Surging or Slipping Problem?

[DucksnDiesel]

Had an unusual problem last week on an extended highway trip. While on cruise control at 65 mph, when heading downhill, the truck was surging so bad it was almost bucking. It was very noticable to both me and my wife. It did this on almost every hill. I watched the tach and and speedometer and they did not appear to move so maybe it was the allison slipping. I don't know but with 59,000+ miles, it has never done this before. Fuel filter has less than 10K on it, using Total Power additive, and the Allison filter has less than 5K on it. No performance mods. Any thoughts on what the problem might be?

[More Power]

This is a puzzle.... I would have asked when the Allison was last serviced and whether you were running any performance products, but you already answered those questions.

Is the ATF level where it needs to be? Does this occur when in Tow/Haul? Does it do it when not using cruise?

MP

[DucksnDiesel]

Well the problem is occurring quite often and it did it again today on cruise at about 60 mph. I am beginning to believe that the problem is more related to the Allison rather than the DMAX. Checked the fluid level on Wednesday - right where it should be. Mobil 1 ATF fluid. If the hills where steeper and it was grade braking that would be understandable, but this is definately something else. This is just coast, then jerk, coast then jerk until it reaches the bottom of the hill (all while staying in OD).

When it's not on cruise and I begin to notice the problem, I ususaly will try to slightly accellerate to alliviate the problem. I guess I will just wait until it does this everytime I drive it, then there would be a 25% chance of the dealer finding it.

[More Power]

I've sent this forum thread to Allison. Let's give them a few days to comment....

MP

[RocketsDmax]

I think there was a program update for just such a condition, with cruise on. Have your dealer check for any current updates for your TCM and PCM.

[More Power]

Allison's recommendations:

"In regards to the discussion we had via phone today, here are some comments and steps that should be taken to address some of the Allison Transmission concerns.

There were updated service calibrations released the first half of CY2002 for the transmission. There are different calibrations for the 2001 and 2002 trucks. The dealer can not load the calibration for an '01 into an '02 and vice versa. The service calibrations were released to address various shifting and system communication issues. The transmission calibration updates are performed by reprogramming the TCM (Transmission Control Module). When this procedure is performed at the dealer, the technician should also check for the most recent engine calibration update. The updates for the engines are performed by reprogramming the PCM (Power Control Module). This is an important step to make sure that both the transmission and engine calibrations are updated and the most current.

After this has been done, the truck should be driven and allowed to re-learn the shift points. On the 2002 truck and later, the latest calibration updates will allow the technician to evoke what is known as "Fast-learn". That allows the transmission to re-learn most of the initial shifts without having to drive the truck several miles. Unfortunately, this feature is not available for the 2001 truck which means it takes longer to re-learn.

Should the problem still exist after the unit has been re-calibrated and driven, I would then recommend that the owner take the truck to the dealer and request that they contact the General Motors Technical Assistance Center. When that is done, the Technical Assistance Center starts a case and works with the dealer and technician to resolve the problem. If they (GM-TAC) needs help, they contact a group of Allison service engineers (I'm one of four) who deal with the GM Heavy-duty pickup platform regarding the M-74 (Allison) transmission.

Best regards,
Allisontransmission.com"

[DucksnDiesel]

Thanks. I will give it a try.

[More Power]

A followup from Allison:

" I would ask the guy if the situation occurs when NOT in cruise (i.e. going downhill and trying to hold the speed in another fashion). The only problem with this is the fact if he trys to hold the vehicle with the brakes, he'll have to do it with his foot in the throttle so that the tranny sees the throttle percentage that it "normally" sees while in the same situation in cruise. If he simply holds the vehicle back with the brakes, he will be sending zero throttle signal (i.e. idle) to the TCM and it may not act up.

Hope this helps.

Allisontransmission.com"

[MikeT]

I had the same problem with the vehicle bucking when going downhill with the cruise on. The problem was the splines on the center yoke of the driveshaft. This has been a problem since day 1 of the HD models. About every 15K miles, the dealership has to pull my driveshaft and grease the splines on the yoke. Afetwards, it drives like a dream. Then one day, about 15K miles later I start feeling it again. The longer I wait to get the yoke greased, the worse it gets. The dealership keeps telling me that this is a normal routine maintenance item. I keep telling them that if it were a routine maintenance item, it would be in the maintenance schedule and the yoke would have grease zerts, and of course, neither are there. So, the dealer keeps doing it for me. Last grease job was at 44K miles. I keep wondering if they are going to continue doing it for free. So far, they have.

Mikey

[Roger Lamothe]

DucksnDiesel

What is your status on the problem. I have an 02 2500 2wd and have the same problem. It's almost like it's dragging and the cruise control keeps nudging to maintain speed downhill. It's mostly on long downhill runs at highway speeds. I can't duplicate off of cruise.

I am planning a month long trip to the west this summer and I hope this isn't anything major.
Thanks :confused: