-
Mountain Driving Question
-
I want to see a letter from Detroit Diesel saying that 2400 is ok as long as it is not under load. Good luck getting one. And BTW, the engine does not control the transmission. They do talk on the data link, but the transmission decides when to shift, not the engine. The transmission ECU is in control of what the transmission does.
-
Of course the transmission ECU controls the transmission. It also controls at what RPM the transmission will shift, whether up or down.
-
I’m really troubled by the suggestion the transmission controls the engine RPM.
I understand that going down a steep hill a coach gaining speed can push engine RPMs past the 2100 RPM limit, but I was unaware (if I understand what is said) the transmission can call for a downshift to a lower gear when the resulting RPM will exceed 2100.
Something is wrong either in my interpretation on what is being said or with the programming.
-
MAX RPM Question
Their seems to be a lot of opinions about the max operating rpm of the Detroit Diesel Series 60 engine. The answer may possibly be listed on the “OPTION LABELS” on your engine. The attached excerpt from the Detroit Diesel, Series 60, Engine Operators Guide, may shed some light on your specific engine operating parameters.
The example below lists all the options on this particular engine as installed at the Detroit Diesel factory. Additionally shown is the horsepower rating at what RPM, injector setting, exhaust & intake valve lash, and the max rpm with NO LOAD. The Engine Option Labels were attached to every Detroit Diesel Series 60 engine made.
I know what mine reads. I know what NO LOAD (NL) means to me.
I am not trying to convince you how to operate your coach or to change your mind. I’m just trying to point out the location of some information to consider.
As always, Your Mileage May Vary.
-
The 2225 RPM might be the no-load, foot to the floor limit. On a previous coach I had some engine work done and before I took the coach on the road the Detroit dealer had me out in their parking lot with my foot to the floor for about 15 minutes. The RPM I saw at that time was close to or the same as that stated value. But when in gear, unless the RPM was being pushed with a downhill the most I could ever get out of the coach was around 2125 to 2150 based on the tach.
-
Deleted.
-
Jon,
You have it backwards; If the RPMs get too high, the transmission will call for an upshift to a higher gear to lower the RPM.
-
There should be only two ways a coach can see in excess of 2100 RPM when being driven. The first is if the supplemental braking programming is causing a downshift into a lower gear when the engine speed in that gear is higher than 2100 for the speed at the downshift. I have never seen that occur but that does not mean it cannot happen.
The second is if the driver allows a downgrade to push his engine speed and I know from posts that this can happen and I assume it is because the transmission is manually put in a lower gear and will not upshift or the supplemental braking as programmed will not allow an upshift. This is a case of a driver having to monitor the coach and do what is necessary to prevent the overspeed.
-
So I decided to do a little research on this issue of maximum RPM for a Series 60 Detroit Diesel. Despite what you all are saying, it made no sense to me that the transmission would allow the engine to overspeed while under computer control. First, running the engine under full throttle in neutral is not the same as having the rear wheels drive the engine when the Jake brake is used. When running the engine under full throttle in neutral, the engine is under load from the combustion in the cylinders, and the engine computer will limit the RPMs to 2,100. When you are running downhill with the Jake brake on, the rear wheels are driving the engine, and it functions as an air compressor/vacuum pump. A different kind of load than full throttle application.
So, I called Detroit Diesel for some answers. The customer service representative I talked to tried to help, but not sure he understood the question. He was able to tell me my engine had 500 HP at 2,100 RPM—(pretty sure I knew that!), and he kept calling up a technical representative—spent at least half an hour on the phone with no real answers. I asked to talk to the technical representative, and all he could do is tell me they would call me back. Figured that would be the last I heard from them, but was surprised when I got a call back from Chris. Chris related Detroit Diesel only programs basic parameters into the engines. Shift points are programmed in by the OEM—in this case Prevost. He did relate by the way that Series 60 engines typically don’t “blow up” until well over 3,000 RPM—I would not recommend trying it, unless you are a racer who knows what he is doing!
Tried Prevost in Quebec, not much luck getting through, so I tried Jim Malcomb, Customer Service Rep for South Central U.S. He was not sure about who programmed shift points, but did relate remembering seeing a bulletin from Detroit Diesel around 2003 that specified RPMs for a Series 60 while using the Jake Brake could go up above 2,100 RPM—he thought it was 2,400.
Tried to find that bulletin without luck, but then I went to the on line Detroit Diesel Series 60 Operator’s guide, and lo and behold, on page 25 it states:
“When descending a hill with Cruise Control OFF, do not allow the engine to exceed 2,500 rpm under any conditions. Failure to observe this precaution can result in overspeeding and serious engine damage.”
