2004 Volvo having cruise and engine brake issues

Check the throttle position sensor. I had an International that did something similar. Ended up showing a .02% throttle which was just enough to keep the cruise and jake from working right. I found that if I put my toe under the throttle and pulled back on it, everything worked right until the next time I hit the gas. Once the throttle position sensor was replaced everything works fine.

 

It's actually called the Accelerator Pedal Position Sensor, which could be bad, or the Accelerator Pedal itself, which starts getting play in it.

 

Does the truck have any fault codes like for epg or something else, if not then i would start at you're clutch pedal switch and make sure that's not causing any issues try to hold the clutch pedal up to since you have same switch like on the brake pedal if not then two options either your epg is going bad or the cylinder behind the turbo is acting up most likely i would bet money its the valve under the starter the issue is that there is really no way to check it out and make sure without volvo software that only shops usually have.

 

I had a freightliner do that
the brake light switch does TWO things..(2 circuits) 1st it turns on the brake lights and 2nd and more to the point it tells the ECM you hit the brakes.... a bad wire on circuit #2 and the ECM my think you are tapping the brakes.... ie jakes cutting on and off/ no CC..
on my freightliner it was a bad pin on the plug going to ECM

 

the epg is the the valve that control the first position of the engine brake it is also used for a warm and hold function to help the truck warm up there is also more uses for it but not related to the problem you are having it is located right under the starter it has one plug going to it and it has a 1/4 air line for supply and there is another line going out under the starter and toward the back of the engine. should be about $500-$600 at volvo for a new one depends on how good you can get the pricing