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Old 11-03-2014, 07:11 PM   #1
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Speedometer off after ECU update

I can't swear to it, but I'm pretty positive I had previously verified the speedometer in my 2013 Interstate read within 1mph of the Kenwood/Garmin GPS. Can someone check theirs and report back?

I took the Sprinter to the Mercedes dealer in Atlanta for my Service A update last week, and was informed they needed to apply a ECU update. I'm a big fan of if it ain't broke don't fix it - but went ahead and allowed the update.

After realizing I was running in limp mode 30 seconds after leaving, I returned for them to reapply the update which they said failed for some mysterious reason. After the 2nd effort, things appeared to work and I was on my way. Except now I notice when my speedometer says 53mph the GPS says I'm going 60mph. Great way to get a ticket had I not noticed this!

Mercedes says I have to bring it back as they aren't sure what would cause this. Seem like it would be a setting to compensate for rear end ratios or tire size variables, but they say there is nothing like that I can set in the ECU which I'm not sure I believe.

Anyone have deeper knowledge here? I may post something in the sprinter forums as well but thought I would check here first.

Brian
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Old 11-03-2014, 07:56 PM   #2
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Mine reads 71 when Garmin says 70.

If the tire size, rear end ratio, etc is not in the ECU, then I wonder where they stashed that info as it is certainly req'd to make the calculation.
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Old 11-03-2014, 08:06 PM   #3
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Mine has been pretty much dead on between the speedometer and the Garmin. We had the ECU updated in July with no change.


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Old 11-03-2014, 08:09 PM   #4
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I wonder if they installed an update intended for a different model - like a 2500 - if that's possible.
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Old 11-03-2014, 08:16 PM   #5
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Just so you are aware, GPS is government controlled satellite navigation supported for military use. Therefore the signal, hence accuracy can vary significantly from day to day, hour to hour, etc. I don't know that I would expect my GPS to be more accurate than my vehicle on any given day.
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Old 11-03-2014, 08:30 PM   #6
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Quote:
Originally Posted by bweybright View Post
Just so you are aware, GPS is government controlled satellite navigation supported for military use. Therefore the signal, hence accuracy can vary significantly from day to day, hour to hour, etc. I don't know that I would expect my GPS to be more accurate than my vehicle on any given day.
This is not accurate. The feature you describe (called "Selective Availability") is technically possible, but was discontinued in May of 2000 at the direction of President Clinton. The next generation of GPS will not even have this feature.

GPS.gov: Selective Availability
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Old 11-03-2014, 08:58 PM   #7
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Before I got the RennTech ECU tune, my analog speedometer, digital speedometer, and Garmin GPS were all within ±1 mph of each other. After I got the RennTech ECU tune, it's still the same, all within ±1 mph of each other.

Mind you, that's an aftermarket ECU tune, not a Mercedes ECU tune, even though I had it done at my local Sprinter dealer.
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Old 11-04-2014, 12:24 PM   #8
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The truck I use to tow the Airstream with is calibrated properly. The speedometer agrees with GPS.
My 1999 Nissan is off by 2 mph. 55 on the speedometer is 53 on Garmin. I believe Garmin because it is triangulating 3 satellites. I never have to worry about a speeding ticket in this car.
Our friends who purchased a 2004 Interstate over the summer have about a 1 mph variance in the speedometer versus Garmin GPS. When Garmin says 60 the speedometer says 61.
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Old 11-04-2014, 02:07 PM   #9
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I can also report the same thing......
WTH...same issue here.
I had my Schedule A done last week, along with software updates et al. Went on a trip to the Smoky Mountains this past weekend (yes it snowed a bunch) and my speedometer was off by about 5-8 mph. My wife called me from her car and warned me I was going like a bat out of hell through a known speed trap.
I turned on my GPS and it confirmed what my wife said. I was traveling much faster than my speedometer indicated...!!!!!!!!!!!
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Old 11-04-2014, 02:09 PM   #10
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and...I did verify it was dead-on with previous trips!!!
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Old 11-04-2014, 04:12 PM   #11
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Would appear that MB has a glitch in their software update.
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Old 11-04-2014, 05:36 PM   #12
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Quote:
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Would appear that MB has a glitch in their software update.
Guess it's a good thing I got the RennTech ECU tune so that MB doesn't tune it anymore…
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Old 11-04-2014, 07:23 PM   #13
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So you aren't or can't let MB do updates?
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Old 11-04-2014, 07:33 PM   #14
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So you aren't or can't let MB do updates?
If MB does an update that changes any of RennTech's programming, RennTech will redo it, free of charge. But since I had the work done through my local Sprinter dealer— they removed the ECU chip and sent it to RennTech— they have it on file that the program was installed, and they shouldn't accidentally undo it.

Since the RennTech tune provides greater fuel economy (only a couple miles per gallon, but still…) plus more horsepower and torque, it has proven well worth the money.
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