Max speed you can drive with exhaust brake "on"?

[bpu699]

2000 winnebago ultimate advantage, cat 3126, allison 3000.

I usually only have the exhaust brake on if going down hill, and below 55mph.

Even then, when it downshifts, the motor obviously zings to redline when braking...

I know some folks leave the exhaust brake on all the time. But if you are travelling 70mph, and have it on, then what happens? I would think you would zing waaaaay past redline...

Now, I believe that during braking the motor can in fact go past redline without damage, as the redline for braking is much higher than the redline for driving...

But still, wouldnt that risk damaging your transmission and/or motor?

What am I missing here?

[wolfe10]

Your Allison 3000 transmission should NOT be programmed to allow engine overspeed EVER.


Said another way, the transmission should only downshift when that lower gear will not overspeed the engine.


What is the highest RPM are you seeing?


While I don't leave the exhaust brake on all the time (eliminates my favorite "gear"-- COASTING and causes way too many up and down shifts), doing so should not overspeed the engine.

[bpu699]

Originally Posted by wolfe10

Your Allison 3000 transmission should NOT be programmed to allow engine overspeed EVER.


Said another way, the transmission should only downshift when that lower gear will not overspeed the engine.


What is the highest RPM are you seeing?


While I don't leave the exhaust brake on all the time (eliminates my favorite "gear"-- COASTING and causes way too many up and down shifts), doing so should not overspeed the engine.

From memory, at 55mph its about 1700 rpm, and then goes to 2200-2400 or so I believe, when it downshifts...

So if it were applied at 70mph, what happens? The Allison doesnt downshift?

[MooseCountry]

It won't shift down with the RPMs too high by design with the Allison.

Last year I drove a fully loaded 26 foot Penske truck from the East Coast to Wyoming through the Rockies with the Allison with the exhaust brake always on. You still have to start out going down slow on grades in a lower gear. The truck had a 70 MPH speed limit governor, but on the steeper mountain grades it could exceed 70 since it would rev too high to downshift even with the exhaust brake on. Got it going to 76 MPH out East in North Carolina on an Interstate grade. Had to use the air brakes to slow it back down to below 65 and it then shifted down a gear. I never allowed it to do that again.

We tow with a Aisin transmission with an exhaust brake and reside off of a 21.4 mile continous grade of up to 10% dropping 5,800 feet in elevation. The speed limit is 50 MPH, but RVs fly pass going much faster with smoking brakes. Go slower than 70. We have fatalities, but it's far too rural to make the news.

Jackson, Wyoming RV crash news...

"I think by the time they realized they should’ve been in a lower gear, they weren’t able to [downshift] because of their RPM range,” Brackin (Lt. Matt Brackin of the Highway Patrol) said. “If they had pulled out of any gear to try to downshift, there’s no way to get it into a lower gear without blowing up the transmission.”

https://www.jhnewsandguide.com/news/...2ce4d7933.html

[bpu699]

Originally Posted by MooseCountry

It won't shift down with the RPMs too high by design with the Allison.

Last year I drove a fully loaded 26 foot Penske truck from the East Coast to Wyoming through the Rockies with the Allison with the exhaust brake always on. You still have to start out going down slow on grades in a lower gear. The truck had a 70 MPH speed limit governor, but on the steeper mountain grades it could exceed 70 since it would rev too high to downshift even with the exhaust brake on. Got it going to 76 MPH out East in North Carolina on an Interstate grade. Had to use the air brakes to slow it back down to below 65 and it then shifted down a gear. I never allowed it to do that again.

We tow with a Aisin transmission with an exhaust brake and reside off of a 21.4 mile continous grade of up to 10% dropping 5,800 feet in elevation. The speed limit is 50 MPH, but RVs fly pass going much faster with smoking brakes. Go slower than 70. We have fatalities, but it's far too rural to make the news.

Jackson, Wyoming RV crash news...

"I think by the time they realized they should’ve been in a lower gear, they weren’t able to [downshift] because of their RPM range,” Brackin (Lt. Matt Brackin of the Highway Patrol) said. “If they had pulled out of any gear to try to downshift, there’s no way to get it into a lower gear without blowing up the transmission.”

https://www.jhnewsandguide.com/news/...2ce4d7933.html

Wow, 10% grade...ouch

On a 6-7% grade, at 50mph, the exhaust brake keeps it from gaining speed downhill. So that's my preferred speed...

Some of the mountain roads going to the east coast, are VERY long, with no exits. No place to pull over... and no passing...

And the UPS/Fedex guys feel VERY comfortable doing 70+mph down hill...

[Skip426]

Ok similar engine/trans set up .

Governed RPM 2500 .max RPM 2640 and once when I was slow to get to the brake on Cabbage Hill ( I 84 OR ) the transmission program , upshifted for engine protection.
Then I really had to get on the brakes to get down to my control speed .45 MPH on that hill.
Exhaust brake , shift program , for most gears ; RPM between 2000 and 2500 no shifting , below 2000 downshift to next lower gear .
On steep downhills I get to 45 MPH and manually select 4th at the summit , exhaust brake on and drive by the tach down the incline RPM at 2500 brake applied down to 2000 then coast back to 2500 to allow brake cooling .

