Allison overdrive lockout switch

kmart · April 3, 2004

Chevy has a service bulletin out that installs a switch to lockout the shift into overdrive, I cannot recall the number just now. It keeps OD off when heavy pulling hills to prevent short shifting.

I am considering this mod for my 2002 2500HD 8.1. Anyone have this mod installed?

I am interested in any problems encountered and what cost was involved before I ask my dealer to do it.

kylant · April 3, 2004

I have had this done. I had the dealer do it per the TSB. They performed it under warranty

It works great. It just keeps the truck from shifting into over drive. Basically it stays in 4th gear.

I have the same truck as you. I have had no problems with this mod.

BlackSilver · April 3, 2004

That feature is standard on my 2003 Allison, but I pull some heavy loads in sometimes difficult grades and have never had to lock out OD.

kmart · April 3, 2004

Is it a normal looking switch or a radio shack toggle? Can you give me some details? My dealer seems less than intelligent when I talk to the service guy.

BlackSilver · April 3, 2004

Is it a normal looking switch or a radio shack toggle? Can you give me some details? My dealer seems less than intelligent when I talk to the service guy.

On my 2003 it's the same button you press for Tow/Haul --- you just hold it longer. Short tap puts it into Tow/Haul, longer press locks out OD. You get a cluster light showing OD locked out. If your dealer doesn't know this, then you need to immediately find a different dealer. Your current one is an idiot who doesn't know his own product line.

Shaners · April 3, 2004

Hey kmart...

I hate to break in on your thread, but I live in Grove City, just North of you.

Welcome to the forum!!!

bluenote · April 3, 2004

On the 2001 and 2002 trucks, you can have an OD/5th gear lockout installed, but it's not done through the tow/haul button like the 2003 and newer trucks. I had it done at the dealer, covered by warranty, although they weren't supposed to do so. You won't be able to get this covered, since GM has now clarified that it is a non-warranty item in all cases. I'm not sure if I'll ever find the need to use it (haven't needed it yet), but since I was getting it for free, I wanted to have the option. Here's a really out of focus pic of the install on my 2001 crew cab.