Engine/exhaust brake question

Im guessing your shifter looks like this?? If so you push the down arrow to limit the highest gear the truck will shift into.

 

Sounds like the issue he’s having is he’s using an auto and it’s automatically upshifting when he wants to keep it in gear while he’s going downhill. And he doesn’t have any Engine retarders at all.

OP I would install one better to have it and not need it than otherwise. Plus it saves a lot of brake wear even though brakes are cheap the labor isn’t if you don’t do your own work. Also tip for mountain driving if you don’t have retarders is to start slow and keep slow a lot easier to stop a truck going 25-35mph than a truck going 70 by the time you feel the fade you’ll likely have enough heat to spare to stop the 35mph truck while the 70mph truck will probably lose most its braking when it drops around 40mph if he waited too long.

 

2700 is screaming.... Even the newer motors only claim to be able to sustain 2400rpm, 2400 without applying throttle, in an engine braking situation. "sustain" meaning to not void the warranty.

Is that a Cummins or Caterpillar in the fl70? I think you are playing with fire taking it that high. Even the new motors with auto transmissions will go past "redline" on a downgrade, if you let them. They still aren't smart enough to let you not worry about it.

 

I don't know which specific model of allison you have mated to that ISC engine, but in any case, the transmission manual for the MT653 (That's a common allison model for that year) states that the transmissión will only hold the gear until the engines reach its governed speed, then the transmission will upshift, so there is nothing you can do, appart from use the service brake to mantain the engine bellow the governed speed.

Only one question. ¿Is your allison equiped with hydraulic retarder? If so, you must have the control in the dashboard for the retarder and can use it, or it may have blended pedal switch which works when the foot brake is applied. The dashboard switch may look similar to this one:

 

I am trying hard not to pull out stories of derring do and 3grand 100+ mph runs of eons past.

But looking at your truck with a cold eye, I would not want to drive it. It's NOT a attack or dig or anything like that. I must have a tractor or truck that can mind my thinking in terms of where the engine needs to be at downgrade.

It to me was pounded into my head as a very important lesson. Don't do it just right, you are going to be dead or someone will be or worst. Now this is not really a lesson on you all here familiar with that engine and braking. Just try to add just a few pounds of air and keep it there just so. What you are doing is trading a very slow rise in thermal. IF you do this gently enough you will not get too hot due to the airflow in there unless you got hung with all sorts of pretty areodynamic coverups...

Just a thought.

I don't know too many desiels today that will accept 2700 with several VERY old exceptions from long ago. One basic reason would be the mains inside the crankshaft where the shaft rests on very precise strips of metal with oil in between. IF the RPMs are too high the Shaft cannot stay balanced and start eating into the mains.

Sooner or later you have no engine.