Delayed Downshift clunk when stopping - 2006 Allison 6-Speed

[RayMich]

I have a 2006 GMC LBZ with the Allison 6-speed transmission.

The truck has been serviced by me religiously approximately every 7500 miles, the Allison external oil filter was replaced every time and about a 1/2 pint of Dexron VI was added to replace what was lost in the external filter.

This truck is 100% stock and now has approximately 62,000 miles on the clock. It has never been raced or abused and the only trailer towing it has seen has been a couple of hundred miles towing a 1,500 lb flat-bottom boat and about 500 miles towing a U-Haul trailer weighing in at around 2,000 lbs.

I did have some small transmission oil leaks TWICE but The oil level never dropped to the ADD line before My GMC dealer replaced all of the transmission cooler lines. The dealer had to replace approx. 6 to 8 quarts of transmission fluid every time they replaced the oil cooler lines. (At least that's what I remember seeing on the repair ticket) The last time was two years ago and I have not had any leaks since then.

At 4,300 miles I changed all fluids to Mobil synthetic oils except for the transmission oil. The transmission oil was replaced with BP's TES-295 approved synthetic oil at around 52,000 miles.

Seven and a half months ago (approx. 10,000 miles ago) I had the transmission serviced by an Allison shop but not a GM dealer. I had the transmission oil replaced with BP's Autran Syn 295 synthetic oil which is TES-295 approved (TranSynd equivalent) Less expensive than TranSynd but still expensive.

The transmission was flushed at that time per the "Hoot" procedure and new internal and external oil filters were installed. Inspection while the pan was off revealed no clutch residue in the pan and a healthy transmission.

Although I don't think my problem is related to the transmission being serviced, I have included this information to give a better history of this transmission's usage and service life.

Shortly after the transmission was serviced, I began to experience a delayed downshift clunk when coming to a stop after driving only about 1 or 2 miles, but this would generally go away when the engine and transmission warmed up.

Lately, I've noticed this delayed downshift clunk much more pronounced and it does not go away even after driving for for 10 to 15 miles.

At first I thought the problem might be the brakes, but when I move the shift lever into neutral before stopping the clunk is not there. However, when I stop with the transmission in gear the downshift clunk returns.

I have not experienced any CEL during this time, but I admit that I have not yet checked for codes with my scanner.

The truck is still under the GM Extended Protection Plan (This extended plan is good until this coming December)

Any comments and suggestions to help me try and figure this out will be greatly appreciated.

I would take it back the the Allison shop that serviced the transmission, but it is 130 miles and two hours away, so I'm hoping to be able to figure this out without having to drive that far.

Thanks.

 

[Ed HD]

Dealer. You're still under GM's bill, make them pay to fix it.

 

[BoostN]

Mine does this. Only when I'm making a sudden stop though..

I'm not sure why

 

[btfarm]

It's been an off and on thing for me as well and was on my LLY too. Comes and goes when cold. It will especially happen when stopping on a downhill slope. I've chalked it up to a relearn issue if I drove somewhat differently for a couple of days but that's hardly scientific... Changing fluid and the spin on never has seemed to have anything to do with it.

 

[saratoga]

IIRC there's a tsb for an intermittent harsh 2-1 downshift. I'll see if I can find it.

Mine does it sometimes also; has since it was new.

 

[RayMich]

IIRC there's a tsb for an intermittent harsh 2-1 downshift. I'll see if I can find it.

Mine does it sometimes also; has since it was new.

Yeah, if you can find it for me, it will really be helpful.

Thanks.

This is really pissing me off, now that the extended warranty gets close to expiring, I start having problems.

On another note, does anyone know where I can buy another GM extended warranty contract without having to pay full retail price for it? -- My current contract expires on December 11 and I just got a card from them telling me that I must extend while there is still at least 2 months left or I can't renew it. I can't afford to get another truck so I have to try to protect myself the best way possible in case anything else goes wrong with this truck.

 

[trotorx2]

I hope you get it figured out Ray...knock on wood...mine is shifting fine but I'm thinking I may be having converter issues.

 

[stacks04]

Ray what is the price of the renew they sent you, your really not going to get much better than gmpp. They are the biggest one we deal with here and by far the best. They do very good by us and approve most everything that comes through our door even some vehicles that are "less then stock".

 

[stacks04]

#PIP4144: Allison LCT1000 Harsh 3-2 2-1 Downshift And ABS May Activate - keywords cold hard shift - (Jun 4, 2007)

Subject: Allison LCT1000 Harsh 3-2, 2-1 Downshift, And ABS May Activate​

Models: 2001-2007 Chevrolet Silverado Classic​

2007 Chevrolet Silverado​

2001-2007 GMC Sierra Classic​

2007 GMC Sierra​

Equipped With the Allison LCT1000 Transmission RPO M74, MW7​

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern:

The Allison LCT1000 transmission combined with 8.1 (L18) or 6.6L (LB7/LLY/LBZ/LMM) engine may experience ABS activation during a harsh 3-2 or 2-1 downshift and or harsh garage shifts. May experience delay or "hanging" upshifts when not using Tow/Haul. The coolant temperature gauge in the IPC may read cold. The Engine Coolant Temperature Sensor (ECT) may be reading cold [approximately 15.5 degrees C (60 degrees F)] or may be reading erratic when the engine is at full operating temperature.
Recommendation/Instructions:

The ECT sensor should be monitored with the TechII Scan Tool and replaced as needed.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.

WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION
​

 

[BoostN]

Yep, that's for the TSB. Mine will even do it and the ABS starts to engage that's how bad it is...

 

[btfarm]

This TSB brings to mind the immediate question... WTF does the ECT sensor have to do with it?

 

[Mad Maxx]

This TSB brings to mind the immediate question... WTF does the ECT sensor have to do with it?

