Exhaust Brake not holding

[Rburian]

Steve, gear lowers to #2, 2005 400hp ISL

[Twilightzone]

>"When the throttle is closed, as it must be for the exhaust brake to operate, there is no boost with an exhaust brake."

>"This cannot happen with a closed throttle."

There is no throttle in a diesel.

[mackwrench]

Originally Posted by Rburian

I agree with everyone's responses, but what would cause the rpm,s to not increase when the gears are reducing.

RPM is relative to road speed, as coach slows down, rpms decrease till transmission pcm drops to lower gear, coach slows more, trans downshift and cycle starts over.... exhaust brake on or off don't matter

..... exhaust brake like a garden hose, I can put my thumb over hose and stop/restrict flow but pressure builds to what ever hose psi is.....

The exhaust brake has a "blade" that like your thumb, stops the flow of exhaust, this creates back pressure relative to piston cylinder pressure on strokes of engine that causes engine to slow rapidly....and as that rpm gets to a programed number, the transmission will down shift....

Any this help?

[mackwrench]

Originally Posted by Twilightzone

>"When the throttle is closed, as it must be for the exhaust brake to operate, there is no boost with an exhaust brake."

>"This cannot happen with a closed throttle."

There is no throttle in a diesel.

Not to get overly Technical, but modern engines have a TPS, Throttle position sensor, now I get where your coming from but I think the thought is the foot feed is now the "Throttle"..... Lol

[Rburian]

Could the ECM be calibrated to increase the RPMs at lower gears ?

[Steve Ownby]

Originally Posted by Rburian

I agree with everyone's responses, but what would cause the rpm,s to not increase when the gears are reducing.

They must increase. I believe you have an Aladdin system so you should be watching that screen as it's much more accurate than the dash gauges.

Here is a typical exhaust brake scenario. You are traveling at approx 60 mph in 6th gear and start down a fairly long 5-6% grade. You have the exhaust brake switch on so you lift your throttle foot. The brake activate and selects 2nd gear. At 60 you will probably see a downshift to 5th. 5th gear at under 2000 rpm won't generate much braking power so if the grade is steep enough and your speed starts creeping up, you may have to stab your service brake for 5 sec or so which will drop you speed & rpm enough to get into 4th gear. At this point, with rpm up over 2000 in 4th gear and speed around 45-48 your exhaust brake should be slowly dropping your speed. If you are decreasing speed too fast, you can toggle the brake off & then back on when your speed increase a few mph. Remember, just because the Allison is showing 2nd gear selected, it will only downshift when road speed is low enough to downshift a gear without over revving the engine. I'm guessing that the ISL will allow closed throttle rpm of 24-2500 before an up shift. That also means that your exhaust brake will be most effective up at 22-2400 rpm.

[mackwrench]

Originally Posted by Rburian

Could the ECM be calibrated to increase the RPMs at lower gears ?

Perhaps, but you also have a PCM, powertrain control module, more than likely that's where the shift points are, but I don't think it could be this big a deal....plus fuel mileage might suffer, possible engine over speed.

Cummins and Allison put a lot of engineering in this, pacbrak is kinda a add on component that "aids" in braking.
I'm not one to mess around with programmers, shift kits and most performance parts...but to each his own, I can respect it....

[Twilightzone]

>"... but modern engines have a TPS."

Probably a better term is 'Accelerator Pedal Position Sensor'. (APPS)
Modern computer-injected Diesel engines... that's about the only input you've got!

[Rburian]

I have not tried shifting to First Gear, to see if RPM,S increase, is it ok to go use first gear on Streep grades,

[RKins]

Originally Posted by Twilightzone

>"... but modern engines have a TPS."

Probably a better term is 'Accelerator Pedal Position Sensor'. (APPS)
Modern computer-injected Diesel engines... that's about the only input you've got!

My old C8.3 mechanical has a TPS that I just had to replace - codes said it was bad. Pricey little pups.

[ladagobago]

When in Florida you won't find any hills Unless you go up to Tampa where they have the Florida Mountains (very high overpasses). When I travel in PA hilly areas I have to use 1st to help hold back the MH on very steep back roads.

I can't imagine the engine NEVER increases in rpm when downshifting unless the pac is not closing completely. I mean that when that suck closes it creates a lot of back pressure.

[Old Scout]

OK "dumb" question here but was kinda thinking the ISL[400] had a true compression brake [Jake] while the ISC [350] had the pac-brake???? Was the pac also an option on the ISL?????

[Steve Ownby]

Originally Posted by Old Scout

OK "dumb" question here but was kinda thinking the ISL[400] had a true compression brake [Jake] while the ISC [350] had the pac-brake???? Was the pac also an option

The ISL could be equipped with either. Some coach builders used the Jake vs exhaust brake as a feature difference between models.

[Rburian]

Some ISL 400 have PAC while others Jake, depends on the Rv Brand and model.