Cummins 400ISL with Allison Jake Brake issues

[Jtmoney714]

Correct.  When I activate the engine brake switch at the drivers seat the trans starts it down shift program.  IE its trying to head to 2nd gear.  I had a buddy drive the coach and activate the switch.  I manually jumped the solenoids while in the engine bay with the hatch open and the engine brakes activate and you can feel and hear the coach slowing.  I disconnected it before he hit the gas peddle and everything seemed to work as normal.  Trans went back to 6th gear selected and operated normally.  This is why I think I could I could jump around this for the time being.

I’d say that it wouldn’t hurt it. I’d almost rather have a momentary switch I could press to trigger when I want. I’d just make sure there isn’t any goofy timing or tune thing that happens. 

 

[dunefinkster]

I thought about the potentiometer maybe failing but I had ruled it out because the smallest press of the gas pedal and the transmission will go back to the current gear. Off the gas and it will go back to selecting 2nd and start its downshifting again.

I will take a look at it closer now that you have brought it up.  Maybe I can find a diagram on how to ohm it out.

As stated earlier  the output to Jake solenoid is from the ECM. If you are not getting  the output you will need to verify output with a scanner/computer. It is possible one or more parameters are not met to engage Jake. Also the Trans must engage lockup to allow engine braking. If the RPM fall below around 1000 it will not engage.

 

[L.R.S.]

My Dutch Star with the ISL and now my Mountain Aire with the ISX both have 'real' Jake brakes with low/medium/high switches.  You can barely hear it.  I assume they go 2/4/6 cylinders for low/medium/high but never looked into it much.  I do know that they work really, really, REALLY well.  

You are correct about the "2" meaning the Allison is being told to go to 2nd, when RPMs allow it.  That way it'll downshift as aggressively as possible when the Jake brake is on, bringing the coach's speed down as quickly as possible.  I don't think I'd want to change it to 4th...

-TJ

Out of curiosity, and extreme boredom, I did some more reading on iRV2.com and found a couple threads with guys that had the dealership/maintenance shop switch the Allison PAC brake to 4 and they hated it and eventually switched it back to 2.