Skip to main content ↑ ↓
Topic: Dash vice Engine (VMSpc) differences change with mileage (Read 450 times) previous topic - next topic

Dash vice Engine (VMSpc) differences change with mileage

I keep an Excel log for all our travels recording all sorts of stuff including both Dash & VMSpc mileage and hours.  From the start it was obvious that there was an offset between the readings.  However, I had not noticed, until now, that the offsets (deltas) were growing with time/mileage. 

Curious I expanded the spreadsheet to compute, for each entry, the differences between the two mileages and the two hour readings and then plot them vs dash mileage.  The results are quite interesting.  See the enclosed plot.  My comments and observations:
    • There are 398 sample pairs in the plot. 
    • The mileage deltas are quite noisy but somewhat approximate a straight line.  Obviously the engine computer can only estimate mileage based on what it knows or can measure which cannot include the actual tire circumference.  Thus I would expect a linear relationship between the two readings.
    • I can't explain the noise in the mileage.  No doubt some of the them are errors on my part.  But given the almost 400 readings occasional errors would not look like this plot. 
    • What really intrigues me is the deltas between the dash & engine(VMSpc) hours.  There is a relatively linear growth of the differences until about 110K miles.  Then it begins a noisy somewhat exponential growth.
    • I'm wondering if how the miles & hours are accumulated/stored (number of bits & format) in the engine computer and my random sampling only at the end of a travel day have something to do with the noise.  Also the format of the data passed from the engine computer to the pc could be a factor.
Has anyone else noticed this phenomenon?  Any ideas?
Audrey & Harvey Nelson
2000 U-320, 40' towing 2014 Jeep Grand Cherokee
nelsonah@ieee.org

Re: Dash vice Engine (VMSpc) differences change with mileage

Reply #1 –
Pretty neat, Harvey.  I have no idea how to explain the difference.  Are you reading miles and engine hours from dash gauges?  I am not sure where they get their data. 

VMSpc is reporting what the ECM is putting on the data bus.  Engine hours are what the ECM accumulates.  Total fuel is what the ECM adds up as fuel is used.  Miles must be some thing that gets reported based on a factor for axle ratios and tire size and the number of transmission output turns.

If I am not using the generator or the Aquahot and I am careful about adding fuels consistently then the fuel used reported by VMSpc is very close to the fuel added, sometimes a bit +, sometimes a bit - but usually within 1 or 2 %

Our dash odometer has always been about 400 miles behind VMSpc.  If it is changing it is very slow.

Miles per gallon is just calculated.

60 hrs out of 3100 is less than 2%.  No idea about accuracy or stability of engine hour meters.

96 miles out of 138497 is less than 0.1%.  Pretty good, I would say.

I will look for you at the GV.  Maybe figure it out there.  Cummins may be able to give us a tutorial on the ECM.  I would like that.

Roger

Roger Engdahl and Susan Green
2001 U320 3610 #5879 (Home2) - 2014 Jeep Cherokee or 2018 F150
Hastings, MN

Re: Dash vice Engine (VMSpc) differences change with mileage

Reply #2 –
Wish my 401(k) looked like that! 

Re: Dash vice Engine (VMSpc) differences change with mileage

Reply #3 –
Remember that changes in those ranges can happen with full or empty fuel, fresh, gray and black holding tanks. Ambient temperature can effect tire efficiency as well as pressure (read diameter and associated calculations) and road surface/condition will make changes in all the inputs that come into play with such long-term reporting. I say that if it gets me to point B from point A without spending a coach buck each time let it go. ^.^d
Larry
1996 U295 36'
Build # 4805
Actually we sold it but just like to lurk

Re: Dash vice Engine (VMSpc) differences change with mileage

Reply #4 –
Quote from: Roger & Susan in Home2 –
Are you reading miles and engine hours from dash gauges?  I am not sure where they get their data.
  Yes.

This brings up another thought.  My dash gauges very occasionally go blank.  This may help explain the deltas but not the noise.

Note that this is not bothering me but rather just causing me a big curiosity about how the ECM and the dash guages work.

Audrey & Harvey Nelson
2000 U-320, 40' towing 2014 Jeep Grand Cherokee
nelsonah@ieee.org

Re: Dash vice Engine (VMSpc) differences change with mileage

Reply #5 –

VMSpc offers the M11 in the engine selections but not the ISM. Wonder if using the M11 choice on an ISM affects any of the readings.
2003 GV320 4010