Volvo VNL 780 loss power and speedometer

Your Accelerator Pedal and/or your Pedal Position Sensor could be bad or going bad.

The symptoms you are describing are common when those parts are failing.

It could also be your Speed Sensor failed, in that case your Speedometer will not work and your truck will go into Limp Mode, but your truck should still accelerate.

 

Acceleration pedal sensor, speed sensor, engine break switch connected to the VECU. From VECU to ECM all signals transmits in digital mode by J1939. Terminating resistors sometime can create this problem.
How to check J1939 terminating resistors - Forums / Diesel and Chassis Electrical and Electronic Systems / Troubleshooting the J1939 Data Link with a Calculator - Truck & Diesel Mechanic Forum - ASE Medium/Heavy Duty Truck Forum

 

What's the code that's popping up when this is happening first of all please check u don't have a shorted Qualcomm cable for one maybe an old one. Second make sure the plug in the speed sensor it's making proper contact. I've never replaced one accelerator pedal in 10 years working of Volvo they almost never go bad

 

I guess you've been lucky, I had to replace mine about 5 years ago because it would intermittently cause my cruise control and engine brake not to work.

When that happened, the check engine light would come on until I pressed on the pedal again and the ECM received a signal again and it would start working normally again until the next time.

It eventually got so bad I finally replaced it and it has been fine since.

My friend's 200O Volvo Accelerator Pedal is starting to go out on him too, same symptoms that I had and his code says Accel Position Sensor.

It ECM doesn't set a hard code at first, but it will eventually.