1991 Beaver Contessa Maiden Voyage

[Gerald Farris]

Michael,
There are several variables that you need to answer before I can give you a definitive answer to your question. If your transmission has never been reprogramed to default to a different gear upon activation of your engine Jake Brake (compression brake), it will downshift to 4th gear when the you turn on the Jake if it can do so and keep the engine RPMs below about 2500. So you will need to slow to a speed that will keep the engine at your desired RMPs when the transmission is in 4th gear. However if the transmission has been reprogramed to a higher gear, you only need to slow to the speed that you are comfortable with since the transmission will not downshift and possibly result in engine RPMs that are above your comfort zone.

The normally considered correct procedure for descending a hill is to slow to the speed that you climbed the steepest part and shift to the gear that you used to climb that part. However, this does not always work because some hills have a slight grade on one side and a steep grade on the other side, so watch closely for road signs that indicate the approaching grade and slow down to a comfortable speed before starting down, and if in doubt, it is always better to be going a little slower than necessary. Remember, when going down hill, it is easy to speed up but difficult to slow down.

Gerald   

[Michael Hannan]

Thanks Gerald.  I'll have to do a test run to see what gear it goes to or stays in when I apply the Jake brake.  On the route I'm thinking of taking has an 8% downgrade for 2 miles.  Will the Jake brake maintain a steady speed or will it gradually pickup speed?

MJ

[Jerry Emert]

Could be Jerry.  We have the same year of coach.  I've refrained from venturing too far in my coach as I don't have to go too far before I start getting into the mountain and some long steep grades.  I remember seeing some of the crashes they showed on my air brakes course when the air brakes are used improperly and the drum heats up and moves away from the pads.  I decided to ask as I rather know before starting down a grade and the last thing out of my mouth is "oh mess!"  Now that I know the key is the RPM limit governs when to use the Engine Brake switch I should be ok.  I hope.

MJ

Dave apparently answered again about the time I did.  I've been told the "red line" on a C-12 is 2300 RPM but only 2100 if a Jake is installed so thats why I quoted 2100 rpm.  Dave just mentioned 1800 RPM for the Jake.  It goes without saying that I would trust Dave with this one and stick with 1800 RPM.  It confuses me because I'm told that our transmission will not let you over RPM the engine but it will easily go up to 2400+ when the Jake engages and downshifts to 4th.  I pretty much turn mine off in the mountains because anything over 48 mph kicks the RPMs over 2100.  Now I have to go even slower.
Good luck clearing the confusion.
Jerry

[Jerry Emert]

Thanks Gerald.  I'll have to do a test run to see what gear it goes to or stays in when I apply the Jake brake.  On the route I'm thinking of taking has an 8% downgrade for 2 miles.  Will the Jake brake maintain a steady speed or will it gradually pickup speed?

MJ

MJ, I frequently have to speed up going down steep grades.  My Jake will keep me under 50 on about any steep grade I've run into to out West or on the way to Alaska and back.  I don't use the brakes much. My problem is I tend to pass most traffic, even some cars on the way up and then get scared and slow way down when I hit the crest.  Good luck.
Jerry

[Michael Hannan]

Thanks very much Jerry.

[Gerald Farris]

Michael and Jerry,
The redline on a over the highway C-12 is 2,100 RPMs under power. The marine version of the same engine has a redline of 2,300 RPMs. The issue here is what is the redline when descending a hill with the Jake brake on, and as much as I have researched this issue, I have never found the answer in any Caterpillar documents. I discussed this subject with an Allison engineer once and he told me that the deceleration redline that they had from Cat for their transmission programing purposes was 2,500 RPMs for RV use, but he did not have access to that Cat information at the time to show me. So at this time, everyone will just have to use the RPM redline that they are comfortable with when operating their coach.     

Remember, if you try to research this issue yourself, there are 5 different common applications that the C-12 is used in with the marine, stationary (generators and pumps) and heavy equipment versions being the only ones still in production. The over the road engines that ended production in 2004 are broken down into 2 categories, trucks and RV/firetruck. The Truck engines deliver less horsepower and may have a tighter redline restrictions than the RV and firetruck version because the truck engine is expected to deliver over a 1,000,000 miles before overhaul, but the RV version rarely ever sees 1/3 to 1/5 of that. Therefore Cat is able to turn up the horsepower ratings, but no where near the water to water cooled marine version that can hit 700 HP.

