Transmission question.

[COElkFreak]

So we had the transmission rebuilt in our 2018 Tahoe 4x4 with the 6L80

A guy at work suggested that around town we drive with the tow haul mode on the keep line pressure up and keep from going into OD?

Any thoughts or opinions?

 

[RoadTrip]

Tow/Haul mode raises the shift rpm’s and keeps the engine in V8 mode (suposedly). Line pressure is a function of rpm’s so driving in a lower gear technically raises you line pressure.

The only time the increased pressure may be advantageous is if your transmission was starting to slip. Since yours is freshly rebuilt, there would be no advantage.

If you live in a mountainous area, the higher shift points may help your throttle response.

Hopefully, the transmission shop also replaced the torque converter for you since those are the main cause of trans failure in these vehicles.

 

[swathdiver]

So we had the transmission rebuilt in our 2018 Tahoe 4x4 with the 6L80

A guy at work suggested that around town we drive with the tow haul mode on the keep line pressure up and keep from going into OD?

Any thoughts or opinions?

That's a terrible suggestion!

Tow/Haul is meant for use when you are at 75% or more of your GCWR. You do not receive any benefit from running it at lower weights. For me, the performance is worse, shifts are delayed and wonky. When turned on at over 10,000 pounds, the transmission feels like normal driving to the store.

One thing kills the 6L80 in the K2 platform, heat! The transmission thermostat keeps the temperature about the same as the coolant and over time this causes the clutch material in the torque converter to fail which FODs the whole transmission requiring a complete overhaul and cleaning. It also of course wears out the fluid faster.

Put in the lower temp thermostat and run the severe service schedule and enjoy the benefits both overdrive gears.

 

[Marky Dissod]

So we had the transmission rebuilt in our 2018 Tahoe 4x4 with the 6L80E
A guy at work suggested that around town we drive with the tow haul mode on the keep line pressure up and keep from going into OD?
Any thoughts or opinions?

You'd be better off using M5 to avoid 6th under 50MpH.
But you're still stuck with GM OE programming, which sacrifices transmission durability & longevity for 1-2MpG.
Have your ecm & tcm tuned. It'll do a far better job at extending the life of your engine & transmission.

 

[swathdiver]

You'd be better off using M5 to avoid 6th under 50MpH.

Why?

 

[Marky Dissod]

... guy at work suggested that around town we drive with the tow/haul mode on the keep line pressure up and keep from going into OD?

You'd be better off using M5 to avoid 6th under 50MpH.

Why?

OP wanted to avoid 'OD'. GM OE transmission 'tune' upshifts as early as possible. Net result is excessive shifting. Less unnecessary shifts, longer transmission life.
Avoiding 6th by using the shifter is not nearly as effective as a thorough tune, but it is free.

 

[Doubeleive]

So we had the transmission rebuilt in our 2018 Tahoe 4x4 with the 6L80

A guy at work suggested that around town we drive with the tow haul mode on the keep line pressure up and keep from going into OD?

Any thoughts or opinions?

"a guy at work" well.... lol no, while you could do that there is no logical reason. The only benefit to using tow mode even without towing something is on steep downgrades it helps save your brakes because it holds the gear therefore you apply the brake less.

1. How it Works​

When the vehicle is on a downhill grade and the computer detects that you are applying the brakes to slow down, the Engine Control Module (ECM) and Transmission Control Module (TCM) work together to:

• Downshift the Transmission: The vehicle automatically shifts to a lower gear.
• Increase Engine RPM: By shifting to a lower gear, the engine spins faster. The internal friction and compression of the engine act as a "brake" on the drivetrain.
• Reduce Brake Fade: By using the engine to slow the vehicle, the physical brake pads and rotors don't have to work as hard, preventing them from overheating (which causes "brake fade" or loss of stopping power).

