Jake Brake and Cat C12

[Keith Oliver]

Ed:
Thanks for that.  I have scratched a bald spot over this, now I can preserve what is left of my hair.
I have a lowly exhaust brake on my coach, I have had Jake envy, but as I see it, I would need more horses before I would get into Jake territory, so I guess I really have hp envy.  The way mine is programmed, I leave the cruise control and the exhaust brake both on the whole time I am on the open road.  I touch the brakes, the EB engages.  I press the brake hard, the service brakes, trans ED and Tow dolly brakes all combine to drastically slow the coach.  It all works!  When I examined the brake pads, I found hardly any wear after 40000 miles, obvious efficiency of the EB.  I presume from this  ( and other) discussion that Jake provides much the same performance on heavier (more hp) coaches.

[Joel Weiss]

Joel,

I have been paying more attention to this subject given my coach has the C12/Allison 4000 combination and is still programmed to go into 4th when the Jake is selected. I have found that 2100 RPM is reached at around 57MPH on my coach.

Driving along I-90 through MT today with virtually no traffic I played with my SilverLeaf and Jake and discovered that in 4th gear my coach turns 2100 rpm almost precisely at 65 mph.  I verified the speed on both the SilverLeaf and my GPS.  I guess Beaver had different differentials in the Marquis and Patriot Thunder because of the weight difference (?).    Anyway the effect of this is that I can engage the Jake at speeds up to ~65 mph and not exceed the max rpm for the C-12.  I may still have Allison reprogram the default Jake gear, but it's nice to know I have less of a problem than has been reported here.

[Edward Buker]

Joel,

That is interesting, your RPM at 65MPH vs mine. Those numbers would match my 5th gear pretty well. I assume that you do get two upshifts when you come off the Jake Brake and that a prior owner has not had your coach set to 5th with the Jake already.

 It is too bad that the Allison display does not show the gear we are in, that would keep this simple. I wonder what other owners RPMs are in 4th gear vs coach speed.

Later Ed

[Gerald Farris]

Ed,
The Silverleaf display tells you what gear you have selected as well as the gear that the transmission is actually in at the moment.

Gerald

[Edward Buker]

Thanks Gerald,

I never noticed that. I'll have to take a closer look. Without my glasses I read the big numbers, the little ones remain a mystery most of the time....

 This coach has always done very well on climbs. My thinking was maybe the 505HP version C12 was a significant factor, but maybe I am actually geared lower with my differential. Not sure if the 505HP C12 was geared differently than earlier Marquis. Gerald, any idea on this one?

Later Ed

[Dick Simonis]

Regarding the max C12 engine speed, I was reading the Cat manual and noticed they give two differant RPM limits for the engine.  One is 2100 rpm if equipped with a Jake brake and the other was 2300 if not equipped.  I take this to mean that if the Jake engagement and supsequent downshift results in an engine speed of 2300, you are not over speeding the engine.  It makes me wonder if the ECM will not allow the transmission to downshift if said downshilft will cause the engine to rev above 2300.

Just a thought and cause for another question.

[Joel Weiss]

Ed,
The Silverleaf display tells you what gear you have selected as well as the gear that the transmission is actually in at the moment.

The data I quoted was obtained directly from the Silverleaf computer.  I correlated the Silverleaf's MPH data with my GPS and they were the same, as they should be.  Then I set the Silverleaf so I could toggle between RPM and the normal gear selection display.  My Jake most definitely drops the transmission to 4th gear and the  engine speed in 4th is almost precisely 2100 at 65 mph.  

I had plenty of time to fool with this while cruising I-90 through Montana and Wyoming; there's no traffic to speak of and lots of hills on which to apply the Jake.  This is a marvelous road on which to exercise a C-12; it loves to climb hills!  Max temp was briefly 197; max transmission temp 192.

I'm probably still going to get my Allison service center to reset the Jake's default gear to 5th so I can engage the Jake at speeds above 65 without concern and so the "jolt" of the Jake isn't so severe at high speeds.  But until then, at least my concerns about this issue are significantly reduced.

[Edward Buker]

Joel,

Thanks for the info. I will have to do that same test and verify my gear vs, speed and RPM. I love the C12 also, just an amazing engine in my opinion. Before I am done I am sure I will be getting my Allison reprogrammed also. Thanks for your insight on the test and the Silverleaf gear info.

Dick,

Based on what you wrote I would interpret that to mean, if you have the Jake Brake mechanical extras installed, then they want you to limit your RPM to 2100. If you do not have the extra mechanical components, which means you probably have an exhaust brake, then your max RPM is 2300. Some component within the Jake Brake System must not be able to tolerate continuous use at RPMs above 2100 would be my take.

