C-12 Engine Brake and RPM

[Dick Simonis]

I've been a bit concerned about the engine brake taking the rpm to ~23-2400 rpm when it downshift so when I took the coach to Pacific Truck Center in Coburg, OR last week for service, I posed that question and asked if that was a programming variable.  The Allison tranny guy...Steve(?) told me that this question comes up often in his seminars and both Cat and Allision standards are full no load RPM plus 500.  So, in the case of a C-12 that would be 2600 rpm max.  This was the first time I've heard that so I wanted to check with others. Now that I've changed the default to 4th vs. 2nd the issue should be minimized anyway.

Apparently we will be at the Seven Feathers Rally in some capacity so someone might corral him and get more info.

FYI, Pacific Truck is a pretty good outfit...neat, quick, and friendly even if they did put 42 qts of oil into a 36 qt sump.

[David T. Richelderfer]

Well, if a little is good, then a lot must be better, eh?

There has been a lot of discussion about this programmable transmission downshifting when the Jake is engaged.  My coach switches to 4th gear from 6th or 5th.  This can be changed.  My understanding is this is software controlled, and can be changed to downshift to any gear you like.  If memory serves me when I discussed this with the technician, he said it can also be programmed to not shift.  That is, the transmission will stay in whatever gear it was in before the Jake is actuated.

At some point, hopefully on the near future, I am planning a software change.  I don't like hearing or watching the engine slam from 6th to 4th in about 1 second after I engage the Jake.  It scared me to death the first time it happened.  Now I try to manually shift from 6th to 5th, then after a few seconds I will engage the Jake only if the engine RPMs are well below 2,000... say 1,600 or so in 5th gear.

I would prefer no change to the current gear upon engaging the Jake.  I would prefer downshifting manually.  Unless I get convinced otherwise, when I get the software change, this is what I will request.

[Edward Buker]

Dick,

I certainly have gotten wrong information from a large Cat dealer service manager who had been there for over 20 years. He said the Cat C12 had a 40qt sump and they have been filling them with 40qts for years there. After a bit of digging I found that the sump was really 36 quarts for sure. Not sure how they got 42 quarts in yours. These questions come up and someone will profess to know the answer and we just can never be sure if it is straight from the horses mouth or the other end.

I have heard 2100RPM max, then 2300RPM without load, and now 2600RPM as a general spec. Cat should have a written spec on the C12 motor by version. I have never actually seen a load and a no load spec documented for a Cat or a Cummins engine and have only seen the 2100RPM spec for the C12. I have seen a 2300RPM C12 spec in a 700HP marine version. If the max RPM  engine limitation is a heat driven issue then it would make sense that there may be a load, no load max RPM spec. If the max RPM spec limit is a mechanical failure part limitation at RPM then load vs no load may have the same limit. I for one do not know so I have been for the most part managing the RPM to the 2100 spec.

Later Ed

[David T. Richelderfer]

I agree, Ed.  I not only prefer the maximum RPM of 2,100 to 2,200, but also I prefer to NOT see the RPMs go from well under 2,000 to near or over 2,400 in the flash of a second when the transmission software drops the transmission from 6th to 4th gear.  Even if it would first shift to 5th, then wait a few seconds before shifting to 4th it would be much better.

[Edward Buker]

David,

I had the 4th gear original program changed to the 6th gear and the good side of that is I never even think about RPMs when engaging now. Longer gentle hills on the interstate that would have you gather speed can be held in check in 6th at lower RPMs and I find that useful. The downside is that you have to manage the braking level with the transmission shift pad to match the hill condition yourself which takes a little getting used to. Sometimes I drop into 5th to raise the RPM and get more braking if the hill is steeper. It is rare that I downshift into 4th manually but certainly on a steep grade that may come into play. In my use of the Jake now maybe 90% of the time I just use 6th, maybe 8% of the time I downshift to 5th, and 2% of the time I go down to 4th in my total use on hills. Slowing to a stop the Jake is literally always in use and shifting down by itself.

On balance if I did not want to fuss with this much I would opt for a 5th gear shift with the Jake engaged as long as the 2100RPM speed in 5th covered the usable speed band that you drive. I think that would be in the 70s somewhere. I would run up to 2100RPM in 5th in your coach and see where you are. You can always rock between Jake off, Jake 3cy, or Jake 6cy to modulate the level of braking in 5th as opposed to shifting. No right and wrong here just I think the straight 5th downshift is probably a simpler way to go overall....They are both way better than the 4th gear slam bam thank you mam approach that we had.

Later Ed

[Dick Simonis]

In my case, since I seldom drive more that 65, and never that fast going downhill where I might need the Jake, 4th gear seems to be best.  That said, I haven't driven it anywhere except back to Lebanon so I'll just wait and see.

Part of the logic is that 4th at 2100 is over  55 and 5th well over 65, I always downshift to 5th anyway approaching a long downgrade...in OR I'm seldom in 6th since the speed limit on most of the roads is 55.

As Ed said, no right or wrong just different driving styles.

Oh, regrading the 42 qts, seems the tech was told I had a C-13 which holds more oil.  I found out when I got the bill and asked how 42 qts went into a 36 qt sump.  They really jumped at that and sent a guy right out to check and than drain the excess.

[Joel Weiss]

If you read material available online about Jake brakes you will learn that the braking force of the Jake is directly related to how close the engine is to its "red-line".  So intentionally slowing the engine down before engaging the Jake results in a significant reduction in braking force.

What I like to do when I'm doing serious mountain driving with a lot of Jake action is to manually downshift to 4th so that the Jake engagement is not accompanied by a downshift.  My PT is geared so that I can easily do ~55 in 4th and I'm unlikely to be doing that in the type of mountain driving I'm describing.  So forcing the transmission to 4th doesn't have any downsides with regard to performance and, in fact, doing so puts the rig roughly at the max HP point of the torque curve.

