Exhaust Brake Gear Selection

[russleonafowler@peoplepc.com]

I have a 1998 American Dream with a Cummins 325. When I activate the exhaust brake it shifts down to 4th gear if my speed is below 70 mph. I am uncomfortable with that high RPMs. Can I have the transmission reprogramed to allow me to make the gear selection? I would like to be able to manually select the gear I wish.

[wolfe10]

Welcome to the FMCA Forum.

Let's clarify exactly what is happening. Your Allison ECU is programmed with a 4th gear PRE-SELECT for the exhaust brake.

That does not mean that it down-shifts to 4th at 70 MPH when you turn on the exhaust brake and close the throttle. What it means is that is down-shifts TOWARD 4th gear, but with the caveat that it will not down-shift to a lower gear until it is safe (from and engine RPM standpoint) to do so. Compare your max RPM from your tach with your engine's no load maximum permissible RPM-- you should be fine. Maximum permissible RPM is NOT the same as governed speed, as your engine maximum permissible RPM under no load will be higher than under load.

But, "for the rest of the story", yes any Allison dealer can reprogram your ECU for whatever "exhaust brake pre-select gear" you choose. Cost should be around $100.

Higher pre-select gears (like 5th) give a lot of driver control, but require more driver input (use of down arrow) when coming to a stop or on steep descents.

A lower pre-select gear (like 2nd) is great for coming to a stop, but often "over-brakes" in regular driving.

FYI, ours is programmed for 5th-- just my personal choice.

Brett

[Guest Wayne77590]

Today I hit the Mountain Springs, CA 6% grade, up then down. I've already posted on the "up."

My Allison 2500 MH is programmed, apparently I found this out today, for 4th gear. I am totally satisfied with the performance of the Exhaust Brake on the decent today. It kept me at what ever speed I wanted to travel down. sometimes I choose 62 MPH, let off the accelerator pedal and it would hold. Sometimes I slowed it to 55 and it would hold. I think I'll leave mine where it is. 12 miles of down grade sort of said it was set right.

[russleonafowler@peoplepc.com]

Thanks for the comments. I will check with an Allison dealer to see if I can get reprogrammed to 5th. I would be more comfortable with the lower RPMs as I have heard about hydraulic pump problems with higher RPMs. I have no problem with more driver input shifting down further.