So by Detroit Diesel standards, overspeeding of the engine does not occur until 2,500 RPM. Also, if you manually downshift with the Jake brake on, the transmission will not downshift until it is safe to do so. You can downshift to second manually, and the transmission will only downshift to the next lower gear when the engine RPMs in that gear is below 2,500. I suspect it is the same for a Volvo engine, but you would need to do your own research to find out.
-
Put me in the category of being “chicken”. This is especially so when things are written, such as the 2100 RPM limit. I have no doubt what Per was told was believed to be true by the people he spoke to but the fact there is nothing to support what they say in writing by Detroit Diesel makes me want to stick with my cautious attitude about engine speed.
The second reason is I have concerns about a downshift slamming an engine from whatever the RPM was in the higher gear up to 2300 in the lower gear. It is one thing dropping a gear without having a corresponding rev match when the RPM spread is relatively low, but I can feel the harshness of a downshift that slams the RPM so high and that can’t be good for anything in the drivetrain.
Does my caution indicate I think Per’s engine or transmission is going to end up in pieces on the interstate? No it does not. But do I think what he is experiencing carries a little more risk than maintaining lower rev limits? Yes I do.
But keep in mind that parachuting and bungee jumping are sports with low risk but I am not prepared to accept even that small risk.
-
Great information Per, thanks for the research.
Roy
-
Jon,
I guess you did not read my post. The quote is straight out of the Detroit Diesel operator’s guide for the series 60. I don’t know how much more in writing it can be. I just think the participants on this forum should get the right facts. If you choose to drive your coach in a more conservative fashion than Detroit Diesel recommends, that is your prerogative.
-
https://ddcsn-ddc.freightliner.com/cps/rde/xbcr/ddcsn/DDC-SVC-MAN-0006.pdf
That is my operator’s manual for my DDEC VI engine and the same 2500 RPM warning is found on page 36. That is a warning, not a permission to exceed 2100 RPM. The limit is as I stated in the link in a previous post.
But it is your coach and if you want to exceed limits and feel comfortable because you are not exceeding a warning, that’s OK with me.
-
My goal is always to not exceed 2100rpm braking with the jake. But let me tell you a story.
Several years ago, a couple of months after we got the bus, I had the bright idea to take bus down the back road in our development because it would save us a few miles of travel on narrow roads. The back road has a couple of curves with a 16% incline maybe a quarter of a mile, total drop maybe 500 feet. No problem I figured. Take it slow, shift into first or second, put the Jakes on high, help with brakes if necessary.
It was just like landing a plane for the first time. Things happened a lot faster than my brain was ready for. As soon as I hit the steep part, the bus accelerated with a vengeance despite me standing on the brakes as hard as I could. The engine howled, the transmission shifted up with a loud bang, the bus picking up even more speed. Somehow I managed to make it around the last curve without taking out the gate.
I fully expected to find some transmission and engine parts on the road, but there were none and they have been fine for the last five years. But that was the last time I went down that road with the bus. And, yes, that road is a riot in winter.
1999 XLV X-6872, liberty #440 -
Jon,
I can understand you want to be conservative, but I think we are doing a disservice to the Prevost Community members if we don’t have the facts. We have established overspeeding the engine does not occur until 2,500 RPM are exceeded, and this was confirmed by Jim Malcomb, who stated on info from Detroit Diesel that the ECM does not register an overspeeding condition until 2,500 RPM is exceeded.
I suspect the 2,100 RPM allowable limit is for when the engine is under load. I would think with a stick-shift transmission under full power, it would be fairly easy to exceed 2,100 RPM, especially on a downhill. Our automatic transmissions, however, would upshift to stay within 2,100 RPM under power. Also, I’m not sure if all of our members are aware you can be driving along at 70 MPH, downshift to second or first gear, and the Allison six-speed transmission will not downshift until the engine RPMs are within limits. If the Jake brake is on, this could result in downshifts that can hit 2,400 RPM. If this is unsafe for the engine, why did Prevost program the transmission this way? I don’t believe for a minute a backyard mechanic changed the shift points—our friend’s coach does the same thing. And I am willing to bet other Prevosts with the Series 60/six-speed Allison combination shifts this way as well.
Jon, you probably have more contacts in the Prevost world than I do, so my challenge to you is to find out why Prevost programmed their coaches this way. I will never buy a Volvo-equipped Prevost, but it would also be interesting to find out if those are also programmed in the same manner. I believe it is in the forum’s best interest to get the facts out to our members.
Log in to reply.