2013 LML Reverse Only, No forward

[kyledeg4]

Hey all, new here but long time Duramax fanatic and fiddler. I have had some issues with my LML trans and can't seem to figure out where to go next, looking for some insight and direction! Any help is appreciated as I am a technician but not an allison guru! Long story and some weird stuff, so buckle in!

Initially pulled trans to build with DHD kit Alto/Ratiotek PRX and 1057 Suncoast- trans worked/shifted great for about 6 months of pretty hard abuse and lots of tuning clearances were within alto spec, front play was good (measured before/after). Running behind a compound turbo setup with 45 overs 700hp range.

Problems started after I did two 1-5 gear pulls a few minutes apart. No abnormal shifts/noises heard throughout the pulls. Trans shifted down through the gears fine, after I slowed to make a left turn, went to pick up the throttle at about 10mph. About the time she wanted second gear it threw a p0776 and limp 3rd. Luckily only a mile from the house, went home backed into garage. Cleared codes and restarted, put it into drive and shift range inhibited blinking indicator no forward gear. OK, tried reverse and no problem there works every time. Figured I toasted her without having much shift defuel or broke something...

Dropped pan, No metal, Transynd fluid nice bright red. Tested solenoid and circuit - all good. New filter, and preventative disassembled VB for cleaning/stuck valve inspection (VB was clean no sticking valves). Replaced ALL solenoids, harness, pressure switch w/ GM / Allison parts. Filled and fired up, still has reverse only. Drive= Shift range inhibited. Out she comes.

No oil in connectors, no issues in wiring harness as I have removed all loom and inspected harness from trans plug to tcu to ecu for frays/cuts/rubs.

Upon disassembly, C1 had slight heat discolor on one backside of friction, c2 clutches were about .03 thou worn on 3 of the discs the others speced as new, C3 and C4 about the same for wear, 3 of each missing a few thou. So, went ahead and did a refresh of all seals, frictions, steels. Alto C1/C2 Excaliber C3/C4 this time. Inspected stator for twisting, holes till at 6/12. Pump looked good no gouging there, all bushings looked good, c2 hub is sleeved and not stripped. Nothing obvious in the hard parts to a (not guru). No shavings, no bushing damage found, torrington's looked great. Went ahead and reassembled unit hoping for best but still get shift inhibited in drive, but no issues for reverse. No codes thrown with this either.

OK- so here is where it gets weird, and I don't know if this is relevant but when I bought the truck the trans temp on the DIC has always displayed -40, even though my logs and OBD port read the trans temp no problem. So, knowing the a50, I bought a new TCU thinking maybe it's going south (also to also have a backup if needed). Flashed new TCU, still get shift range inhibited but reverse is functional. I have tried stock files to make sure it isn't a tuning corruption as well to no avail. Speed sensors were ohmed on disassembly and were in spec. Turbine speed is reading good no dropouts or erratic's and comes to zero when in reverse but no drawdown in drive. I can see the oncoming clutch pressure go to 232psi on the log when put in reverse, but it takes about 3 seconds to build to 232 when putting it in drive and I see no turbine speed drawdown. VSS on my logs is showing some RPMS that I don't think are normal, when shifted into gear it will sometimes read RPM (some occasions 5krpm) on VSS which I know is not correct as I have the brakes on... so I decided to unplug the VSS to see if it would let me select drive (thinking computer seeing too many RPM preventing shift to range) reading zero VSS value and still only have reverse! Also, NO dtc is set by unplugging the vss- which I think is strange. I can also unplug my trans 20-way harness, and I still do not get a CEL...

Also, Investigated the transfer case tone wheel, seems OK, has some fore/aft play on the shaft maybe 1/4" inch or so but do not know if this is in spec. Wheels turn in reverse, so I don't think t-case is exploded or stuck in neutral. Dropped the oil on it as well, no shavngs there.

What am I missing guys? Massive hydraulic leak or comms issue? Why no p0700 dtcs? Thanks for your time if you made it this far... any help is more than appreciated.

 

[Ron Nielson]

I don't know how different your 2013 is vs my 2008, but maybe something here will help.

2008 Chevy Truck Silverado 2500 2WD
V8-6.6L DSL Turbo

No First, Second, Third, and Fourth Gear​

1. Vehicle

• 
  
• Transmission and Drivetrain
• 
  
• 
  
• Automatic Transmission/Transaxle
• 
  
• 
  
• Testing and Inspection
• 
  
• 
  
• Symptom Related Diagnostic Procedures
• 
  
• 
  
• 6L50/6L80/6L90 - Automatic Transmission

1. 
   