Gerald 

[Jerry Emert]

So at this time, everyone will just have to use the RPM redline that they are comfortable with when operating their coach.     
Gerald

Please Gerald, tell me what to be comfortable with!  LOL just kidding kind of!  There is a big difference between the 1800 RPM damage point Dave was mentioning and 2500 RPM deceleration RPM!  I will have to get below 45 MPH to get down to 1800 RPM when the Jake slams it into 4th.  I've never noticed it go to 2500 on the downhill but I have seen it at 2400+.  Soon I will stop procrastinating and get the transmission reprogrammed.  Thanks for your help!!
Jerry

[David T. Richelderfer]

We had our C-12/Allison reprogrammed at the Kenworth shop in Pasco, WA.  It took longer for the technician to round up his laptop and the correct connection wire than to perform the reprogramming.  The whole process took about 20 minutes and we were off their lot.  The technician had a road call just before completing the programming.  He said he didn't have time to make up a bill - so no charge.

Now with the Jake on and in 6th gear, if I drop cruise control, then the Jake kicks on and it stays in 6th gear.  No change in RPMs, therefore no slamming the engine to over 2,000 RPMs as it did before the reprogramming.

By the way, on our coach the programming connection plug was under the removable dash plate in front of the steering wheel.

[Dave Atherton]

Gentleman little technical information that may be of interest. Compression Brake operations,
The compression brake should not be activated when engine rpm is above 1800 rpm. The
maximum power rating is 2100rpm. At this level, the amount of braking that is produced by the compression brake is aproimately 315 to 325 horsepower
Information Systems Operation: C-10 and C-12 Truck Engines  Media Number-RENR2235-06
SMCS-1119, 1129
Dave Atherton Retired Cat Mechanic

[Jerry Emert]

Gentleman little technical information that may be of interest. Compression Brake operations,
The compression brake should not be activated when engine rpm is above 1800 rpm. The
maximum power rating is 2100rpm. At this level, the amount of braking that is produced by the compression brake is aproimately 315 to 325 horsepower
Information Systems Operation: C-10 and C-12 Truck Engines  Media Number-RENR2235-06
SMCS-1119, 1129
Dave Atherton Retired Cat Mechanic

So Dave, why do they program the Jake to come on at any RPM?  Also why would they program it to slam the engine into 4th gear and 2400RPM?  Could it be a difference like Gerald was saying in the engine application ie, truck vs. RV?  It just boggles my mind that they say the Allison is programmed so it will not damage your engine by shifting inappropriately.  I know on a car with auto transmission you can be going 60 and drop it into low.  It won't shift until it's safe for the engine to do so.  Why would Allison/Beaver do it differently?  I don't doubt that the 1800 you quote is the limit but why would they do it differently?  Thanks
Jerry

[Mike Shumack]

I wonder if Dave meant to say "the compression brake shouldn't be activated under 1800 rpm".
The more rpm, the more braking (up to the engine's rpm limit).

From my Owner's Manual:

[Dave Atherton]

Mike, that is correct an reading what you posted says the same on compression brake from
builder of compression brake. Reading again it seems the 2100 rpm and down to 1700 rpms.
What I posted off Caterpillar Service technical information they are saying 2100 rpms down
to 1800 rpm. Again everyone has different thoughts as how his motorhome should operate
and yes there is information from both the engine builder and builder of the Jake Brake.
Dave Atherton Retired Cat Mechanicq

[Dave Atherton]

Jerry, yes you have a very good question. there seems to be Lot of mix thoughts on what is
a RV engine and a truck engine, than there is the off road heavy equipment engines. Will
answer your question so, every Caterpillar engine engine is assigned a serial number and
A arrangement number. In looking for information or parts for that serial number a person
in many cases will find the engine serial number that pertains to 40 engines that is used in
Trucks, R.V., off road heavy heavy equipment that use the same parts. The answer to your
Question Jerry, Caterpillar set a standard as how they want there engine specs set for trouble
free operations. Other company’s like Allison transmission have there own thoughts where
Specs should be set for there product. The jakebrake builder as you can see on post bove this
Specs run nearly the same s Caterpillar, which would tell, person maybe there is something
I may overlooked. Now going back to Allison transmission builder they also have there own ideas
about how high engine rpms will operate within there transmission range. Myself being a
Caterpillar person understanding there engines would go with the technical values on operation
Of there engine as posted when to enguge the compression brake
Dave Atherton Retired Cat Mechanic