Exhaust brake

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BigJohn

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St Charles, MO
When my exhaust brake comes on my tranny indicates 2 & 4. I assume I am in second gear and I could upshift to fourth. If I wanted to hold in second can I use the arrows and lower the top gear to third and then second?
Thanks. John
 

Ned

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The 2 indicates the preselected lowest gear and 4 is the actual gear that the transmission is in.  It won't go any lower until the speed drops enough to let it do so safely.  You can't even use the shift buttons to downshift if it would result in an unsafe engine speed.
 

BigJohn

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St Charles, MO
If the speed builds up then the tranny shifts to fifth? I assume the only way to hold speed down is to use the  service brakes? And use the service brakes to drop it to third? This seems to defeat the purpose of the exhaust brake.
Thanks John
 

taoshum

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I don't know if the exhaust is directly involved... mainly tranny and engine drag?

Anyway, I assume that if the rig gets going too fast  for the current gear, it will shift to a higher gear to save the tranny...or the engine... not me!  If your brakes fail going down a steep incline, the engine brake may not save you.  I guess we could get some retro-rockets and light those babies in an emergency... 8)
 

Jim Godward

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BigJohn said:
If the speed builds up then the tranny shifts to fifth? I assume the only way to hold speed down is to use the  service brakes? And use the service brakes to drop it to third? This seems to defeat the purpose of the exhaust brake.
Thanks John

John,

If the speed starts building in a gear, hit the brakes long enough to down shift to the next gear.  Do not wait for the up shift.  Or you can hit the brakes for a few seconds to drop the speed 5 or so MPH and do this every minute or so.  If the speed still wants to build and you have been hitting the brakes more and more, do it one more time for the down shift.

Stay ahead of the vehicle and all well be well.  :)
 

Jim Dick

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If your transmission is an Allison, it is smart enough to upshift if the engine starts to over rev. Hitting the service brakes to bring down the speed will allow the transmission to stay in the gear you require. The two work in conjunction with each other to provide a safe braking situation without overtaxing either the transmission or engine.
 

Wizard46

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Ever wonder why they have those runaway ramps on long downhill grades, there's always a chance something could go wrong. Always err on the side of safety.

I try to start down the hill in the gear I climbed the hill in. That way a little service brake can usually get me back to the speed where I need to be.

Jerry
 

Gary RV_Wizard

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If the speed builds up then the tranny shifts to fifth? I assume the only way to hold speed down is to use the  service brakes? And use the service brakes to drop it to third? This seems to defeat the purpose of the exhaust brake.

Not defeating it - just complimenting it. The exhaust brake may not be adequate to provide 100% of the braking you need in all situations, so use the service brake to  provide more when needed.

The exhaust brake logic in your coach engages 2nd gear when the brake activates. As the others have explained, the transmission will prevent over-revving of the engine, so it doesn't always go immediately to the target gear. If the grade is very steep, the exhaust brake may not be slowing the vehicle enough to allow further downshifting, so you can give it some help with the service brakes. Once you get the speed down enough to allow a downshift, the exhaust brake provides additional "drag" and is probably adequate to hold it there, but sometimes on a steep downgrade you might need an occasional further assist from the service brake.  That's still a lot less service brake use than without any exhaust brake at all.

Exhaust brakes vary in the amount of drag they can provide. If you want to use less service braking, consider upgrading the exhaust brake system to a more capable model.
 
B

bucks2

Guest
The exhaust brake is designed to HELP stop or slow the rig. Anyone who totally relies on it to keep them at a safe speed down every hill is going to be disappointed. The exhaust brake works well if the driver uses the big rig and state approved method of starting down the hill in the same gear as would be required to go up it. (note that this is sometimes a guess as you don't always go up the same side of a hill as you are going down and the grades in each direction may be very different) Look at many summits and you'll find suggested speeds for various weight rigs to descend the hill at. Some suggest as low as 30 mph in order to keep control of heavy rigs.

Also take into consideration that the gearing of your rig determines the effect of the exhaust brake on the vehicle. The lower the gear, the more braking effect the exhaust brake has. For example you may descend a hill in 4th + exhaust brake @ 45mph and have to use the service brakes often to keep the speed in control. Or, you could descend in 3rd + exhaust brake @ 35 mph and never touch the service brakes. Watch the professional truckers and see how often the good drivers use their brakes. They slow down at the top of the hills and then glide down in complete control with no smell of overheated brakes. RV drivers would do well to observe how the good pro's do it. Also remember that there are some real jerk pro drivers out there too. Look for the really nice looking independants, those guys are paying for their own maintenance and rigs, they take care of them like you take care of your RV. There are also some company drivers that are excellent, but it's easier to spot the independants.

Ken
 

elkhornsun

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Dec 10, 2011
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There is an electronic governor that will keep the engine from reaching too high an RPM level. A diesel may be operating ordinarily at 2000 RPM and go to 3000 RPM with engine braking on a downgrade but with most engines there is no fuel being supplied at this point, only air. No need for combustion when the engine is serving as a brake.
 
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