Allison will not shift manually

[Beekeeper]

My 2006 Chevy 2500 HD with Duramax and Allison will not manually shift. Put it in "M" and it goes straight to "6" and shifts just like it is in Drive.

Can anyone help me?

Beekeeper

[lmholmes11]

Any codes?

[DmaxMaverick]

Any persistent transmission faults should trigger the SES and store a code. If the SES isn't on, any faults would be interpreted as temporary, but a soft code would still be stored.

That said, many tranny features are not available until the engine/tranny are up to operating temp. I don't know for certain if the manual shifting is included with this condition, but it's a starting point.

Also, even in manual mode, the transmission won't shift to a range not permitted (you can't shift up to 6 at 15 MPH). If your "up" button is stuck, or otherwise not working properly (perhaps a damaged harness or poor connection), a selection to manual, and a perceived (by the TCM) request for "6" is present, it would shift as if in drive mode, advancing gear ranges as they are "permitted". If a "downshift" selection does nothing on the display or with shift behavior, I would suspect this to be the case.

[Beekeeper]

There are no faults displayed on the panel. I don't have a code reader. Any suggestions as to what I should buy would be appreciated.

Engine is definately up to op temp.

I have tried mashing the downshift button with no resulting change.

Beekeeper

[DmaxMaverick]

Kennedy has a simple reader for not much $. Autozone carries several Actron brand scanners, and they are pretty good for the money. I think your problem is going to be button or wire harness related.

You might try disconnecting the harness under the dash. If it is a button or a wire in the column or lever, it shouldn't default to "6" when you move the lever to manual. Early model Tow/Haul mode button wires would occasionally break at the moving point where the lever meets the column. It isn't common, but has happened. The harnesses are similar and can suffer the same fate. In your case, I don't know if the lever is a single assy, or can be repaired.

[Beekeeper]

Thanks for the info. I will try disconnecting the cable.

I really wanted one of those super-duper things that read codes and gives a bunch more data while driving. I read in an RV Forum (I pull a 5th wheel) that ScanGauge has a good one.

Beekeeper

[JohnC]

If it goes right to 6 regardless of what gear you're currently in, I doubt it is the switch, unless it somehow detects a switch failure. Normally, if you are currently in (say) 4, shifting to "M" will result in the gear indicator showing "4".

[a5150nut]

BeeKeeper,

Have you looked on line to see if there is a program available for a lap top? The OBD2 cables are available with a serial connector on the other end.