My 2003 C3500 spent 5 days at the dealer for 3 trans problems. Intermittant U2106 for the last year or so no O/D lockout, and a 500rpm flare up when downshifting in Tow/Haul. TCM was changed for the U2106. They tell me O/D lockout works but I get no lite on dash or rpm increase when pushing button twice. The 500 rpm flare up happens in Tow/Haul, when you let up off the pedal,trucks slows, then the rpm goes up 500 or so then it downshifts. It does not always do it. If you hit the brakes it usually will do it also. They tell me this is normal, but I don't recall it doing this before the OBD2 reflash they did in August. Is this normal?

The O/D lockout is not enabled by pushing the button twice. It is enabled by holding down the button for 2-3 seconds.

Thanks Max, It works fine.I so very seldom use it, I forgot. How's bout the flareup? I don't remember it doing that before.

I think the 500 rpm flare up is to closer match the downshift rpm (the amount the engine has to come up to compensate) vs. abruptly clunking into the next lower gear. think of a manual trans and dumping the clutch from 3rd to 2nd you could lock the rears for a short time.http://www.dieselplace.com/forum/smileys/smiley23.gif