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Speedometer reading way too high, and transmission not shifting up

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I have had this happen to me three or four times in the past six months.

When first starting out on a drive, the speedometer would read way too high (like 80mph when I am only driving about 30), and the transmission not shifting.

What id did to resolve this issues was to pull over, turn the engine off, and then restart. The problem went away.

I am concerned that one of these times I may not get so lucky.

Anyone have something similar happen?


Cummins 5.9L ISB 26 /275LT /300


Allison Transmission

Model 2000 MH


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I have had a frequently occurring issue like yours with speed being high about 15 mph. I didn't do a stop start as you did. Mine settled down after driving for a while. 

My current issue is high RPM being way too high. Sometimes it settles down when I speed up the motor and other times it continues for a good while.

My feeling is that in my 96 Dynasty these are caused by my alternator.

I'd love to hear more about these problems and fix them.

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  • 2 weeks later...

I had this issue again, and thought about it a little more.

I have a steady speed readout on my speedometer normally, once I am underway without the speed increase and have no issue. 

When I have the problem, I know right away. 

I see the speedometer read way to high, like 80 mph when I am only going maybe 20.

This has happened  4 or 5 times now.

I just stop, shut the engine off and then restart. (that seems to reset the problem)



Does the speed sensor go directly to the speedometer, or does it go to the engine or transmission modules first?

I heard somewhere that the signal to the transmission or engine module comes from the speedometer.

My thought is that perhaps I have a speedometer issue.



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For what it's worth, the tach on my 97 Dynasty (mechanical C8.3) has an issue where it will jump up very high and immediately settle back to the correct RPMs. This occurs frequently when the engine is cold, and far less frequently when it is warm. From what I've read it appears likely to be an issue with the alternator, though a dodgy ground might also be a factor?

I replaced the belt and tensioner and noticed when I hand-turned the alternator that it made a ticking sound every rotation, but there was no play in the pulley and I couldn't "feel" anything corresponding to the ticking either, but now I noticed the tensioner is jumping a bit when it's running. I think I'll pull the alternator off and get a rebuild kit while it's in winter storage.

My speedo has been reading a little high, usually 2-5kms hour over. Last trip though it also started to jump way up and then back down, though staying high for minutes at a time. Not sure what could be causing this either, though dodgy ground connection seems likely. 

I will be refreshing my ground bus connections at the rear of the coach, and if there is one in front of the front firewall in the spring to see if that clears up the speedo issue.


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Thanks for the response

I am going to replace my VSS (Vehicle Speed Sensor) this weekend and give it a try.

However, my speed reading is steady and accurate going down the road, when I am not having the issues described.

I am hoping it is nothing more costly like my ECM.

My next step may be to find a Cummins shop




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