Cummins 400HP 3000 trans downhill questions

[Ray-IN]

re: your RPM getting high. Cummins says the higher RPM when slowing is not harmful like it is under power.
The TCM and ECM "talk" to each other to ensure neither the engine nor Allison are damaged by over-revving. When RPM reach the maximum safe level the Alllison will upshift, regardless of your wishes, to protect the engine.
This leaves you with only the service brakes to regain speed control.

 

[A Traveler]

The maximum RPM for the ISL under power is 2,100. That is the RPM where the horsepower peaks.

However, the maximum approved RPM under braking is either 2,300 or 2,400 RPM. (I don’t have the Cummins ISL Manual at hand.)

 

[SargeW]

Here is an excerpt from my Chassis manual regarding the operation of the Allison MH3000 and Cummins ISL.

Driving Conditions 

Normal driving – best fuel economy o Select “D” and “Mode On”  Performance o “Mode Off” o For mountain driving, select lower gears to maintain 2000+ engine RPM

Hill climbing on hot days

Keep RPMs high to cool engine Driving Tips with the Allison 3000MH Transmission: The points at which shifts occur depend upon predetermined speeds and other operating conditions. A transmission “shift calibration” includes several sets of shift points used according to current or anticipated operating conditions, such as engine or transmission fluid temperature. You can change shift schedules using the MODE button. The transmission control module (TCM) includes the capacity for two separate and distinct shift calibrations, one for use in “Primary Mode” of operation and one in “Secondary Mode.” Primary – This shift schedule is typically used for all normal vehicle operations. Secondary – This is an alternate shift schedule that the TCM uses upon request. This is operator
controlled using the MODE button. When you are driving under normal road conditions, the DRIVE mode is recommended for the best performance and fuel economy.

The MODE switch should be set to ON for economy mode, but MODE off should be used when climbing hills and when extra performance is required. The display screen on the shift control pad will indicate the highest selected gear for the transmission. When mountainous or up-and-down terrain conditions are encountered, you should manually select a lower gear, preferably lower than 5th gear. This can be done at any road speed by pressing the down arrow repeatedly until the desired gear is indicated in the window of the shifter pad. When your road speed decreases to a safe point, the transmission will downshift at a higher RPM than normal. This will decrease the use of overdrive while pulling hills, which can result in excessive heat build-up in the transmission, and keeps the engine operating at peak horse power and performance.

When ascending a grade, maintain engine speed to within 400-500 RPM of governed engine speed. Governed speed will be 2200 RPM on the Cummins ISL engine model. Road speed may decrease, but the engine will be at its peak in the power curve. It is especially pertinent to monitor your water temperature gauge when climbing steep grades. Keep in mind that it is not uncommon for the temperature to increase, especially in hot weather. If the gauge reaches the end zone or if the temperature warning light on the gauge panel should come on, reduce your road speed, shift to the next lower gear and keep your tachometer within 500 RPM of engine governed speed. In many cases this will stabilize the water temperature. If the temperature gauge continues to rise, pull to the side of the road and shift the transmission into neutral. Bring the engine RPM to 1,700—2,000 RPM until the temperature drops down into the normal range. This should occur in a relatively short period of time. If the temperature gauge does not begin to drop, stays in the red zone, or continues to rise, shut down the engine and allow it to cool. After the engine is allowed to cool check the fluid level in the reservoir and add coolant if needed.

 

[unni]

