Jakes engaging lightly in low gears

Hey Guys and Merry Christmas!

I got a 1999 Freightshaker Century 12.7 S60.

Recent work includes:
new injectors and injector harness, new ECM

However, I had an ECM update recently to remove a "Reward for Mileage efficiency" software update. Now I can go 75 mph all the time.

Problem is: the jake brake comes on lightly when starting out from a stop in 3rd gear. Any ideas what could be wrong? Overhead maybe?

Thank you for any help you can give. Have a great Christmas and New Year.

 

Will do, that sounds like a good starting spot. Thank you.

 

Yes standard trans, and started right after "software update", NOT after new injector harness, injectors and ECM. I do have a bad hole in muffler by corner of sleeper driverside. The jakes sound like they come on for 1 second but not enough to pull me down in speed, just a jake noise. Fuel economy has not changed. Jake switch is off totally. Thank you BRS for the help.

 

OK now that you mention it, I have noticed a "movement/vibration" at my feet on the floor. Thought it was a worn shakle, but I will check the motor mounts tomorrow too. You may be right that the noise is the rusted muffler moving sounds like the jakes. Thanks for the ideas, gonna check it out tomorrow.

 

OK great insight, I will look for a white puff of exhaust next time I hear it. I'm gonna pull the stack/muffler also to make sure the rusted out grinding isn't the "pseudo-jake sound". I appreciate the advice.