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Author Topic: ECM VS SPEEDO/VSS  (Read 1461 times)

Offline plymouthrunner

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ECM VS SPEEDO/VSS
« on: December 26, 2014, 04:49:48 pm »
coming over from DF looking for answers/suggestions .have an 04 ram 1500 4X4 5.7 ST for a couple of years with no problems . threw a egr code ,now fixed ,but noticed ecm is reading slower than speedo approx. 25 mph at 65 mph haven't noticed any issues just bothers me. have been told it was tires and pinion factor needed reset/checked .took it in to dealership reflashed ecm and set tires to largest they could 275R75/17 running 285R75/17 and no change in speedo/ecm readings . worried it may be damaging trans due to lower shifts any suggestions ?

Offline bikemancs

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Re: ECM VS SPEEDO/VSS
« Reply #1 on: December 26, 2014, 11:34:58 pm »
I don't think your 285s should be creating a 25 mph difference in speed. I would say the real issue would be in your mileage accumulation.  Is the odometer rolling normally? or is it the possibility it's the actual speedometer that is showing the wrong info? Does it feel like the shift points are in the right spot when you are accelerating?


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Offline plymouthrunner

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Re: ECM VS SPEEDO/VSS
« Reply #2 on: December 26, 2014, 11:57:01 pm »
yes the odometer is working correctly ,previous owner ? had the ECM replaced as it Chrysler marked ,so the odometer is about 70 miles off and is staying the same since I noticed this .

Offline bikemancs

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Re: ECM VS SPEEDO/VSS
« Reply #3 on: December 27, 2014, 04:41:00 am »
maybe there is an issue in the actual speedometer. like a broken spring or something... if the odometer is rolling correctly, then there *shouldn't* be an ECM issue... I think.  Sucks as to diagnose you'd probably have to take apart half the dash... but... If you have a GPS it might be worth it to just use the 'dashboard' feature on the GPS to give you actual speed.


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Offline Drew

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Re: ECM VS SPEEDO/VSS
« Reply #4 on: December 28, 2014, 03:22:38 am »
That slight change in tire size won't affect anything. Going from 265's to 315's was a 5mph change at 70 for me.

Anyway, what it sounds like is you have a speed sensor that has gone bad. There is one in the rear axel, check that one out first. I don't have the manual in front of me but you should see 55-90 ohms if you put a multimeter to it. Once again I could be wrong as I'm not looking at the manual. second, check the ABS sensors in the front. They do play a role and one could be bad.

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Offline plymouthrunner

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Re: ECM VS SPEEDO/VSS
« Reply #5 on: December 28, 2014, 11:31:23 pm »
will have to check on tomorrow ,rainy today . I had to build the front end shortly after buying as it was shot .I question that at the dealer if possible VSS was the problem . Have been trying to locate wiring diagrams for it to chase this down . ie where does ecm and speedo get their readings from sensor wise . thanks will check back and update on what I find .

 

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