code 1781

[chessy56]

2001 2500HD, LB7 w/291,000 miles
The last couple of times the truck has gotten "cold" (low teens), I noticed that the tranny would not shift out of 5th to OD while on the highway. After running the highway for several miles, the tranny finally shifted into OD. Later, after checking for DTC codes, I came across this P1781. All I've found so far talks about an improper voltage @ the TCM. If the temperature is up a bit in the morning, it will shift into OD w/o problems. Battery voltages are good.

No MIL lamp appears. Any thoughts?

[rapidoxidationman]

I think all you have in the '01 allison is 5 speeds. What you might think of as "overdrive" is just the torque converter locking up, which it won't do until a temperature sensor is satisfied that things are warm enough. Someone else can chime in on where the sensor is, but if it is faulty then maybe that's the problem.
My old '03 and my present '05 are both 5 speed allisons and will not lock up the TQ until the engine temp gets up, just like it said somewhere in the owner's manual. Takes about 5-10 miles of highway speeds, depending on ambient temperature.

[chessy56]

Well, it's gotten a buch colder around SE Wisconsin than before and I'm not seeing this DTC set again. What I did notice, was a somewhat sluggish cranking speed when it got cold. I found a poor electrical connection on the negative terminal @ my second battery. I put a new terminal bolt on and tightened it down good- cranks without problems again, even in the coldest temperatures and the DTC hasn't returned. Looks like I might have found a potential problem!

[More Power]

I think all you have in the '01 allison is 5 speeds. What you might think of as "overdrive" is just the torque converter locking up, which it won't do until a temperature sensor is satisfied that things are warm enough. Someone else can chime in on where the sensor is, but if it is faulty then maybe that's the problem.
My old '03 and my present '05 are both 5 speed allisons and will not lock up the TQ until the engine temp gets up, just like it said somewhere in the owner's manual. Takes about 5-10 miles of highway speeds, depending on ambient temperature.

What chessy56 reported sounds normal. In my trucks, the torque converter won't lock till the transmission temperature gauge begins indicating temp... An unlocked TC allows the trans to build heat a little faster. Still, it takes longer the colder it is outside. The "winter front" can help to speed that process when not towing. Don't use a winter front when towing...