2. When does it activate?​

The system is smart and looks for specific "trigger" conditions:

• Grade Detection: The vehicle uses an internal inclinometer to know it is on a slope.
• Brake Input: It typically only activates if you have your foot on the brake pedal while descending. The longer or harder you press the brake, the more aggressively it will downshift.
• Cruise Control: If you have Cruise Control set, the system will downshift automatically to maintain your set speed without you ever touching the brake pedal.

3. "Normal" vs. "Tow/Haul" Mode​

While Grade Braking works in normal driving, it is much more aggressive in Tow/Haul Mode.

• Normal Mode: The system is conservative to keep the ride smooth and quiet. It might only drop one gear.
• Tow/Haul Mode: The system assumes you have a heavy trailer pushing you. It will downshift much sooner and more aggressively, sometimes causing the engine to "roar" at high RPMs. This is normal and safe for the engine.

4. Common Driver Concerns​

• "The engine is screaming!": Drivers often think something is wrong because the RPMs jump to 4,000 or 5,000. This is intentional. The engine is designed to handle this high-RPM "coasting" to save your brakes from melting.
• Unexpected Downshifts: If you tap the brakes quickly on a hill, the car might stay in a lower gear even after the road flattens out. To "reset" it, simply tap the accelerator, and the transmission will upshift back to a normal cruising gear.

 

[tom3]

I do drive in M5/L5 quite a bit. Even on the highway at normal speeds the engine rpm is well below 2000 rpm which is fine for these engines. Would run 300k miles at those rpms. And this keeps the engine in full V8 mode which eliminates all the locked up converter shipping with the cylinder cutouts and ins. I also don't like using that 4 cyl mode with a cold (winter cold) engine. On long trips I do run it in regular drive and let it work its magic with fuel economy, which is really excellent.

 

[tagexpcom]

I tow a 5500lb 7 x 14 cargo trailer (camper) - about 20K miles now. I often forget to set tow mode when I first take off but then notice a bit of sluggishness plus a low sounding, drive train vibration (I imaging it's straining?) at 60mph.

Tow mode clear this up (sluggishness and low vibration) and the breaking feature works well going down our long (10mile, 6% grade) mountain passes.

 

[Rygrego]

That's a terrible suggestion!

Tow/Haul is meant for use when you are at 75% or more of your GCWR. You do not receive any benefit from running it at lower weights. For me, the performance is worse, shifts are delayed and wonky. When turned on at over 10,000 pounds, the transmission feels like normal driving to the store.

One thing kills the 6L80 in the K2 platform, heat! The transmission thermostat keeps the temperature about the same as the coolant and over time this causes the clutch material in the torque converter to fail which FODs the whole transmission requiring a complete overhaul and cleaning. It also of course wears out the fluid faster.

Put in the lower temp thermostat and run the severe service schedule and enjoy the benefits both overdrive gears.

I installed the lower temp trans thermostat and a Mishimoto trans cooler and my trans temperature never exceeds 170 degrees (most of the time it runs 154) even while towing a 24 ft tritoon in Arizona 115 degree heat.

 

[swathdiver]

OP wanted to avoid 'OD'. GM OE transmission 'tune' upshifts as early as possible. Net result is excessive shifting. Less unnecessary shifts, longer transmission life.
Avoiding 6th by using the shifter is not nearly as effective as a thorough tune, but it is free.

Mine sounds like a manual transmission rowing through the gears when you listen it, completely natural to my way of thinking.

I do not understand how staying out of overdrive or limiting shifts extends the life of the transmission.

 

[Doubeleive]

Mine sounds like a manual transmission rowing through the gears when you listen it, completely natural to my way of thinking.

I do not understand how staying out of overdrive or limiting shifts extends the life of the transmission.

it doesn't, but driving around the city in L3 is pretty fun you get a nice rumble and good pedal response

 

[swathdiver]

I've held 1st and 2nd before when playing around with the kids. They stayed in auto and I could get the jump on them until their 6.2s wound up and blew past me! LOL

 

[Doubeleive]

3rd is good to 100mph with 3.42, just for reference I believe that puts you at about 5800rpm or so

 

[Marky Dissod]

... do not understand how staying out of overdrive or limiting shifts extends the life of the transmission.

