Couple of issues when in 4 low

[H3slate]

I wanted to get some opinions on a couple of issues I am having with my truck in 4 low. First my truck is a 2009 H3 Champ. Ed 5.3L V-8 w/ 6L80e 6 speed auto transmission. I am running 35" tires and have 4.10 gears.

While in 4 low the speedometer is not accurate, it will often get up to 30 mph even though I am going no more than 5 mph. My only concern with this is that the incorrect speedo then makes the odometer incorrect. So I don't know actual miles traveled for navigating or fuel consumption. The speedo is accurate in 4 H and was adjusted to match the 35" tires by the previous owner with a tune by HP tuners. On my previous truck I believe it did this as well, just not as bad, and more accurately represented the speed of the wheels turning in loose or muddy terrain.

My second concern is when using the engine to control a decent in 4 low. On a steep downhill I will shift the transmission into 1st, transfer case in 4 low, and let the engine control the speed. The only problem with this is that while decending the revs will drop suddenly causing a jolt of the rig slowing down suddenly. The revs will go back to normal idle, normal decent utilizing the engine, then it will do it again. I have not experienced this rev dropping issues in any other situation. 4 low climbing or flat ground the revs are fine. In 4 H locked and 4 H unlocked I have not experienced the revs dropping. My main concern is that the revs dropping creates a situation where I have to consider not using 4 low for decents, or maintain a slightly higher than idle revs while using the brakes to help control the decent.

Looking for feedback and hopefully solutions to these issues.

 

[CaseyS]

The first should be related to your transfer case electronics. In 4lo, a signal is sent from the xfer case control unit to the ecu to adjust the pulse train from the vss since it should be 4x less pulses per wheel rev. The 6l80e swap should not have messed with this. But I do know on the i5 this info is used to adjust the transmission shift points since the tcu is all together with the ecu.

For the second issue, need more info. Is it like the trans is kicking in overrun clutch braking? This will likely be the 6l80 tcm cal somewhere. You say the revs drop, then go back to normal idle which is not making sense. Descending the engine will be revving up to the point that the engine compression provides enough resistance to gravity
How is it revving down under this condition?

 

[H3slate]

The first should be related to your transfer case electronics. In 4lo, a signal is sent from the xfer case control unit to the ecu to adjust the pulse train from the vss since it should be 4x less pulses per wheel rev. The 6l80e swap should not have messed with this. But I do know on the i5 this info is used to adjust the transmission shift points since the tcu is all together with the ecu.

For the second issue, need more info. Is it like the trans is kicking in overrun clutch braking? This will likely be the 6l80 tcm cal somewhere. You say the revs drop, then go back to normal idle which is not making sense. Descending the engine will be revving up to the point that the engine compression provides enough resistance to gravity
How is it revving down under this condition?

Thanks for the response Casey. On the transfer case electronics, how would I go troubleshooting/adjusting the settings to correct the speedo?

For the second issue, the trans is not shifting, it is in first. The engine revs go up as it is trying to control the speed of the vehicle down the hill. The speed increases a little as gravity tries to take over. Then the rpms drop to about 600 rpm and there is a noticeable "braking" of the vehicle. It will continue to do this as you go down a hill, with no input on the accelerator or brakes as you let the gearing control the decent.

I will try to get some video of this later this week.

 

[atvspeed4]

I thought I posted this up but I am in the middle of doing the 6L80 swap. I have confirmed with the company doing our programing that a fully stock functional 6L80 swap in an H3 is not possible. PRND works properly but PRND321 is not possible due to a OS not existing that will allow this. It's possible to do PRND"M"21 using a modified Corvette OS but this does not allow 4lo tables.
I honestly was wondering if your vehicle was the missing link that somehow found a way around this...

 

[CaseyS]

You really only need a PRNDM with some means of tap up/down shifting. Your 3 position will be the M position. I think that's what I'm going to plan around.

I need to get to the details but I think you have to wire the tap signals to pin 7 straight on the trans connector.

As far as the speed, the xfer case should still be sending the 4lo signal to the bcm independent of the transmission so I'm not sure why the speed doesn't work.

In my application I attempted to wire the 4lo to my standalone to get a 4lo shift table to work but the signal didn't not work.

If you are in 4lo, manual shifting would be preferred anyway.

 

[H3slate]

I thought I posted this up but I am in the middle of doing the 6L80 swap. I have confirmed with the company doing our programing that a fully stock functional 6L80 swap in an H3 is not possible. PRND works properly but PRND321 is not possible due to a OS not existing that will allow this. It's possible to do PRND"M"21 using a modified Corvette OS but this does not allow 4lo tables.
I honestly was wondering if your vehicle was the missing link that somehow found a way around this...

Adventr did the swap on my truck. I know he spent some time getting the programming to work. I have the stock shifter, and it functions as PRND"M". So when i drop it below drive it holds the gear that I am currently in. On the road I can drop it to 3rd, and it will downshift, but not above 3rd. On the trail I can shift to "M" and it will hold first or Second. I believe that Adventr used a GM tcm programmed as a 2009 Denali to have it work with the pcm.


I wonder if this is where my issue with the speedo in 4 low comes from

The 4l60 uses a t42 tcm that is external to the transmission. The 6l80 uses a t43 transmission controller that is built into the Transmission and controls the valve body directly.

 

[atvspeed4]

I'm familiar with your swap and true blue Motorsports who Adventr purchased the wiring harness. The issue is that any other vehicle than the Corvette needs the bcm to use "M" I think that when you are shifting to M it is defaulting to 5th and is getting into an issue. When you offroading and in 4lo it is going to 1 or 2 because the minimum rpm is not being met in 5th so it is dropping to 1st or 2nd but it is not getting the signal to run off 4lo table for shift points

 

[atvspeed4]

Just to follow up with more info. Since denalis and escalades did not come with 2 speed transfercases they lack the shift tables for 4lo. There are 4 available shift tables but the denali/ escalade OS is not set to access a 4lo table. To get this to work someone would need to write a custom OS based on the corvette OS that swaps out the shift tables for those of a silverado that has 2 speed transfercase...the company that is building our harness for the swap is trying to sort this out but it is uncharted waters