DMax/Allison owners manual shift issue

[rhidalgo]

On our last trip out the manual shift feature seamed to work when it wanted to. Sometimes putting the trans in manual resulted in the up/down shift functioning as it should then the next time -- nothing. I could find no pattern, it seemed to be completely random. It really seems like an electical issue but I am calling on the vast experiences out there for help. Anyone else experience this and have any ideas other than to start changing expensive parts?

[jsmitfl]

I would unhook the batteries and let the computer reset. See if it works and if not go to Chevy garage.

[Phil P]

Hi

Be very careful of “unhooking” the batteries. You must use a very specific procedure or you will set off the OnStar emergency battery backup system and there isn’t anyway to reset it except the $600 exchange.

With that said.

I have been having problems with all kind of electrical equipment in our 2009 Duramax.

AM radio quit receiving anything but noise, the OnStar turn by turn quit working, the transmission wasn’t shifting correctly in “tow / haul” and if the truck sat for 3 day the engine cranked very slow to start and if it sat for 5 days wouldn’t start at all.

The dealers (more than one) couldn’t figure out what to do they all said they couldn’t find anything wrong with the electrical system.

I purchased the manuals. ($240.00)

After reading about the electrical system I found there is a specific way to disconnect the batteries to avoid problems with OnStar and also that the truck can’t be stored without running for more than 21 days without disconcerting the batteries or putting a maintenance charger on them.

So I went to a friends Starter / Alternator shop and had him check the batteries. He used one of the new state of the art battery checking tools and found nothing wrong with the batteries and charging system.

Well being an older ASE certified master truck mechanic I ask him if he had a carbon pile battery tester. He did, we did and guess what the passenger side battery only held a 50-amp draw for about 15 seconds before the voltage started falling off. Then when increased to 100 amps the battery really went south.

I took the truck to the dealer and instructed them to replace both batteries (let them do it so if OnStar goes south they can fix it).

You guess what!

My entire electrical system problem went away. OnStar went back to working the AM radio works almost as good as the FM radio the transmission shift is back to normal and the truck starts after sitting for a week or so.

Phil P

[robert285rld]

I had a shifting issue when needing a down shift on hills or simply wanting additional speed. No one could locate the problem. Then a second problem developed - the change oil light came on every time started the engine. Researched the the problem on the internet. Determined both these electrical circuit are controlled by the accelerator pedal - three circuits run through this control module. Returned to the dealer who located the problem in the wiring harness where it is routed thru the firewall. Three wires had shorted out on the metal firewall. Easy to repair when problem found.

[rhidalgo]

Now I've noticed an occasional issue with the cruise control and wiper functions. Seems more and more like a short or a grounding issue somewhere. Now it'll be to find it. All these features are on the column so it's the obvious place to start looking. Thanks for the help guys.

[bethandkevin]

Since the tow/haul button was put on the shifter, there has been an ongoing issue with broken wires leading to it. This goes all the way back to 1999. I've repaired many of them. That issue is caused by the constant flexing of the wires when using the tilt function and the motion of the shifter. I would, like you, check the steering column area for broken wires. Take each wire between thumb and fingers and tug on them, they are known to break inside the insulation and not show. Be sure to remove the covers from the column and look there also. Good luck.

[Hooker]

I just came from an Allison seminar. The Allison will not let you do something that will damage the transmission. If you are on the interstate and try to downshift to a gear that is too low, it won't do it, but waits until the speed is okay for what you want it to do. You may not be doing this and this may not be your problem, but it's worth knowing.