You already know that GM's OE 'tune' tries to upshift as soon as possible to try to find the last MpG. Depending on how one drives, terrain, traffic, etc,
the transmission may be needlessly 5-6ing & 6-5ing, when it could just stay in 5th instead.
One of the biggest benefits of a tune is that the transmission will not shift as often as GM wanted it to. This alone extends transmission life,
nevermind the other various improvements that are less obvious.

(How much longer would a transmission last if it shifted half as many times as that other transmission each and every day?)

 

[COElkFreak]

Tow/Haul mode raises the shift rpm’s and keeps the engine in V8 mode (suposedly). Line pressure is a function of rpm’s so driving in a lower gear technically raises you line pressure.

The only time the increased pressure may be advantageous is if your transmission was starting to slip. Since yours is freshly rebuilt, there would be no advantage.

If you live in a mountainous area, the higher shift points may help your throttle response.

Hopefully, the transmission shop also replaced the torque converter for you since those are the main cause of trans failure in these vehicles.

They replaced everything in the tranny!!

 

[swathdiver]

You already know that GM's OE 'tune' tries to upshift as soon as possible to try to find the last MpG. Depending on how one drives, terrain, traffic, etc,
the transmission may be needlessly 5-6ing & 6-5ing, when it could just stay in 5th instead.
One of the biggest benefits of a tune is that the transmission will not shift as often as GM wanted it to. This alone extends transmission life,
nevermind the other various improvements that are less obvious.

(How much longer would a transmission last if it shifted half as many times as that other transmission each and every day?)

Well, I like the shift schedule and don't mind going into 6th gear at 40 mph. It seems very natural, sounds like I'm driving with a manual transmission. Holding gears longer would make it feel like running in Tow/Haul mode without enough weight on. The 6-speed is designed to keep the engine in its power band. If you delay the shifts, it will not run in the ideal powerband to my way of thinking. It would act like an old 3-speed with the rpms dropping over a 1,000 rpms per shift.

Granted, I live where it's flat and our only hills are highway overpasses, so maybe you guys in the hills experience things differently.

 

[Joseph Garcia]

They replaced everything in the tranny!!

I think that he was referring to the fact that the torque converter is not part of the transmission itself. It is an additional part that is located outside of the transmission.

 

[Marky Dissod]

Holding gears longer would make it feel like running in Tow/Haul mode without enough weight on.

Tuned properly, the shifts may happen later, but the shifts don't necessarily have to feel like Tow/Haul mode shifts in Normal mode - again, if tuned properly.

The 6-speed is designed to keep the engine in its powerband.

These days this statement can't make any sense for any automaker's transmission -
program in the TCM decides when and how to shift, transmissions just follow tcm commands based on FILTERED driver requests.
Changing the program changes when and how it shifts. Also, in Normal mode, at light/lazy throttle angles, the TCM upshifts too early / downshifts too late ...
because that's how CAFE MpG scoring skews shift schedules.

If you delay the shifts, it will not run in the ideal powerband
to my way of thinking.

GM's OE 'tune' may be closer to your way of thinking than mine? Especially when the axles are under-geared.
And once steep and/or long hills come into the picture ...

 

[Marky Dissod]

Well, I like the shift schedule and don't mind going into 6th gear at 40MpH ...
Granted, I live where it's flat and our only hills are highway overpasses, so maybe you guys in the hills experience things differently.

For whatever it's worth, I once tuned an LT1 F-car with 3.42 & 235/55R17, that lived in San Fran.
HAD to ask if he wanted to be able to use Normal mode to climb up & down those hills ... did NOT get the tune right on the first try,
but he told me that resorting to Performance mode managed the hills sedately enough ...
so I tuned his Normal mode more like Performance mode, and Performance mode more like 'Bullitt mode', in case he wanted to REALLY enjoy the hills ...