33000 is the main deciding factor here. As you start gaining momentum, barakes takes longer to apply in slowing down and can end up heat glazed and will go down on its braking capability. Sice you have a Jake brake, the Allison shifts down to 2 by default. This can be programmed to change to 3 or 4. But leaving it at 2 is the best choice. As I mentioned before, 33000 is the deciding factor. On that tired drive, when you are a bit careless, default jake brake shifting down to 2 will give you peace of mind.
Saying that, it does over heat the engine on a hot day for a long drive downhill. You will have to learn to drive down. The best way to do that is by learning to use a combination of brakes, jake and downshifting. The following is the learning procedure. Keep a note of how hard you have to slam the brakes to get the vehicle under control (back to 50 mph) from 60-65. Do this once or twice along a straight, safe slop just to get yourself an understanding of your brake over your 33K. Always lower the gear to 5 and sometimes maybe upto 4 downhill with intermitent use of brakes. As you start applying the brakes and once the vicle starts slowing down, switch on the jake brakes. Now your vehicle wont rev up as much and you brakes wont take too much load to get it under 50(or 45). Once it hits the slow mark, switch off the jake and release the brake. Let the vehicle coast down a bit and continue the process. This may sound complex but can be learned as you drive. You are now preserving your engine, your tranny and your brakes and best part is safety. Since you are not always on brakes, brakes dont overheat and glaze. Since you are not always on jake, engine doesn't overheat. Since you are always on 5th gear, tranny will agree with you too.

Personal advice is never to trust automatic systems. They are built for your safety at the expense of your equipment. But Allisons are notorious for being selfish and will take down your engine if it has to preserve itself. Manual downshifting with intermitent use of brakes and jake is always best for your coach. Good luck.
By the by, I have had jakes brakes and exhaust brakes, have glazed my brake rotors, overheated my engine and allison tranny. Have survived most crazy situations and at times got stuck too. Never let myself slip on icy roads, as that can go sideways real fast.

 

[Gary RV_Wizard]

But Allisons are notorious for being selfish and will take down your engine if it has to preserve itself.

I think that's obsolete information, but it never hurts to be cautious.

Any modern transmission is computer controlled and works closely with the engine computer to manage rpms and torque. It won't ever over-rev and engine. In particular, an Allison 3000 or 4000 won't even allow you to manually select a gear that is potentially damaging to engine or tranny. If you put the selector in gear "2", the Allison merely takes that as advice from the driver, not an absolute command. It will do it only if the engine computer concurs.

Sice you have a Jake brake, the Allison shifts down to 2 by default.

This is NOT universally true - many engines with Jake brakes are programmed for 4th gear when the brake activates. It's purely up to the powertrain engineer who sets up the chassis.

 

[A Traveler]

There is so much incorrect information in Unni’s post (two above this one) that I can’t even begin to comment on It. I’d be here all day.

My best advice is to ignore that post.

 

[Larry N.]

33000 is the main deciding factor here.

I don't understand that statement. Where does that number come from and what does it mean? And I can tell you for a fact that the Beaver I used to have did, indeed, select 4th gear, not 2nd, when its engine brake was selected -- the factory setting.

 

[Ray-IN]

Gary is exactly right!
For instance, one time I reached to push the economy mode button without looking. I accidentally pushed the reverse button. The Allison 3000 just whined as it bypassed the valving; the engine continued at the same RPM without hesitation.
But the next rest area was an underwear change.

 

[unni]

I don't understand that statement. Where does that number come from and what does it mean? And I can tell you for a fact that the Beaver I used to have did, indeed, select 4th gear, not 2nd, when its engine brake was selected -- the factory setting.

"2005 Winnie Vectra with Cummins 400HP 3000 trans. 33,000 GVW." - Original post begining. Yes, your RV was programmed at factory. Allison's safety default is 2nd gear by engineering.

 

[unni]

There is so much incorrect information in Unni’s post (two above this one) that I can’t even begin to comment on It. I’d be here all day.

My best advice is to ignore that post.

Prove me wrong. No offense meant, I would like to know.

 

[unni]

I think that's obsolete information, but it never hurts to be cautious.

It is not obsolete. Allisons engineered default is 2. I am not going to the detailed engineering math but will try and explain without it. 1st gear is the ultimate stopping power, but it is one the lower side of gear/engine rpm ratio. So it is never used. 2nd gear is the first gear that is on the higher gear/engine rpm ration. So the ultimate stopping power that can be safely used is 2nd gear without damaging the engine or transmission. But in the event if the rig has to be saved from crashing at uncontrolled speed and if any has to be sacrificed, it is better the engine than the transmission. If the engine fails and transmission is still getting battery to TPU, you are still in the safety zone. If transmission fails, you are dependent purely on brakes and that will not save you if the RIG is heavy at uncontrolled speeds.
Usually transmissions are re-programmed to 4 by modern standards if there are 6 gears to be in a middle ground especially for Cummins. Caterpillars and engineered to be more rugged and super heavy conditions that they go with defaults in most untouched cases. Newer rigs however have a slightly toned down engines in RVs that they compromise this and go for 4th gear at Jake brakes.