[Edward Buker]

Dick,

I guess for me, if I want to turn the Jake on I do not want to be looking away from the road to see if I am at 57mph or 63mph and trying to make decisions about do I need to use the service brake first to get to the Jake safe RPM. I just want to hit the button when I need it and keep my eyes on the road. I also think from a safety point of view, in case my speed for some reason was to ever get out of control, I would want the Jake available without question. This is just an opinion and as we said no right and wrong here.

Later Ed

[Steve Huber Co-Admin]

Dick,
I got the same answers you did re Jake brake/RPM from 2 different Allison/Cat service centers in MN this month.
Steve

[Dick Simonis]

Ok, so we have yes, no, and maybe.  If anyone is going to be at the Seven Feathers Rally and, if indeed this chap is attending, perhaps he can be asked some specific questions and etc.

I recall he asked me if I was going to be there and said if I wasn't happy with the change, let him know and he would change it.  Based on that comment I got the impression he might be giving a seminar or some such....which apparently he does on a regular basis.

[Mike Humble]

They will have a rep at Seven Feathers.  They aren't scheduled for a seminar, but will be available to answer questions and do some diagnostics.
Mike

[Dick Simonis]

They will have a rep at Seven Feathers.  They aren't scheduled for a seminar, but will be available to answer questions and do some diagnostics.
Mike

  Perfect, that would be him....quiz the devil out of him and let us know what he says.

[Edward Buker]

I decided to call Caterpillar Tech support and have them look up very specifically what is the documented spec for max RPM for a Caterpillar C12. From their operations and maintenance manual for a C12 they have a maximum spec of 2100RPM not to be exceeded if equipped with an auxiliary braking system. I asked about why they used the term auxiliary braking and he said they use that term to mean either exhaust or Jake brake. Many times the term Jake brake is used in error by a customer for an exhaust brake and they wanted to be sure and clarify that any engine braking system is the issue and chose to use auxiliary to clarify that. The Cat C12 if not equipped with an auxiliary braking system is not to exceed 2300RPM. To me this makes sense in that there is more mechanical complexity in the valve train or in the case of an exhaust brake higher cylinder pressure involved. I do not see why they would up the RPM with those issues beyond the max design spec. I can see without those complexities allowing a 200RPM adder as rational. I asked if there was any information about an adder for just braking and he said no.  I know many of us have exceeded the 2100RPM especially with the Jake tied to 4th gear and gotten away with it. Cat may be conservative on the spec in general for long term reliability but I would live by the 2100RPM as a good rule. If someone at a seminar tries to tell you + 500RPM to the spec while braking then I would ask them to produce the Cat documentation of that limit.....my guess is that this is a well traveled urban legend at the dealers like the 40 quart sump.

Later Ed

[Edward Buker]

I liked Joel's comment on braking and getting the coach into 4th with a manual shift before engaging the Jake brake. I never liked the abrupt high RPM downshift from 6th to 4th and at times it was like a slam into 4th gear when you hit the Jake. While it is very true that you lose braking efficiency at lower RPM, it is also true that depending on the hill and speed situation that may be exactly what fits the hill situation the best to not have to invoke the use of the service brakes and maintain speed control. The vast majority of the time for interstate driving the 4th gear Jake selection for me was the wrong choice and way too aggressive for braking at interstate speeds. I now use 6th most of the time, then 5th, with a rare 4th. As Joel points out for serious mountain driving just staying in 4th and using the Jake as needed is a great technique. For those of you with just have the straight 4th gear option when the Jake engages, you are either using extra fuel over braking or wearing service brakes more than you need to in order to lower the speed enough to have the Jake safely available RPM wise at interstate speeds. I waited several years of owning this coach before I bothered to make a change and after having done that I know I should have done it sooner.

As you can tell I am a conservative sort and we probably have not heard the end of the C12 redline RPM saga. I am sure that there is something behind the tolerance for higher braking RPMs being proposed by some. For all of us, given the expense of these engines, see if we can get to some documented real source from the seminar folks....challenge them a bit for the proof and see where that leads us. If we can run higher RPMs braking we should be able to know that and be confident of the testing that concluded that, as well as the CAT source that was behind it. We are 15 years or so into C12s on the road and it seems silly to me to have to debate where the redlines are at this stage of the game.... I for one am drawing a line in the sand at 2100RPM and will wait to hear from you folks at the seminar if you can coax out a documented source for another spec. Maybe unlike congress we can get something done here on this matter....Time for a cold beer.

Later Ed

[Joel Weiss]

Ed:

From having read other posts on this subject, one of the key differences between your rig and mine is that mine is geared so that I can safely engage the Jake up to 65 mph and still not rev beyond ~2150 in 4th gear.  From what Gerald and others have posted the Marquis reaches this same point at ~57 mph which is a lot more of a nuisance.  I rarely drive above 65 mph since my toad is limited to that, so I'm really never at risk with respect to the 4th gear issue.  I don't have to use my service brakes to slow down before engaging the Jake so leaving it set for 4th gear isn't a real issue.  I figure the fuel economy difference between running in 4th or 5th with the Jake is insignificant.

With my gearing hill-climbing in 4th at 50-55 mph is fantastic since the engine is turning ~1500-1550 which close to its max HP point.  A couple of times I've forgotten to take it out of 4th when the road leveled out and I can tell you that running a C12 in a 36,000 lb GCVW regime at 1,500 rpm makes for some pretty exciting performance.  According to one of the techs at BCS, Beaver used to have "drag races" with the PT's at some of its rallies back in the good old days!

Joel