Community Forums

Leibercoach · 06-15-2018, 10:27 AM

Hi, my husband and I have had several motorhomes all with less horsepower and always pulling either our boat or horse trailer and never had a problem. We can pull the hills up no problem but downhill the Retarder doesn’t hold at all and then it forces us to use our breaks and they get hot quick. Has anyone else had his problem and if so what have you done.

MSHappyCampers · 06-15-2018, 10:41 AM

Hi !

Welcome to IRV2! We're sure glad you joined the gang!

You need to get the "retarder", which I assume is either a jake brake or exhaust brake, fixes ASAP! You can also keep the transmission in 2nd gear going down grades.

Good luck, happy trails, and God bless!

theroc · 06-15-2018, 05:54 PM

Welcome to the forum.

Just curious, do you have a Discovery with a Cummins ISB or a Discovery LXE with a Cummins ISL? If an ISL, does it have an exhaust-brake or an actual compression brake (both are available on the ISL)? If a standard Discovery, then you should have an ISB with exhaust brake.

Cummins does use the term "retarding power" to describe the HP of the stopping power of their exhaust brake (go HERE --and even though Cummins uses a Jacobs-branded exhaust brake, it isn't a true "Jake brake") so don't feel bad using the term but usually, a "retarder" is used on the transmission to slow the vehicle down. I've driven commercial buses with transmission retarders and I've never liked them as the ones I've driven have all been hard to modulate for smooth operation which is important in operating buses.

No matter, it sounds as if it may not be doing the job the way it should and perhaps you should have it checked. In the meantime, lock in a lower gear on steep inclines and keep the RPMs up, because you do not want to over-heat the brakes. I've attached the chart from that Cummins page in the link above to show the obvious ...more RPMs = more braking HP.

.

Toribk · 06-15-2018, 07:52 PM

What engine do you have in your Discovery?

alank · 06-15-2018, 09:47 PM

The Discovery has a transmission retarder controlled by a switch just behind the transmission gear selector. With your foot off the gas and just a little brake use, it should be downshifting the transmission. If you don’t have a good enough feel to identify downshifting, look at the gear numbers to see if it is working. You don’t have an engine brake per say.

Toribk · 06-16-2018, 05:32 AM

The Cummins ISC 8.3L engine uses a VGT (Variable Geometry Turbo) and it is my understanding that both the turbo and the Allision 3000 transmission work in unison to provide supplemental braking.

Toribk · 06-16-2018, 05:42 AM

Quote:

Originally Posted by Toribk

The Cummins ISC 8.3L engine uses a VGT (Variable Geometry Turbo) and it is my understanding that both the turbo and the Allision 3000 transmission work in unison to provide supplemental braking.

Here is an excerpt I found from some Cummins literature I had stored on my "RV Stuff" thumbdrive for the ISC 8.3L engine:

"The VGT works in conjunction with the engine and transmission to provide an integrated*braking system*used to help slow the vehicle. This is commonly called exhaust braking. This braking power is achieved by modulating the turbo sliding nozzle ring to restrict the flow of exhaust gases from the engine, which in turn creates a high back pressure on the engine. The high back pressure creates a high level of resistance to the motion of the pistons within the engine and this resistance is used to reduce engine speed and thus vehicle speed".*

Leibercoach · 06-16-2018, 10:07 AM

Thank you all for the responses. It is a Discovery with the ISL 8.9. We have taken the rv into freight liner and they drove it down the freeway not under a load and said it was fine. We also took it into Cummins to try to have the gears reprogrammed but not much success. If you come up a large pass and start out very slow at the top before going down you can hold it in 2nd gear but only if you start out extremely slow...like 25mph. We are currently at lake Powell and driving here at 65 came up on a descent and could not get the coach slowed down at all and this thing is a run away train. Freight liner yesterday said that the braking system in the new rvs .... the retarder works with the turbo but say you have to use your brakes well as soon as you hit the brakes under a load you start smelling brakes. It just doesn’t seem right to us and it is very scary. In our previous rvs I hardly ever hit the brakes the engine brake always slowed us down in this thing it’s completely useless

Leibercoach · 06-16-2018, 10:09 AM

ISL 8.9

MikeO58 · 06-16-2018, 10:14 AM

I stab and release brake. Keep it slowed down. You are correct that retarder does not really hold back coach from picking up speed.

Toribk · 06-16-2018, 12:15 PM

Gear down manually on a crest to control your descent if you hadn't already before climbing.

Keeping the transmission geared down keeps the RPM's up on the ascent which aids in better cooling....and your ready to descent.

Takes a little practice if your use to driving the majority of the time on flat terrain.

Fiesta48 · 06-17-2018, 12:32 PM

I agree my '14 8.9 ISL with retarder brake does not brake enough on long steep slopes. Must keep RPMS up, over 2k to work better. And must still use service brakes. Just did several mountain passes. Need to be in 4th gear or lower. If you get speed down enough by hard service braking it will go into 3rd. Even lower speed into 2nd.
This retarder it not the 3 stage type like on larger heavier trucks with Jake brakes.
If you gain too much speed it can go up a gear to save the engine.

FIRE UP · 06-17-2018, 01:56 PM

Quote:

Originally Posted by Fiesta48

I agree my '14 8.9 ISL with retarder brake does not brake enough on long steep slopes. Must keep RPMS up, over 2k to work better. And must still use service brakes. Just did several mountain passes. Need to be in 4th gear or lower. If you get speed down enough by hard service braking it will go into 3rd. Even lower speed into 2nd.
This retarder it not the 3 stage type like on larger heavier trucks with Jake brakes.
If you gain too much speed it can go up a gear to save the engine.

Ok,
Just so I'm clear on this, are you saying that a 2014 Coach, with an 8.9L Cummins, DOES NOT HAVE A JAKE BRAKE? And that it in fact, has a TRANSMISSION RETARDER? I just want to make sure we're talking apples for apples and not apples for oranges here.
Scott

NevadaNick · 06-17-2018, 02:44 PM

380hp ISL has exhaust brake and 450hp has engine brake from what i could gather. In our Phaeton with 380hp the ex brake was marginal at best. You have to learn its capabilities and go with it. For me it was usaully in 4th gear and stab braking to keep it from upshiftingdependingbon the grade.