LML ABS Causing Strange Bump at Stop Lights

snowman22

Member
Jan 30, 2018
297
1
18
SoCal
So this has been a long saga and I’ll try and make it short because I’m out of ideas and need some help.

Two weeks after getting my trans built by Mike it started this strange free roll and harsh stop at the last 3-5ft coming to a stop light, but an intermittent issue and varying severity. The best way I can describe it is it feels like a harsh 2-1 downshift where it goes into neutral and loses engine braking and grabs first abruptly and then the truck instantly stops. Mind you, all this happens at less than 3mph and isn’t violent per say, but noticeable and concerning with a trailer.

I originally thought it was something with my brakes sticking because I hadn’t changed the original pads rotors. I had been pretty rough on the truck after getting the trans done, so I wasn’t too concerned. Fast forward a few months and I replaced all 4 rotors and pads. Didn’t help one bit and everything was moving freely when I pulled it apart.

Then I thought it could be a sticky valve or something in the trans (not my expertise at ALL). I called Mike a few times and ended up getting a stock file from Mark for the tuning just to test. No change. I ultimately made the drive down to Mike because I just wanted him to feel it. Once I was able to find a time for us to look at it he discovered a communication code with the body module to the ECU. Cleared it, drove the truck and couldn’t make it repeat. He was also pretty confident that the trans couldn’t cause what I was describing. Found out the code was caused by the MotorOps tune I had previously run, but no longer set on Marks tune. MotorOps was a joke and didn’t want to help at all and just said they couldn’t fix it after a half assed attempt, I digress. After clearing the code and running the truck for a few weeks, no codes are being set with Marks tunes, but the issues remains.

Then yesterday I got an idea that it could be caused by the ABS motor pulsing right before a stop. I figured the best way to see would be to pull the ABS motor fuse and see what happens, problem GONE! The ABS light never comes on the dash when the issues is occurring, but clearly is within this system.

Any ideas on where to start or what could be causing this issue? I think it happening after the trans build is just a coincidence, but is there a speed sensor that could be causing this inside the trans?

If you made it this far, thanks.
 

gmduramax

Shits broke
Jun 12, 2008
3,773
51
48
Nor cal
Jack the truck up and see if either have play in them. Will the tech II show wheel speed? You should see it cutting out.
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
18,297
488
83
Phoenix Az
usually, if its a front wheel speed sensor, you get a steering yank one way or the other when almost at a stop. a rear speed sensor could cause what you are talking about if its seeing funky things almost at a stop. may not be t-case related as i forgot you will have speed sensors at each rear wheel unlike the older gens

do you have the ability to log speed sensors?
 

snowman22

Member
Jan 30, 2018
297
1
18
SoCal
usually, if its a front wheel speed sensor, you get a steering yank one way or the other when almost at a stop. a rear speed sensor could cause what you are talking about if its seeing funky things almost at a stop. may not be t-case related as i forgot you will have speed sensors at each rear wheel unlike the older gens

do you have the ability to log speed sensors?

I have a GDSII, but figuring out how to make it work again will be the tough part. It most definitely does not pull to one side when it happens.
 

snowman22

Member
Jan 30, 2018
297
1
18
SoCal
It doesn't appear to be the wheel speed sensors. All 4 corners read within 1km/h of each other. There were a few times one would go to zero a split second before the other three, but it wasn't the same sensor every time.

I'm going to watch it again on the way home. I was hoping it was going to be an easy speed sensor, doesn't seem I'm going to get so lucky.
 

six5creed

Member
Jan 6, 2016
1,037
30
48
N.C.
Sub'd.


Mine does the exact same thing and started after having the trans built about a year ago. I've only driven it 3,500 miles in a year but it does it often.
 

gmduramax

Shits broke
Jun 12, 2008
3,773
51
48
Nor cal
It doesn't appear to be the wheel speed sensors. All 4 corners read within 1km/h of each other. There were a few times one would go to zero a split second before the other three, but it wasn't the same sensor every time.

I'm going to watch it again on the way home. I was hoping it was going to be an easy speed sensor, doesn't seem I'm going to get so lucky.

