I have a 2008 Suburban that I had the 6.0 gas swapped to a 2008 LMM Duramax. I am having an issue with a hard down shift when coming to a stop. If I completely stop and immediately start again it will slam into 1st. If I stop and give it 2 - 3 seconds it will not happen. Any thoughts?
Hard Down Shift from 2 - 1
- Thread starter arich4387
- Start date
You might give some more information. is the transmission transmission shifting correctly otherwise? How many miles since conversion. You don't mention what trans you have. I guess we are to assume that it is the Allison.
And have you carefully checked the fluid level.
And have you carefully checked the fluid level.
ESPESCIALLY LMM's. I know mine does some REALLY wierd crap, and is why I have to run an 01 OS(tried later OS's, and they all freak out on me).
It is an Allison. Fluid is good. Donor engine/trans had 44k miles at conversion. They have 53k on them now. Conversion was done last June. I have talked to a couple mechanics on it and one suggested to check my wheel speed sensors. One may be faulty and not telling the trans to shift down soon enough. It is shifting correctly otherwise.
I believe Eric has had to do A50 TCM swaps to a few of these for weird shifting issues. Also make sure you're not experiencing a bump from a sticking slip yoke instead of a bad 2-1 downshift.
do you have the tire size "RPMS" calibrated properly in both the ECM and TCM tune? I noticed mine doing something similar to what you described when I changed to 33" tires, adjusting to the correct calibration value in both tunes seemed to alleviate the issue entirely for me. Hope this helps.
Messing with the speedo, gear ratio, and other speedo settings in the TCM is a good way to cause issues like this. The majority of wierd shifting posts I come across besides trannies with clutch problems is because people mess with the speedo settings in the TCM. I leave them alone as GM set them to something different than what the truck came with in the 1st place, and changing them can affect thousands of lines of code you CANNOT see with EFILIVE.
Now that I am back at my computer and have EFI live tune tool open, the value I was talking about changing was (H0136) "tire revolutions per mi/km" they include a calculator that allows you to plug in your current tire size, then it will pop out a corrected number for you to plug into this table. It is actually very accurate from what I have seen in GPS apps and radar equipped traffic signs versus indicated speed on the dash.
There is also a matching table in your TCM tune, (H0136) "tire revolutions per mi/km", changing one without changing the other would probably be a bad idea.
That's as brave as I got in messing with the tune on my transmission, but no issues at all so far, it fixed my hard downshift to first when approaching a stop sign, and I am well over 15,000mi into making the change. I had my trans serviced and transgo jr installed a couple of months ago over at Mike's shop, he said everything was still looking great inside the trans. I agree that messing with anything else in this tune is a bad idea if you don't know what you are doing, but I see no benefit in telling the ECM/TCM I am running 22" tires when I am actually on 33".
For S&G you may want to plug your current tire size into the calculator and see if the calculated value is rougly the same, and be certain they are the same value in both tunes.
There is also a matching table in your TCM tune, (H0136) "tire revolutions per mi/km", changing one without changing the other would probably be a bad idea.
That's as brave as I got in messing with the tune on my transmission, but no issues at all so far, it fixed my hard downshift to first when approaching a stop sign, and I am well over 15,000mi into making the change. I had my trans serviced and transgo jr installed a couple of months ago over at Mike's shop, he said everything was still looking great inside the trans. I agree that messing with anything else in this tune is a bad idea if you don't know what you are doing, but I see no benefit in telling the ECM/TCM I am running 22" tires when I am actually on 33".
For S&G you may want to plug your current tire size into the calculator and see if the calculated value is rougly the same, and be certain they are the same value in both tunes.
If you look at stock tunes, you will find those values don't match to begin with. Leave the tcm alone IMO. Messing with ANY speed tables in it can throw off thousands of others you cannot see with efilive.
That is not the case in my stock tunes, each value is set to 424 in both my ECM and TCM.
In the TCM, the table (H0136)'s description reads "Number of tire revolutions per kilometer used for shift stabilization and grade braking calculations." Mine never does anything weird and I am frequently going up and down long 15%+ grades. Come to think of it, before I had made this change, I did have it kick into grade braking and be temperamental about coming out every once in a while after the change to 33" tires. I know it is part of grade braking, but it is alarming to watch and hear it carry the revs up high like that
I can't say for sure, but I'd bet the technicians at the dealership can change the tire size in your ECM, it would also be just like a tech at a stealership to forget to match the TCM calibration... speaking from experience working at and hearing stories from other dealerships. I'm no diesel tuner, but misreporting the size of what is essentially a large gear (the tires) to the ECM/TCM makes no sense at all IMO. If you have physically changed the size of the tire, why wouldn't you want to change the calibration value in the computers accordingly? Rolling circumference of a tire = Pi (~3.14) * Diameter in inches, so say you have a 22" tire, in one revolution it will travel 69.08", if you change to 33" tires, one revolution will carry it 103.62". So in one tire revolution, your truck would travel almost an additional 3 FEET.
What this table reads is revolutions in a km/mi, a mile is 63,360", divide that by 69.08" and you'll find that it takes ~917.2 rotations of the 22" tire to travel one mile. Now here is where my logic is coming from, divide the same distance by the 103.62" for the 33" tires and you'll find that they will turn out a mile in only ~611.5 rotations. That's a pretty large discrepancy in my mind, I don't know if these tables tie in to ODO, fuel economy calculation, and onboard maintenance scheduling, etc. but I would sure hope so for any tables that use tire size to determine mileage calculations. I see a lot more potential for short shifts, limps, etc. not adjusting tire size electronically when changing tire size physically. I know that is a wall of text and probably nowhere near pertinent to the OP, but I didn't just go in and change these two values without a good rationale behind doing so... and I am a math nerd, so I thought I'd share :thumb:
Not trying to start a fight over it, just wanted to share my logic on it.
Out of curiosity though, do you know how much weight difference there is between the donor truck and the new rig, OP?
Edit: Mine is an LBZ duramax with the A40 6 speed BTW
Ok, so that certainly isn't a point of concern. Out of curiosity, did you at least look at what I recommended? I know a reset of the TAPS is never advised, but maybe a reset of the TAPS as a last ditch effort so that it can re-learn its shift points. Do you know if there were any modifications made to the ECM/TCM tune in the past? Someone did make a good point before in verifying your drive ratio in the suburban physically against what is written in your tune. I still really lean toward checking what I originally suggested, otherwise maybe try logging some data to see if you arent getting any signal from a related sensor.
I have not done anything with ECM/TCM. I don't have the tools to dive into that. I had the swap done by Eric at Duraburb. I have asked him a couple times about it but he is a pretty busy guy. I'll have to bug him some more about it. It is a 4 X 4.
It is an awesome camping rig, the problem is I had to go buy a bigger camper........
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It is an awesome camping rig, the problem is I had to go buy a bigger camper........
Sent from my iPhone using Tapatalk