Any modern transmission is computer controlled and works closely with the engine computer to manage rpms and torque. It won't ever over-rev and engine. In particular, an Allison 3000 or 4000 won't even allow you to manually select a gear that is potentially damaging to engine or tranny.

Easy to prove you wrong. One example is Allison MD3060 programmed in oshkosh truck chasis used for RVs. Engine, Tranny, Rig, Body, Paint none of it matters compared to the safety of the passenger. That is DOT standard. The EPU and TPU must be programmed to sacrifice its own if it comes to the point of driver safety. That is why VW was shot down because they played with their EPUs.

If you put the selector in gear "2", the Allison merely takes that as advice from the driver, not an absolute command. It will do it only if the engine computer concurs.


This is NOT universally true - many engines with Jake brakes are programmed for 4th gear when the brake activates. It's purely up to the powertrain engineer who sets up the chassis.

'programmed' is the word. RV manufacturers are allowed to program as long as they submit the details to DOT and is approved.

 

[unni]

Gary is exactly right!
For instance, one time I reached to push the economy mode button without looking. I accidentally pushed the reverse button. The Allison 3000 just whined as it bypassed the valving; the engine continued at the same RPM without hesitation.
But the next rest area was an underwear change.

You had the higer intervention at that 'one time'. If ever you sell your RIG remember to not mention this 'one time'. Also never mention the identifying detail of your RIG in this forum too if you want to sell it for a good price to a dealer. A knowledgable dealer can read that 'one time' from the TPU conditions. That piston took the entire hydraulic force of the fluid against it combined with the momentum of your RIG at your speed in terms of hydraulic pressure. Those pistons have a patented synthetic resin ring, which I am sure is damaged.

 

[Ray-IN]

Easy to prove you wrong. One example is Allison MD3060 programmed in oshkosh truck chasis used for RVs. Engine, Tranny, Rig, Body, Paint none of it matters compared to the safety of the passenger. That is DOT standard. The EPU and TPU must be programmed to sacrifice its own if it comes to the point of driver safety. That is why VW was shot down because they played with their EPUs.

What Gary wrote is what my Cummins owner manual states. When you are descending a steep grade and the engine RPM reaches(my ISC) 2,400 RPM the Allison will upshift to prevent engine damage.
Gear 4 is straight drive, 5 and 6 are overdrive gears in the Allison 3000 series. I have driven a steep enough grade that I had to go to gear 2 to hold speed only once.

 

[Gary RV_Wizard]

It is not obsolete. Allisons engineered default is 2.

Sorry, that is simply not true. Allison makes several different Transmission Control programs available and the chassis builder can select any of them. The gear for an exhaust or engine brake is one of the several variables in those programs, and its is also individually selectable. The chassis builder can also choose to program his own tranny profile and it will be Allison-approved as long as the programming is done with Allisons software.

Easy to prove you wrong. One example is Allison MD3060 programmed in oshkosh truck chasis used for RVs.

I don't see any "proof" there. What/where is this "sacrifice"? I assure you the MD3060 will upshift before over-revving the engine. And VW got in trouble for finagling their EPA fuel economy testing performance, not for passenger safety considerations.

 

[Ray-IN]

The term "Jake brake" refers to Jacobs Vehicle Systems engine compression brake. The company also produces exhaust brakes that restrict exhaust flow, which are often referred to incorrectly as a Jake brake.
The PacBrake is an exhaust brake, like the Jacobs exhaust brake.
The Cummins engine company only installs engine brakes on ISL and larger engines, smaller engines are fitted with an exhaust brake and/or a VGT turbocharger.