Previous Jake Brake thread

[Joel Weiss]

A couple of months ago there was a thread in which concern was expressed about C-12 engines being subjected to "higher than redline" situations when using the Jake (compression) brake under certain circumstances.

There is a similar current thread on the irv2 forum in which the same concerns are expressed relative to Cummins engines.  The situation being described is exactly the same as we had discussed here--whether the 4th gear downshift when the Jake is activated can result in an "over redline" situation.  The key difference is that Cummins has stated that there are different redlines for acceleration and deceleration and that the engines can withstand significantly higher rpm's in the latter case without concern.   http://www.irv2.com/forums/f123/engine-brake-usage-110054.html

I was curious if this issue had ever been discussed directly with someone at CAT?  It's hard to imagine that  CAT and Cummins engines would be significantly different in their behaviors with regard to this issue.  I personally don't have a problem since I can engage the Jake at speeds up to 65 without concern about exceeding redline anyway, but I thought raising this issue in the context of the Cummins information might be useful to others.

[Ken Buck]

My Cat C9 will not accelerate past 2150 with the foot throttle. The book says not to rev it past 2900 going downhill.

Ken

I guess I should add that 2150 is what it looks like the tach reads. I should look at the Alladin I guess and find out exactly some day. Also I think all the C9's technically use an exhause brake, not a true Jacobs exhaust valve opening Jake brake.

[Dick Simonis]

I seem to recall my Cat C-12 manual says something about that.  I'll double check but it stuck in my mind that the RPM limit was 2300 without a Jake brake and 2100 with a Jake.  I have noticed that the downshift will not occur until the speed is low enough to not exceed 2300.  Intuitively, it would seem that if Cat were concerned they would have dropped the shift point so a donwshift would simply not occur that allows an overspeed.

[Joel Weiss]

I seem to recall my Cat C-12 manual says something about that.  I'll double check but it stuck in my mind that the RPM limit was 2300 without a Jake brake and 2100 with a Jake.  I have noticed that the downshift will not occur until the speed is low enough to not exceed 2300.  Intuitively, it would seem that if Cat were concerned they would have dropped the shift point so a donwshift would simply not occur that allows an overspeed.

That's what I would have thought, also.  But there were some pretty strong comments posted here about engine damage. I think the issue started with one or more individuals who believe their engines were damaged from an overspeed situation.

[Marty and Suzie Schenck]

There are under load and under no load rpm's for diesel engines. The under load rpm for the C-12 is 2100 and the under no load rpm is 2150. This is right from CAT @ 1-877-777-3126. So if you are going 65mph or faster and engauge the Jake Brake on a Thunder or Marquis and it downshifts to 4th gear you run the risk of over reving the engine. That is one reason I had the default gear changed to 6th. No chance of over reving and I can control the braking gear. Marty

[Joel Weiss]

There are under load and under no load rpm's for diesel engines. The under load rpm for the C-12 is 2100 and the under no load rpm is 2150. This is right from CAT @ 1-877-777-3126. So if you are going 65mph or faster and engauge the Jake Brake on a Thunder or Marquis and it downshifts to 4th gear you run the risk of over reving the engine. That is one reason I had the default gear changed to 6th. No chance of over reving and I can control the braking gear. Marty

Thanks for the data; I'm surprised that the load/no-load range is so much smaller for the CAT than for a Cummins engine.  I had considered having the default gear changed, but reconsidered after reading a Jacobs paper which explained that the braking effect of a Jake is highly related to how close the engine is running relative to red line.  After a summer in the mountains of WY, ID and MT, I'm quite comfortable with the current Jake/Allison setup.

[Marty and Suzie Schenck]

Joel, The load no load specs for the C-12 are about the only ones that are that close. My old Cat 3126 was 2100-2800rpm's. We each learn to drive our coach's in a manner that we are comfortable with and that is another reason I changed the defaut gear to 6th. With truck driving being the trade I retired from, I thought it odd that the transmission would downshift from 6th to 4th. I delivered lot's of brand new class 8 trucks to dealers with Allison transmissions and I can't remember a one that defauted to 4th gear. It's nice to be going down a grade and take your foot off the throttle and hold that speed without any downshift. If that won't hold the speed just hit the down arrow on the shift pad and it will drop to a lower gear. I can understand the 4th gear drop for a Pac-Brake but the Jake Brake is so far superior that the downshift just is not necessary (personal opinion). There are many owners of C-12's that have changed the default to 6th and love it and I have heard of none that have gone back. Marty

[Phil Sales]

Marty and I are of the same opinion. As a commercial driver, I too wondered why the 2 gear down shift. In the trucks the engine rpm would not allow a two gear down shift, so why do it with an automatic.
I too had my Allison 4000 reprogrammed to NOT shift. It shifts down on it's own when  the coach slows down enough to 5th, slowing more then to 4th. As Marty said, it is nice to go down a grade and just take my foot off the pedal, and the Jake holds me all the way. This is personal preferance of course. I too will never go back