My Freightliner chassis turns on the brake lights when the exhaust brake is active , so I have the exhaust brake off when on cruise to avoid interruption on short downhills .

[d23haynes57]

For that era the exhaust brake has some programming options, and it can operate differently with the cruise control in standby. I had mine set so that with the Cruise switch on, even not active, the exhaust brake would not turn until the brake pedal was activated. For normal driving conditions I rarely used the exhaust brake. You will never normally wear the brakes out but over the years they will need service more from non use. Anyway, the exhaust brake is also a wear item. The butterfly valve can wear and burn out and that cylinder can fail. I had the rear pin mount break causing the brake to slam closed.

For engine speed the engine is governed at 2,500 rpm for power and for braking 2,700 rpm. At high speed when the exhaust brake is activated it will wait for the speed to be at a safe level before it downshifts. It also keeps the torque converter locked up.

One of reason that I do not leave the exhaust brake in auto all the time is that I like to lift my foot and let it coast when possible. Stepping on the throttle you are making a fuel purchase. Any form of braking, you are throwing that purchase away. Even with the Cat Soft Cruise I also found that using cruise used more fuel than using a smart foot on the pedal.

[Dutch Star Don]

As noted, the Allison WILL NOT allow the engine to overrev. Here's another way to look at it. You're driving down a grade with the exhaust brake on at let's say 45 mph, and the transmission has downshifted to 4th. If that grade gets steeper and the engine starts to rev higher, unless you hit the brakes, the transmission will actually upshift to fifth to protect the engine.

So......you won't overrev the engine, but you will see rpm numbers you can't reach while driving on flat ground.

With the exhaust brake, you can either stab the brakes to get it under control or start down the grade at a slower speed.

We drive out west with some of the biggest grades in the country. Our 2005 Monaco Diplomat had a 400ISL and an exhaust brake. We were towing a 5K truck. If I started down one of the larger grades at 55mph, I was soon forced to stab the brakes as the exhaust brake is just not that effective. However, if I started down that same grade at 45mph, I had much more control over the coach and very little use of the foot brakes.

The problem with the above, you have truckers with gears and engine brakes running up your backside at 45 mph. You just have to ignore that and do what is safe for you.

I changed my Diplomat exhaust brake out to the more powerful PacBrake II, I think it was called, and found that it did nothing at higher speeds, but was more effective at slower speeds compared to the OEM model. Not much help.

[d23haynes57]

You can only do so much braking by compressing air. Still better than a gas rig where you can only pull vacuum.

[dons2346]

"But if you are travelling 70mph, and have it on, then what happens? I would think you would zing waaaaay past redline..."
You might go past redline but the transmission will shift up to the next higher gear. The transmission is smarter than you think it is and will protect itself.
I leave my brake on all the time and have done so for over 200,000 miles without damage to the motor from over revving.

[bpu699]

Thanks all... I guess I need to be brave and turn it on over 70mph...

[dons2346]

When the brake activates, what gear is it trying to seek? My 3126 was programed to seek 2nd and my ISC is programed for 4th. You might want to get programed to seek 4th

[DarkSideJeep]

Originally Posted by bpu699

2000 winnebago ultimate advantage, cat 3126, allison 3000.

I usually only have the exhaust brake on if going down hill, and below 55mph.

Even then, when it downshifts, the motor obviously zings to redline when braking...

I know some folks leave the exhaust brake on all the time. But if you are travelling 70mph, and have it on, then what happens? I would think you would zing waaaaay past redline...

Now, I believe that during braking the motor can in fact go past redline without damage, as the redline for braking is much higher than the redline for driving...

But still, wouldnt that risk damaging your transmission and/or motor?

What am I missing here?

My Aspire also has the Allison 3000 transmission but has a Cumins 8.9L.

The engine brake system in your coach must be different than on modern coaches. On mine I run with the engine brake on most of the time. It only engages when I lift off the throttle. When I am in cruise it will engage when the speed exceeds the set speed by about 7mph. If I want to maintain 55mph going downhill I drop the set speed to 48mph.

[wolfe10]

Originally Posted by dons2346

"But if you are travelling 70mph, and have it on, then what happens? I would think you would zing waaaaay past redline..."
You might go past redline but the transmission will shift up to the next higher gear. The transmission is smarter than you think it is and will protect itself.

Let me state that a little differently: If you engage the exhaust brake at 70 MPH, and downshifting to 5th would overspeed the engine, the Allison would NOT downshift.


Note-- not a big difference in effective gear ratios between 6th and 5th.


I can tell you on my Caterpillar 3126, 70 MPH was right at the 6/5 downshift point.


As others have mentioned, max governed speed is lower than max no load engine speed.