Silverado Tuning for Off-road behavior

[Wahawk]

I have two unique situations I'm hoping I can tune and make adjustments to the behavior on my 2018 Silverado. I'm looking for feedback from others before I buy more credits. My truck has the 5.3 Flex Fuel V8 and the 6l80 transmission.

The strange behavior I've noticed is when I'm "off-road" (forest service roads) and I'm trying to maintain any speed between a crawl and 15 MPH going downhill.

Behavior 1) I've noticed when the truck is high range on the transfer case and manually set to first gear it will free wheel until it gets to about 15mph, then the transmission will "catch" and start engine braking. The truck does a fairly good job at holding 15mph as it will alternate between braking and freewheeling with the engine/trans. Is there any way to lower this threshold, or eliminate the free wheeling completely? On some FS roads that make up mountain passes, 15mph is too fast. I believe this is a transmission parameter.

Behavior 2) In 4lo range I can only get engine braking at a crawl (2 MPH). It will only work when I am in 4lo, locked into first gear, and pointing down hill. The engine/trans will be engaged immediately and hold the truck at a low speed. If touch the gas pedal it will immediately start rolling ahead and the engine RPM will rise (I've let it get to 4k rpm with zero engine braking before I get the brakes). This tells me the engine and transmission are coupled, but the ECM is holding the throttle open; which is different than in high range. Is there any way to change these parameters so I can maintain engine braking?

This is very frustrating, because every other older GM vehicle I've driven allows me to put the truck in 4lo, pick a gear that gives me appropriate engine braking, and then apply throttle to set my speed. As the truck sits, I'm forced to use my brakes more than I'd like doing down these National Forest mountain passes if need a speed between a crawl and 15 mph. Hill decent control also doesn't like working for long periods of time and it is also the brakes.

[GHuggins]

Here's a stock tune or nearly stock - has tire changes - if you want to take a look. There are basically tcc and dd settings that you can change to help with what you're after. There is also a mountain mode that you can activate, but don't know the extent of what it does as I'm pretty sure it's the downhill decent function, but I don't see adjustable tables for that?

[TheMechanic]

GM Service InformationHome Publications Number Search New Bulletins Bulletin Search Feedback TIS2Web SI Help
2018 Chevrolet Silverado - 4WD | Cheyenne, Sierra, Silverado Service Manual 10994838 | Transmission | Automatic Transmission - 6L80 MYC or 6L90 MYD | Description and Operation | Document ID: 3647691
Drive Range, First Gear Engine Braking
When the gear selector lever is moved to the Drive (D) range from the Neutral (N) position, the transmission will provide engine braking. In this operating range, the normally-low C1234 pressure control solenoid 5 is commanded ON and in the engine braking mode the following changes occur within the hydraulic circuits:

Fluid Pressure is Directed to the 1-2-3-4 Clutch and the Low and Reverse Clutch to Provide Engine Braking
Manual Valve

The manual valve is moved to the Drive (D) position and allows line fluid pressure to enter the drive fluid circuit. Drive fluid is then routed to the clutch select valve 2.

Clutch Select Valve 2

Drive fluid at the clutch select valve 2 passes through the valve and enters the drive brake circuit. Drive brake fluid is then routed to the clutch select valve 3.

Clutch Select Valve 3

Drive brake fluid at the clutch select valve 3 passes through the valve and enters the Drive B fluid circuit. Drive B fluid is then routed to #1 ball check valve.

#1 Ball Check Valve

Drive B fluid seats the #1 ball check valve against drive 1-6 fluid to force drive B fluid into the CB26/C1234 feed passage. CB26/C1234 feed fluid is routed to the #6 ball check valve, through orifice #43 and, to the 2-6 clutch regulator valve. CB26/C1234 feed fluid passes through the 2-6 clutch regulator valve and enters the pressure switch 3 (PS3) fluid circuit. PS3 fluid is then routed to the normally closed pressure switch 3 and opens the switch.

#6 Ball Check Valve

CB26/C1234 feed fluid seats the #6 ball check valve against the 1234 clutch feed passage forcing CB26/C1234 feed fluid through orifice #32 before entering the 1234 clutch feed circuit. The 1234 clutch feed fluid is routed through orifice #33 and then to the 1-2-3-4 clutch regulator valve.

1-2-3-4 Clutch Applies
C1234 Pressure Control Solenoid 5

The C1234 pressure control solenoid 5 is commanded ON allowing actuator feed limit fluid to enter the PCS1234 clutch fluid circuit. PCS1234 clutch fluid is then routed through orifice #35 to the 1-2-3-4 clutch regulator valve. PCS1234 clutch fluid is also routed through orifice #29 and then to the 1-2-3-4 clutch boost valve.

1-2-3-4 Clutch Regulator Valve

PCS1234 clutch fluid moves the 1-2-3-4 clutch regulator valve against 1-2-3-4 clutch regulator valve spring force to allow 1234 clutch feed fluid to pass through the valve and enter the 1234 clutch fluid circuit. The 1234 clutch fluid is then routed to the 1234 clutch boost valve and the 1-2-3-4 clutch.

1-2-3-4 Clutch Boost Valve

PCS1234 clutch fluid pressure acts on a differential area moving the 1234 clutch boost valve against the 1234 clutch boost valve spring. The 1234 clutch fluid passes through the valve and enters the 1234  clutch feedback circuit. As PCS 1234 clutch fluid pressure is increased to a given value, the 1234 clutch boost valve opens the 1234 clutch feedback circuit to exhaust. This results in the 1234 clutch regulator valve moving to the full feed position sending full 26 CL/1234 CL feed pressure (full line pressure) to the clutch.

1-2-3-4 Clutch

The 1234 clutch fluid enters the 1234 clutch housing to move the piston against spring force and compensator feed fluid to apply the 1-2-3-4 clutch plates.

Drive Range, First Gear Engine Braking

[Wahawk]

Thank you for sharing the tune GHuggins! I'll take a look at those settings and see what makes sense to me.

[flyingfool007]

i have a 2wd and it also exhibits a wierd operation when in manual 1st gear. it will roll for a while downhill engaged crawling, then it feels as it lets the clutch out for a couple seconds to coast, then back in gear again.. i also notice the motor will not drop below 1000 rpm while in 1st gear. even if my idle speed is programmed to 550 or 700 rpm. it always goes back to 1000 rpm as a moving idle speed.