I’m betting that’s the problem. The truck sees one sensor go to 0 and the truck thinks a tire locked up.
 

snowman22

Member
Jan 30, 2018
297
1
18
SoCal
I’m betting that’s the problem. The truck sees one sensor go to 0 and the truck thinks a tire locked up.

I would agree, but the strange thing is it's a different sensor every time. I'm going to spend some time with it on the way home and see if the issue happens when a particular sensor gets to 0 first.
 

gmduramax

Shits broke
Jun 12, 2008
3,773
51
48
Nor cal
Stock size tires on the truck? Maybe it’s the abs module causing issues, it is strange that it’s multiple corners.
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
18,297
488
83
Phoenix Az
you scanner may not have a high enough refesh rate either to really see the problem. i have no experience with the module your using to know
 

snowman22

Member
Jan 30, 2018
297
1
18
SoCal
Stock size tires on the truck? Maybe it’s the abs module causing issues, it is strange that it’s multiple corners.

Basically stock tire size

you scanner may not have a high enough refesh rate either to really see the problem. i have no experience with the module your using to know

I'm using the scanner below. It loads a Window XP VM and runs the GM GDS software. It's refreshing the tire speed fast enough I would expect it to show a drop in signal. I can see every incremental change in km/h even at a rapid deceleration.

https://www.amazon.com/VXDIAG-Tech2...ords=vcx+nano&qid=1561061375&s=gateway&sr=8-3
 

Woodster

Computer Nerd
Aug 28, 2009
176
0
16
Utah
I have noticed very similar issues with my truck. I have Mark's tuning and I also had my transmission re-built and transfer case serviced while at Mike's. I had originally thought I was experiencing the dreaded "Drive-shaft clunk" but from what I can tell everything looks solid. I have no play/leaking from any u-joints and everything feels solid. Do you experience any issues with your 3-2 downshift?
 
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snowman22

Member
Jan 30, 2018
297
1
18
SoCal
I have noticed very similar issues with my truck. I have Mark's tuning and I also had my transmission re-built and transfer case serviced while at Mike's. I had originally thought I was experiencing the dreaded "Drive-shaft clunk" but from what I can tell everything looks solid. I have no play/leaking from any u-joints and everything feels solid. Do you experience any issues with your 3-2 downshift?

I don't think my issue should be confused with a harsh 2-1 downshift. My LB7 would do some crazy stuff on the 2-1 once and while after Mike did it. Honestly that felt like a computer trying to compensate for line pressure it didn't know it had.

This is VERY different. The only reason I thought it was the trans originally was the timing, but didn't feel like the trans from day one. I'm not a torque converter guy and thought maybe it could cause my issue if the coupling was causing cavitation which could create a load no load situation on engine braking. Again, I get "how" they work, but the nuisance of them are not my wheelhouse.

Unfortunately after converting the GDS to read mph from km/h I can no longer see a lazy speed sensor. I had originally set it to km/h to make the readings as sensitive as possible, but it reads so close to all the others I don't think it's my issue. It even displays the difference as soon as you start turning.

Not sure what next because when I did the brakes thinking it was my issue. I verified all the play on all 4 corners, looked for rubbed lines, flushed the brakes with the automated process (twice) and don't think my issue is there.

Food for thought, does anyone think it could be the G80? I don't know a whole lot about the actual mechanics, but could the clutches in it cause a loss of drive and then re-apply? This is a LONG shot because the truck drives absolutely perfect otherwise.
 

Dallas S

Member
Jun 17, 2009
446
3
18
Alberta
Pull front speed sensors out of hubs, clean rust off mounting pad, reinstall.

We have seen this many times. IMO the rust is pushing the sensors out of the hub and too far from tone wheel. We can see it on the scanner, front wheels will drop off at low speed. Trans bumps 2-1. ABS pulses.
 

snowman22

Member
Jan 30, 2018
297
1
18
SoCal
Pull front speed sensors out of hubs, clean rust off mounting pad, reinstall.

We have seen this many times. IMO the rust is pushing the sensors out of the hub and too far from tone wheel. We can see it on the scanner, front wheels will drop off at low speed. Trans bumps 2-1. ABS pulses.

I don't lose any signal at low speeds. I can see the sensors steadily run at 1km/h and 1 mph consistently.

There is also nearly zero rust on this truck, it's run primarily in southern California.