2. No First, Second, Third, and Fourth Gear

No First, Second, Third, and Fourth Gear

Document ID: 1593287

Checks​	Causes​
Automatic Transmission Fluid Pan (29)​	Verify proper fluid level and condition, inspect transmission assembly for leaks.​
Control Valve Upper Body (304)	Inspect for sticking, damaged, or worn 1-2-3-4 clutch regulator valve (337) and spring (326).
Control Valve Solenoid Body (314)	Inspect for contamination or debris. Pressure control solenoid 5 commanded/failed LOW (off) or leaking – Perform solenoid valve assembly function test. Refer to Control Solenoid Valve and Transmission Control Module Assembly Solenoid Performance Test.
1-2-3-4 and 3-5-Reverse Clutch (51)	Inspect clutch pack retaining ring (429) for proper installation. If the retaining ring is out of the groove the following parts must be replaced: 1-2-3-4 and 3-5-R clutch housing (427), 1-2-3-4 and 3-5-R clutch bearing assembly (428) and the 1-2-3-4 clutch backing plate retaining ring. Inspect for a cracked or damaged 1-2-3-4 clutch piston (414), piston housing (417), spring (413), and dam assembly (412). Inspect for cut, worn, or damaged 1-2-3-4 clutch piston seal (416), inner seal (423), and housing seals (424 and 425). Inspect for improper selective 1-2-3-4 clutch backing plate retaining ring (429).
Automatic Transmission Fluid Filter (26)	Inspect for damaged or restricted fluid filter assembly (26). Inspect for damaged fluid filter seal assembly (25) or improper sealing interface, leaks, and fluid aeration.
Torque Converter Housing (2)	Inspect for damaged or scored pump housing (203), cover (219), rotor (217), slide (212) and vanes (216).
Torque Converter Housing (2)	Inspect for damaged or leaking 1-2-3-4 and 3-5-R clutch fluid seal rings (230).
Center Support (67)	Inspect for worn or damaged low clutch sprag assembly (467).
Automatic Transmission Fluid Pump Seal (19)	Inspect for damage or leaks.
1-2-3-4 Clutch Hub (61)	Inspect for damaged splines on the hub assembly.

 

[kyledeg4]

Thanks for the info, but looks to be about the 6L series.

 

[Bdsankey]

Start the truck (leave it in park) then crawl under it and unplug the main harness going into the back of the trans.

Please report back if it moves in both directions or just reverse as it does now.

 

[PureHybrid]

Does the turbine speed drop at all when commanding first gear? Even if there's a hydraulic leak in the system it should at least try to engage the c1 clutch, and partially slow the drum down before setting shift inhibit. If it's throwing the shift inhibit as soon as you move the gear lever then there's something else going on

Did you pull the c1 / c2 drum all the way apart?

 

[kyledeg4]

Hey yall thank you for replies!

So I did leave that part out... Tried starting truck, unpluging connector and shifting to reverse/forward... In my case I get nothing... unplugged harness I get no codes either. My gear underline dissappears, no reverse or drive unplugged.

So I get no turbine speed change at all before shift inhibit. C1/C2 was dissassembled, all new seals including white teflon seal. None damaged, new balance piston too just for kicks.


On the logs it looks like it commands "drive 6th", I get no turbine pull down like in reverse, I see clutch pressure up to 232 (albeit slower than when put into reverse) and then it commands neutral and tells me inhibited, to shift to N.

 

[Bdsankey]

Hey yall thank you for replies!

So I did leave that part out... Tried starting truck, unpluging connector and shifting to reverse/forward... In my case I get nothing... unplugged harness I get no codes either. My gear underline dissappears, no reverse or drive unplugged.

Unplugging the harness on a 06+ truck will always remove the PRNDL line as the NSBU (what tells the truck the gear you selected) is internal and on that same harness.

So I get no turbine speed change at all before shift inhibit. C1/C2 was dissassembled, all new seals including white teflon seal. None damaged, new balance piston too just for kicks.

So are you saying this condition existed before or after you tore it down? IE did resealing/rebuilding the input drum cause this issue or does it remain unchanged after this work.

On the logs it looks like it commands "drive 6th", I get no turbine pull down like in reverse, I see clutch pressure up to 232 (albeit slower than when put into reverse) and then it commands neutral and tells me inhibited, to shift to N.

 

[kyledeg4]

Results of unplugging harness I cannot get any gear, no reverse, no forward. Plugged in, I get reverse only.

This condition was the reason for dissassembly and inspection.

Essentially chain of events was:

1) p0776, cleared code and only had reverse + shift inhibit in drive. New filters/solenoids/VB cleaned. NO change just have R and Inhibit but no codes thrown.

Removed trans Refresh/inspect/reclutch

New TCU

Same result shift inhibit and reverse only.