Later Ed

[Jay and Raylene Todd]

I thought I would chime in with my 2 cents worth and expand just a little on what Ed Burker said. With our '02 Thunder, when the Jack was switched on (low or high) the Allison would drop into 4th regardless of the resulting engine rpm and like Ed, I would often find myself starting a downgrade, engaging the Jake and having it slam into 4th with the rpm hitting 2400-2500 rpm. One thing to remember is that the software is designed to prevent damage to the transmission, not the engine! The software is designed however so that in normal operation, the transmission will not downshift until the resulting engine rpm is ~150 rpm below the governed engine limit. When I had the Allison computer reprogrammed to give me all 6 gears when the Jake was turned on, the technician assured me that what I was seeing happen, wasn't really happening. It wasn't worth arguing.

When we purchased the '06 Thunder at the beginning of the month I was wondering if I was going to have to go through the whole thing again and have the Allison reprogrammed. This one also switches to 4th when the Jake is switched on however it follows the normal convention of allowing slowing via Jake in the gear you are in until the resulting rpm of a downshift will be ~150 rpm below the engine max governed, just like normal operation. Not only that, this coach will allow me to operate the cruise control with the Jake switched on (I usually leave it in high while on cruise) so that if I tap the brakes, the cruise disengages and the Jake comes on. Very nice !!!

Don't think I am going to be making any changes to this one ...

[Edward Buker]

Jay,

Your '02 Thunder seemed to mirror what my '02 Marquis does.  I am sorting out in my mind if I want to have the transmission programmed to go to 5th, or have all gears available and use 6th also. I guess I am not seeing a downside to the 6th gear arrangement given it will go to 5th at around 1950RPM anyway. Any downside that anyone has experienced, let me know.

Later Ed

[Jay and Raylene Todd]

Hi Ed,

I was mixed also about having the program change go to 5th or have all 6 available. Marty had his changed to all 6 available and really liked it so when I had the change done I followed his example. I made 2 mountain driving trips in the coach before we got the new one and I had no regrets at all about having all 6 available. You still get the Jake slowing you, even in 6th, and once it slows enough it will downshift to 5th and so on.

You won't be sorry if you give yourself all 6.

[Phil N Barb Rodriguez]

As an FYI from a new guy on the forum regarding the RPM/speed issue, our 2000 Thunder, C-12, shows 2000 RPM at 60 MPH with Jake engaged, 4th gear, according to SilverLeaf.

I realize this is too fast but as a test I have descended the Grapevine North bound on I-5 at 60MPH Jake engaged (4th gear) and 2K RPM towing our Cherokee. Never touched the foot brake brake once.

Don't really like the 4th gear setup and would prefer taking advantage of higher gears in the Jake braking process.

Piil[face=Sans-Serif][/face]

[Joel Weiss]

As an FYI from a new guy on the forum regarding the RPM/speed issue, our 2000 Thunder, C-12, shows 2000 RPM at 60 MPH with Jake engaged, 4th gear, according to SilverLeaf.

I realize this is too fast but as a test I have descended the Grapevine North bound on I-5 at 60MPH Jake engaged (4th gear) and 2K RPM towing our Cherokee. Never touched the foot brake brake once.

Don't really like the 4th gear setup and would prefer taking advantage of higher gears in the Jake braking process.

Piil[face=Sans-Serif][/face]

Phil--

It sounds as if your Thunder is geared the same as mine.  I turn 2100 at 65 mph.   That is almost the redline for the engine but there shouldn't be any harm if you don't exceed that rpm.  The posts about this issue are primarily from people who seem to have gearing that gets them to redline at lower road speeds.

I was all set to have my Allison reprogrammed to a higher gear with the Jake until I fully understood that the braking effect of the Jake is very much related to engine speed relative to redline.  The Jacobs website provides good explanations of this effect.  They make it very clear that you want to operate the Jake as close to redline as possible.  If you engage the Jake in 5th, for example, the braking effect will be significantly less compared with having it downshift to 4th.

We just completed a several thousand mile trip through the Rockies covering a lot of WY, MT and ID.  I had lots of opportunity to experience using the Jake on some very steep descents and am now totally comfortable with the 4th gear shift issue.  I followed an 18-wheeler down Lookout Pass from MT to ID on I-90 at 45mph with the Allison in 3rd and the Jake fully on.   Never had to touch the brakes.

IMHO if I really need the Jake I probably shouldn't be going faster than 65mph.  If I'm coming down a mountain and gaining speed with the Jake on full, that's a good indication I ought to bring the speed down.  

Joel

[Phil N Barb Rodriguez]

Makes total sense Joel Re: RPM and slowing capability. I think I'll unthink the hgher gear option

Next time you are that close to us in Idaho, stop by. We're a little Northwest of Spokane.

Phil

[Joel Weiss]

Makes total sense Joel Re: RPM and slowing capability. I think I'll unthink the hgher gear option

Next time you are that close to us in Idaho, stop by. We're a little Northwest of Spokane.

Phil

We just spent a couple of weeks in Spokane having a residential fridge installed at Truline RV (great folks, BTW).  We'll be back in May to have the flooring in the front of the coach replaced.  We'll give you a call when we're around.

Joel