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Jittery speedometer. Where to start looking?


wamcneil
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Hi all,

I've got an intermittent problem with the speedometer. I suspect it's related to the speed sensor, wherever that is...

It usually starts happening several hours into a drive, and is made worse on a bumpy road. The speedo will drop by about half, then snap back up to speed, then sometimes shoot up 20% and immediately fall back to speed. This happens more or less randomly, but seems to be more frequent when the real axles hit a bump. 

When the cruise is engaged, it causes some jerkiness as the cruise control tries desperately to compensate for the wild speed changes.

Since it affects the cruise control, I'm thinking it's not gauge related. I figure the gauge is reading speed off the data bus.

Where does the system take speed information? I'm assuming there must be a sensor on the output shaft like a car would have?

Do these sensors fail, or is this more likely connections/wiring?

Thanks all,

Walter

 

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Mine was caused by a loose wire on the transmission.  I crimped it tighter and installed some zip ties to secure the wire and it seems to have cured it. 

The sending unit is on the back of the trans which in our coaches is the towards the front.   

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 I'm assuming the "Warning" light does not come on, on the dash, or you would have mentioned that. On my 2001 Dynasty ISC 370, the speedo has not been accurate since I purchased it from the original owner. The original owner said it has not been accurate for most of the time he owned the coach. After owning the coach for 4-5 years the warning light started coming on after about 10-20 minutes in to the drive but there were no performance issues, so I ran it like this for a while until I could get it in the shop. The shop said it was a speed sensor issue and replaced it (don't know where it was). It fixed the issue of the warning light coming on (after about 10-20 minutes in to the drive), but not the speedo issue. After using the coach 3-4 times now the warning light is coming back on again. I don't want to take it back to the same shop as they don't know what they are doing half the time, at least when working on my HWH system. I am planning to buy a fault code reader, but haven't figured out which one to by yet. I had the NEXAS Heavy Duty Truck Scanner NL102 OBD/EOBD+HDOBD Diagnostic Scanner Scan Tool but it could not read the speed sensor fault code when the warning light came on, even while I was driving and the warning light was still on and active, let along read the speed sensor fault code as a "history" fault code. The Cummins shop’s reader could read the code. Since my NEXAS device could not read the fault code, I’m returning it. FWIW, I also tried the VIDENT iEasy320HD Heavy Duty Truck OBD2 Scanner and it could not read the fault code so I returned that also.

 

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44 minutes ago, vito.a said:

Mine was caused by a loose wire on the transmission.  I crimped it tighter and installed some zip ties to secure the wire and it seems to have cured it. 

The sending unit is on the back of the trans which in our coaches is the towards the front.   

That sounds like a plausible source of my problem. It seems to be vibration and maybe also heat related. Mostly shows up on the bumpy last 20mi of the trip home. I'll crawl under there and tighten the output speed sensor connections.

38 minutes ago, Corkman said:

 I'm assuming the "Warning" light does not come on, on the dash, or you would have mentioned that. 

Right, no warning lights. 

I've got a bluefire diagnostic adapter, but didn't have any fault codes. Their user interface is pretty clunky and crude, but it works...

Edited by wamcneil
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Speedometer jumping is often the result of bad grounds. That's the first place I would look. You may have a wire braid from the chassis to the transmission that needs to be removed, cleaned up and reinstalled. I would also check for and clean up any wire braid from the chassis to the engine while you're at It. Good luck.

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I also believe it is a bad ground, because when I turn on my headlights all the readings on the gauges changes.

I'm just reluctant to open up the dashboard and look for it with all the wires so short. I don't think I can pull out the dashboard very far, and I'm afraid I might pull of some wires accidently.

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