Exhaust or jake brake or neither?
How do I know if I have a jake brake, an exhaust brake or is it possible that we don't have either of these? We have a 2001 holiday rambler imperial with an Allison 3000 transmission. Am extremely...
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The Allison ECU with a MD3000 will activate the exhaust brake automatically when the following conditions are met;
Torque converter in lockup,
Throttle pedal released,
Dash mounted exhaust brake switch is on,
Cruise control not active,
ABS not active.
The exhaust brake works independently from the foot brake, but some vehicle manufactures wire a switch into the system requiring the brakes to be applied before the exhaust brake operates. Others will fit a foot operated switch that has to be pressed.
When the ECU activates the exhaust brake, it also switches to a secondary shift schedule that downshifts the transmission earlier. This is done to keep the engine revs higher to increase the exhaust brake performance. It is also possible to force downshifts even quicker by programming the Exhaust Brake Preselect Parameter to a low gear, eg 2nd. The options for this parameter are gears 2 through 6. When the exhaust brake is activated the display will show the preselect gear that is programmed in the ECU. The display will only change if a lower gear than the normal top gear is programmed.
Selecting 2 can be a bit brutal as the transmission will try to downshift to 2nd gear as soon as possible. 4th is often a better choice. At low engine speeds in a high gear the transmission may drop two gears when the exhaust brake activates.
As previously advised it is often suggested to go down a hill in the same gear and engine revs that you would go up it. If you would go up a particular hill at 1800rpm in 3rd, then it is probably a good idea to manually select 3rd and use the exhaust brake to hold the engine speed at about 1800rpm going down. This will depend on how effective your exhaust brake is.
You should always manually select the gear when going down a hill, don't just leave the selector in D. The transmission is likely to upshift, instead of downshifting to keep the engine revs high for maximum exhaust brake retardation.
Torque converter in lockup,
Throttle pedal released,
Dash mounted exhaust brake switch is on,
Cruise control not active,
ABS not active.
The exhaust brake works independently from the foot brake, but some vehicle manufactures wire a switch into the system requiring the brakes to be applied before the exhaust brake operates. Others will fit a foot operated switch that has to be pressed.
When the ECU activates the exhaust brake, it also switches to a secondary shift schedule that downshifts the transmission earlier. This is done to keep the engine revs higher to increase the exhaust brake performance. It is also possible to force downshifts even quicker by programming the Exhaust Brake Preselect Parameter to a low gear, eg 2nd. The options for this parameter are gears 2 through 6. When the exhaust brake is activated the display will show the preselect gear that is programmed in the ECU. The display will only change if a lower gear than the normal top gear is programmed.
Selecting 2 can be a bit brutal as the transmission will try to downshift to 2nd gear as soon as possible. 4th is often a better choice. At low engine speeds in a high gear the transmission may drop two gears when the exhaust brake activates.
As previously advised it is often suggested to go down a hill in the same gear and engine revs that you would go up it. If you would go up a particular hill at 1800rpm in 3rd, then it is probably a good idea to manually select 3rd and use the exhaust brake to hold the engine speed at about 1800rpm going down. This will depend on how effective your exhaust brake is.
You should always manually select the gear when going down a hill, don't just leave the selector in D. The transmission is likely to upshift, instead of downshifting to keep the engine revs high for maximum exhaust brake retardation.
