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Overspeeding the engine

Yahoo Message Number: 11496
Anyone with the Cat C-12 have a problem with exceding 2100 RPM with use of the 'jake' ?

I attened two siminars at the CCI ralley in Eugene and FMCA in Redmond. I also called Allison to confirm what I had heard. The Allison party line is this. The transmission is set to never overspeed the engine. It will not allow RPM greater than 200 over the no load governed speed for that engine.
Now the problem, Cat folks I talk to don't know exactly what that means, they give me the RPM that is published in the Cat manual as the 'no load governed speed', in the case of my engine, a C-12 with Jake, is 2100 rpm.

This is very easy to exceed by driving 62 MPH and having the jake selected 'on' and letting up on the throttle. The transmission goes to 4th gear and the RPM will exceed 2300 RPM if allowed to.

Alison say, "No problem".

Cat says " the engine RPM is being exceeded"
I am scheduled to have Cat plug in the computer and check for 'logged overspeeds' next week.

I also am reading the Cat computer selected to RPM not just the Tach.

I was told by more than a couple of driving instructors to " just leave the Jake brake on all the time, no problem.

Well I'm not so sure now, anyone have any input on this condition?

Thanks, Larry
'99 Affinity 5714

Re: Overspeeding the engine

Reply #1 –
Yahoo Message Number: 11502
Larry,

We have the C-13 in our 05 Mag and if I put on the jake at a speed that will generate RPMs higher then the engine will handle the jake will not kick in. Same thing happens going down a mountain, if the rpms get to high for the engine the coach will up shift out of the Jake and go faster and faster so then I have to use the air brakes to slow the coach back into the jake. It is my understanding that all engines work this way to avoid overspeeding damage. You can lose control on a down hill grade, with the jake on if you let the coach go to fast. And besides our tires are only rated at 65 mph so keeping the rpms under control is important.

Bill G. 2005 Magna #6425

Re: Overspeeding the engine

Reply #2 –
Yahoo Message Number: 11510
Bill,

The jake brake should not disengage when you shift to a higher gear, just like it doesn't disengage when you downshift. However, in the higher gear you have less braking torque to the rear wheels from the engine with the Jake on, so it might feel like the Jake disengaged. Because of the decreased torque, braking ability from the Jake will be greatly reducedat the higher gear.
Per 5259 Affinity

gablerwh@... wrote:

Re: Overspeeding the engine

Reply #3 –
Yahoo Message Number: 11513
Bill, your coach is operating the way Allison says it should. Mine however is letting the RPM go higher than I think it should and the Cat ops manual seems to agree with me. The problem is, as owner/operators we don't have ready info on the 'no load governed RPM' numbers, and from what I have found thus far, neither does Cat.
More info to discover I guess.
Regards, Larry
Affinity 5714

Re: Overspeeding the engine

Reply #4 –
Yahoo Message Number: 11577
Bill G. Permit me a question please. On your C-13 Jake brake do you have 3 selections, and if you do, does the transmission downshift to 4th in all 3 selections?
I ask because the Cat shop updated the engine software on my C-12 and now with Jake selected in 'low' the transmission does not downshift to forth [ I don't see the number in the window nor do I feel the downshift] but in 'mid' and 'hi' position the transmission down shifts to 4th. This is a change in the new software, prior to the update all 3 selections downshifted to 4th. Not sure this is all bad as it makes for smoother operations in traffic and I can still manually select 4th in downhill driving, but if it is a 'glitch' in the software I want to have it remedied ASAP.
As for the reason I first posted, overspeeding the engine, The Cat computer logged no overspeeds on my engine, the good news, but I still don't have a number where the transmission will not allow the downshift, still checking.

Thanks for your help, Larry

Re: Overspeeding the engine

Reply #5 –
Yahoo Message Number: 11582
With every one concerned about Cat engines, smoking, running to hot, and now over reving; I am glad I have a Cummins ISL. Wish it were An ISM. Seems Cummins has their communication with Allison working. No load rev limit on ISC and ISL is about 2650. TWI 11731.

Re: Overspeeding the engine

Reply #6 –
Yahoo Message Number: 11585
TWI, WOW! Hold the Fort! Cat engines are NOT overspeeding!! At least mine is not. I was concerned that my engine might have been overreving on Jake brake application but it is not and the Allison transmission is the controlling factor in this case, not the engine. Allison will look at the RPM and if it is in the range to downshift safety it will, if the RPM is to high for a downshift, the tranny won't shift until the RPM drops to a safe range.
Hope I didn't mislead anyone with my concerns--- I guess that is how rumors get started.
Larry

Re: Overspeeding the engine

Reply #7 –
Yahoo Message Number: 11590
Larry,

I believe my C-13 shifts down to 4th in all 3 positions of the Jake but I would not bet my life on it. I have only driven it 4000 miles but I think it down shifted each time I selected the first position. I only use the first position coming down hills that are not steep grades so I'm not sure but the nect time I am out I'll watch it more closly.

Bill G. 2005 Magna #6425
 

Re: Overspeeding the engine

Reply #8 –
Yahoo Message Number: 11594
Bill, I just talked to Cat about the updated software, and the "Tech" that installed it omitted a few inputs, so it's drive a 100 miles to have it corrected, ya gotta love it.
The Jake should downshift in ALL 3 positions. This per Allison and Cat.

Thanks for the info.
Larry

Affinity 5714