HM400-1 won’t shift out of low gear

[ETMF 58 White]

My Komatsu HM400-1 has decided to not shift out of low gear either forward or reverse. it was working fine until it wasn’t, so I am suspecting a wiring or controller issue. I checked the dump bed raise signal and it showing the correct voltage in and correct signal voltage out at the switch, anyway. I did not check voltage all the way to the controller because I’m not sure I know how to do that.

The truck does have a front brake solenoid disconnection code that the local dealer suggested that I check out. (I am sorry I didnt write that code down, but I can post it after I go back to the job and look). The dealer did not tell me how to check it or even where that connection would be. I do not have a service manual. Perhaps I need to tilt the cab, because when I look at the front brake lines all I see are the steel lines and no electrical wiring. So I suppose the brake solenoid must be somewhere up in the innards of the truck.

Any suggestions would be appreciated. Hauling dirt in low gear is not a viable plan for a haul distance of more than a few hundred feet. Serial number is 1064 and the truck has 17,000 hours and runs fine and dumps and steers and brakes fine.

 

[coalrulz]

In my experience with Komatsu HM trucks, 3 most likely culprits to check if truck will not shift up from low gear:
- Dump body not seated
- Retarder not fully released, ensure lever is pushed all the way up
- Mud under brake pedal, most common

 

[Nige]

Not sure about a Komatsu but if it was a Cat truck I would add the transmission output speed sensor to that list. If the truck can't tell how fast it's travelling it won't upshift past converter drive in 1st speed forward (the lockup clutch won't engage in other words) although it will select reverse.