P0840 Code

[Ne-max]

2005 LLY. Has transgo kit.

Trying to figure out this p0840 code. Truck does not throw it when its empty and drives great. But once gooseneck is hooked to it, it throws the code and has shifting issues in the lower gears. Once it's in 4th and 5th it's fine.
It did this a couple months ago. I cleaned main plug and put electric grease on it. Fixed the issue tell now. Replaced internal harness and the pressure switch inside. Did not help.

 

[Ne-max]

Just got the truck to do it without a trailer. Did not do it tell trans got to 180 degree and was aggresive with it. Now it does it non stop.

P0840-Transmission pressure switch solenoid d. Is what scanner says

 

[Mike L.]

Start checking grounds as this looks like an electrical problem. Could be at the TCM wires 1 through 5.

 

[Ne-max]

Start checking grounds as this looks like an electrical problem. Could be at the TCM wires 1 through 5.

Will do. Thanks sr.

 

[Ne-max]

Just a update on this. Have had 3 guys pm me on this issue. Learned the truck had the Transgo A1 shift valves. Put the stockers back in a it fixed the issue. Also seems to run cooler. Reason it did it towing was trans ran hotter.

 

[Mike L.]

Allisons are easy till they don't work. Now you have to earn your money.
Assume everything is wrong till you prove it right. Measure everything no matter what it is.
We do this on every build.