 

[kyledeg4]

Did some more playing around with the truck with a different ECM, found no faults in wiring harness and still displaying trans temp of -40. Does anybody know the BCM role in shift range inhibited/trans temp sensor logic?

Have some EFI logs if anybody is interested in looking I can email them as it looks like they can't be uploaded here. I did notice some weird numbers for the transfer case output speed PID but I do not have any comps to know what it would normally read. In my opinion it should be the same as output speed. The only trans electronics that have not been changed are the internal nsbu/imu and the turbine speed sensor. Otherwise TCU, ECU, pressure switch, solenoids, internal harness, and output sensor have all been changed.

As of now, I did get the truck to go into drive and shift up through the 6 gears therefore it looks like things are working in the trans OK as no dtcs thrown. I have narrowed down the issue and can replicate it now. It seems that if you drop it in drive without being on the brakes and immediately transition to giving it some throttle it will shift up/down through all of the gears and maintain that status as long as the brakes are not applied as if you were coming to a stop. As soon as you hit the brakes and come to a "stop" I do not feel the truck trying to push through the brake pedal like you normally feel, it feels like it is in neutral, and the turbine speed never comes to a stop like it does in reverse on the brakes. This is when the underline starts blinking and will give you the shift inhibited shift to neutral message with no dtcs. Reverse works as normal, stops the turbine, and gives the normal feel of "pushing/in gear".

 

[Bdsankey]

When you had the VB apart, did you ensure all valves moved freely and "bounced" in the bottom of the bores with everything clean? Ie nothing is stuck/hanging up?

 

[darkness]

I would think you would see something in the fluid, but what are the chances this is a torque converter issue?

 

[kyledeg4]

Yes, VB is clean.. no sticking valves. I mean it is clean clean.

The fluid has been clean this whole time. I was curious on the possibility of a TC issue as well. BUT why would it be normal in reverse and have no go in drive if it was a TC issue id think itd have issues both ways correct?


So, as I sit now I attempted to drive it down the driveway but no luck. I backed it up just fine but when it tried to go into drive it was pulsating and then into shift inhibited. Again no codes thrown.

What are the chances that this is a tcase issue? It has 260k on it... had some metal shavings when I first bought the truck but fluid has been OK since. At this point I may pull it and take a look inside this weekend.

Otherwise with a new tcu, ecu, harness has been looked at, new internal harness, solenoids, clean VB, new ignition switch.... this leaves us with nsbu/imu, TC, or torque convertor. After that.. trans comes out again LOL. Let me know what yall think. At a loss here.

 

[PureHybrid]

If you suspect the transfer case, maybe try sticking it in 4LO?

 

[gmc502]

I'm thinking Tcase.

 

[Ron Nielson]

found no faults in wiring harness and still displaying trans temp of -40

A -40 reading means that the temp sensor is unplugged/disconnected, missing ground, etc I don't see why that would be your problem though.

 

[kyledeg4]

So it does it in 4lo/4hi ranges as well. Shift inhibited. Going to toss an nsbu in this weekend for shits and giggles. If it still does it guess I will be investigating transfer case afterward.

Anybody see a chance the BCM is the issue?

Also as far as the -40 Trans temp reading, I know its not the wiring ecu or tcu. So that would leave me the bcm or cluster itself for that issue because the trans temp data is making it to the tcu/ecu if I can log it. Its getting lost somewhere between ecu/tcu and the bcm/cluster.

 

[2004LB7]

So it does it in 4lo/4hi ranges as well. Shift inhibited. Going to toss an nsbu in this weekend for shits and giggles. If it still does it guess I will be investigating transfer case afterward.

Anybody see a chance the BCM is the issue?

Also as far as the -40 Trans temp reading, I know its not the wiring ecu or tcu. So that would leave me the bcm or cluster itself for that issue because the trans temp data is making it to the tcu/ecu if I can log it. Its getting lost somewhere between ecu/tcu and the bcm/cluster.

Unlikely BCM. Those TCMs can run without them. Also unplugging the transmission should take that out of the equation anyways.

The sensors are not canbus based. They don't report on the canbus to the dash. They are wired to the TCM which then translates the sensor signal to a canbus signal that the dash will then display. If the canbus temp is absent then you'll probably have codes, dashes or a blank temperature. Not -40. A negative 40 is what the TCM or ECM reports when the signal from the sensor is at its lowest or non existent

I believe there is more then one temperature sensor. So make sure you find them all when logging.

 

[JoshH]

When you had the transmission apart, did you check to see if the stator support shaft had spun in the housing? I'm not sure if it can spin to the point of blocking flow to the C1 drum or not, but I have seen a spun stator support shaft cause cooler flow and overheating issues before. Had me scratching my head before I found it. Since all the fluid to the drum runs through it, it would be worth inspecting if you haven't already.