS!

[The Heinz]

Sounds very much so like something that can be resolved with programming. Unfortunately, you'll need a dealer to do the programming for you; requires Premium Tech Tool and knowing exactly what to program (usually involves an eService case nowadays). 

[Nickdieselz]

It sounds like the issue you're experiencing with the MDrive transmission and jake brake could be related to how the transmission control module (TCM) is programmed to handle engine braking at higher speeds. Since it briefly downshifts but then returns to the higher gear, it may be trying to balance engine RPMs and load under certain conditions, which can result in this behavior.

Here are a few considerations:

1. Transmission Software Calibration: Some MDrive systems are set up to avoid excessive downshifting at higher speeds to prevent over-revving the engine. It might be a built-in safety feature, but since you've had this issue since new, it's possible that the programming could be fine-tuned by the dealer or a service center. Updating or reprogramming the TCM could solve this.

2. Load and Speed Conditions: The transmission may prioritize maintaining a higher gear at speeds like 70 km/h and above if it detects that downshifting would result in engine overspeed or excessive braking force. At lower speeds, it might allow more aggressive downshifting.

3. Jake Brake Settings: Some systems have different levels of engine braking (e.g., Stage 1, 2, or 3). At maximum, the system could be trying to provide more braking force through the engine but still limits downshifting under certain conditions. Double-check that you're in the correct mode for your intended braking.

4. Manual Intervention: Since you are manually downshifting to compensate, it may indicate that the automatic control logic isn’t calibrated to your driving style or conditions. Sometimes manual control is preferred in hilly or high-speed braking situations, but it shouldn’t be a constant necessity.

I recommend having the dealer or service center check the transmission’s software and calibrations to see if there's an adjustment that can be made for better jake brake performance, especially if this has been a persistent issue since you got the truck. They can perform a diagnostic to see if the downshift logic can be adjusted to better match your driving conditions.