Engine over heating up hill grades

[Dave Atherton]

Summer is upon us and been getting many calls on engine overheating on up hill grades.
Caterpillar Engines involved 3126A up to Cat C-9 with heui injection systems. Technical
Information as follows. For optimum performance, allow the engine to lug downward to
peak torque rpm ( 1440 to 1550 rpm ) before downshifting. Continue to down shift ( manual
shift Allison trans ) until a gear that maintains the desired speed is reached. Continue to
operate at a low rpm ( 1440 to 1550 rpm ) if motorhome will proceed to the top without
downshift. Begin upshifting as grade of the hill decreases and the engine begins to accelerate
above 2000 rpm. Driving in this manner will provide optimum fuel economy's and performance.

Note: allowing the engine to lug below peak torque is permissible if the motorhome is cresting
the top of grade. However, extended operations at an engine rpm that is below peak torque
( 1400 rpm ) will raise the exhaust temperature and the cylinder pressure and in most cases
overheating warning light and alarm will activate. Remember going down the same grade,
do not allow the engine rpm to exceed 2900 rpm, engine damage can result.
Dave Atherton Retired Cat Mechanic

[Edward Buker]

Dave,

With overheating being the issue, I am a little surprised that you are not recommending downshifting sooner and keeping the rpms up in a higher range. While lugging down to peak torque rpm range would give the best economy, as a trade off, it does not give the best cooling system performance.

Is there some modification to the practice you would recommend if overheating or engine temp is rising too much while following the peak torque down shift routine?

Later Ed

[Jerry Emert]

Dave, what is the maximum engine RPM for a C-12?  I am a little concerned when the Jake kicks in going down hill.  I see RPMs of around 2400 sometimes.  Thanks
Jerry

[Dick Simonis]

I've always had an overheating issue with the Marquis when pulling steep grades at 100+ degree outside temp.  Not sure why since my 2000 PT was fine unless the extra 80 HP and additional 10,000 lbs is a factor.

This reminded me that last year when we pulled into the park in Alamo, NV and a fairly new Newall came into the site next door and we were chatting about pulling the grades going N from Wickenburg, AZ to I40.  He had the same issues I did of having to downshift to keep the RPM at 2000 and speeds around 45 due to high engine temps.  I would see about 217 which is just south of the engine light coming on.  Once the outside air temp dropped into a more livable range it behaved much better.

[David T. Richelderfer]

Jerry - There have been numerous discussions here in the Forum about the Jake Brake versus RPM issue.  Within a year of my purchase of my Marquis, I had the Kenworth shop in Pasco, WA reprogram my transmission to NOT shift down to 4th gear when the Jake Brake kicks on.  I had Kenworth program the transmission to NOT shift out of 6th or 5th gears unless I do it manually, or as it normally does under normal driving conditions.  Now I don't worry about it causing a spike in the engine RPM.

My C-12, 505 HP engine rarely goes above 200 F.  Almost always the C-12 temperature runs between 185 F and 196 F.  On the other hand, my transmission rarely goes above 140 F.  The transmission runs about 40 degrees over outside ambient temperatures.  Only on a hot Summer day have I seen my transmission temperature near 140 F.  Yes, it has an added cooler for the transmission according to the previous owner's remarks.

[Jerry Emert]

David, I have followed the previous discussions but decided to keep the original programming.  I didn't really worry about it until my current trip to Alaska. 
The Canadian Rockies and other roads up here are a bit steeper than I encountered last year in the American West.  Also less warning or actually no warning of serious descents up ahead.   I will have the re-programming done this Winter.  There is no "red line" on my tach so I really don't know what the high end is on the C-12. 
As far as transmission temps go, that is another sore spot with me.  My Allison 4000 routinely runs 250+ in hot weather.  It was about 65 yesterday and it was running about 213.  It does not change with ascending mountains or descending.  It is totally dependent on outside air temp.  In 90+ weather it will creep up to 250+ in 2 or 3 hours.  I've talked to Allison engineers that say that the transmission routinely survives operating temps of 350+ degrees when used on commercial trucks.  I've learned to downshift to 5th when it rises above 250 to bring down the temp which it does rather quickly.  If it raines, temp goes down a lot!  If I stop even for a few minutes the temp goes down.  I'm kind of over it.  The warning light has never come on when I've exceeded 250 so I just deal with it.  I've brought this up on the Forum before with no real suggestions other than "it is what it is."  CAC and radiator have been cleaned.  Only other thing is an auxiliary cooler that I may not really need.  Sorry for the "recap" but it has been a concern.  But back to the question.  Redline on a C-12 anyone??
Thanks
Jerry

[Steve Huber]

Jerry,
This thread may be helpful. It notes that  spec wise max RPM on a C12 is 2100. http://beaveramb.org/forum/index.php/topic,1016.0.html
Steve

[Jerry Emert]

Thanks Steve, those discussions were before my time as a Beaver!  I just noticed the 2400 RPMs while the jake slammed the coach into 4th gear.  I had seen something about 2100 RPM but nothing official.  I guess I'll be looking for a Allison dealer or shop in Alaska that can reprogram it for me.  I should add that this is tough for me to monitor closely because unlike most of you I have to have the Jake turned on in order for my cruise control to work.  I know most are opposite but thats the only way to have cruise.  It does seem to operate a bit more smoothly with the cruise on.  It doesn't slam into 4th quite so abruptly. 
Thanks again.
Jerry