Isn't Engine Power Limited by Engine Temp? Basically as the Motor Warms Up, the ECM will Slowly Allow the Engine to put out more Power. So if the ECT is Bad or Messing Up, maybe it Pulls Power very Quickly, and that Causes the Tranny to Shift Funky.

 

[stacks04]

It also has to do with how the trans shifts because of trans fluid temps ect. The tcm is very reliant on engine parameters to do its job. It is amazing how changing the power curve can alter the shift quality and things of that nature.
I made a tune once with efi live that was soley a boost tune to see what changes happened. All I did was adjust the boost levels down low so it porduced boost alot quicker than stock, the trans was shifting as if it had fallen out of the truck and and was dangling on the ground.

 

[btfarm]

I do know that since I've been running the predator on 85 for the last 5k or so that the trans shifts HARD when it's cold.
So the TCM is highly reliant on the ECT for alot of it's info huh? That explains a thing or 2...

 

[RayMich]

#PIP4144: Allison LCT1000 Harsh 3-2 2-1 Downshift And ABS May Activate - keywords cold hard shift - (Jun 4, 2007)

Subject: Allison LCT1000 Harsh 3-2, 2-1 Downshift, And ABS May Activate​

Models: 2001-2007 Chevrolet Silverado Classic​

2007 Chevrolet Silverado​

2001-2007 GMC Sierra Classic​

2007 GMC Sierra​

Equipped With the Allison LCT1000 Transmission RPO M74, MW7​

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern:http://www.thetruckstop.us/forum/#ss1-1982002​

​

The Allison LCT1000 transmission combined with 8.1 (L18) or 6.6L (LB7/LLY/LBZ/LMM) engine may experience ABS activation during a harsh 3-2 or 2-1 downshift and or harsh garage shifts. May experience delay or "hanging" upshifts when not using Tow/Haul. The coolant temperature gauge in the IPC may read cold. The Engine Coolant Temperature Sensor (ECT) may be reading cold [approximately 15.5 degrees C (60 degrees F)] or may be reading erratic when the engine is at full operating temperature.
Recommendation/Instructions:http://www.thetruckstop.us/forum/#ss2-1982002​

​

The ECT sensor should be monitored with the TechII Scan Tool and replaced as needed.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.​

WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION​

Thanks, I really appreciate it!

This is quite interesting, because during the times when I get this downshift "clunk" I also get pulsation from the brake pedal. That's what originally lead me to think that my problem was the brakes. In reality it is BOTH. :???:

I'm going to try to take it in to the GMC dealer tomorrow with a copy of this TSB.

 

[BoostN]

Thanks, I really appreciate it!

This is quite interesting, because during the times when I get this downshift "clunk" I also get pulsation from the brake pedal. That's what originally lead me to think that my problem was the brakes. In reality it is BOTH. :???:

I'm going to try to take it in to the GMC dealer tomorrow with a copy of this TSB.

Yes, Because you ABS is starting to kick in and therefore you can feel it in the brakes.. I feel the exact same thing in mine.

 

[RayMich]

Yes, Because you ABS is starting to kick in and therefore you can feel it in the brakes.. I feel the exact same thing in mine.

OK, I can see how the TCM might rely on engine temperature to controls shifting, but how is the ABS tied into this? Why does the hard downshift and the ABS act up simultaneously just before the truck comes to a complete stop? I'm only going about 1 or 2 mph when this happens.

Why is it that the ABS does not act up if I shift the transmission into Neutral before I come to a complete stop?

This sure is weird ! -- But it IS acting up exactly as stated in this TSB. :???:

 

[RayMich]

I finally got a chance to take my truck in to the GMC dealer this morning, along with a copy of the TSB that stacks04 posted.

For the first 3 miles or so after leaving home, the problem was there, but by the time I got to the dealer, 10 miles later, the powertrain had warmed up and it was not doing it. Of course, when they took it out for a drive it shifted fine. They checked the ECT sensor, the shift points and the ABS with a Tech2 and again found nothing abnormal. I suspected that this was going to be the case.

The people at the dealer were very nice and told me that my best bet is to leave the truck with them overnight and they will check it in the morning when it's cold. But my problem is that right now this is the only vehicle we have and I can't be without a car. I asked if I could get a loaner or a rental overnight, but was told that GM's extended warranty won't cover it. So now I'm stuck. My extended warranty expires in two months and with my luck, something will fail right after that.

One good thing about this trip to the dealer was that they lubed it and did an engine oil and filter change, with me supplying the Mobil 1 synthetic oil and AC/Delco oil filter, for only $9.50 -- For that much, I may have them do it from now on. Sure beats me getting all dirty and sweaty crawling around under the truck on a creeper.

Have any of you GM techs been able to diagnose and fix this delayed downshift clunk and ABS problem? If so, how did you fix it?

Thanks.

 

[Ed HD]

$9.50?! That's the cost of the filter. I'd definitely do that with how expensive/qt. M1 is.

 

[RayMich]

Ray what is the price of the renew they sent you, your really not going to get much better than gmpp. They are the biggest one we deal with here and by far the best. They do very good by us and approve most everything that comes through our door even some vehicles that are "less then stock".

Sorry I missed this earlier.

They wanted $2,850 for a new 24 month/24,000 mile contract but they would only go with $100 deductible and they would cancel my existing contract which has a $0 deductible. Additionally, the refund for the unused portion of the existing contract they were going to send to GMAC Financial since I had financed the truck through them.

I told them that there is NO WAY I was willing to pay that much for the contract, especially since they were screwing me out of the remainder in the existing contract. They then lowered to price to $2,500 but I still feel that if worse comes to worse, I can put the $2,500 in a savings account toward maintenance. The truck has been almost flawless for the past 65,000+ miles, so I will take my chances.