Speedometer Issue

[tnrvers]

For several years when we take our 3, 4 or 5 month trips, we have periodically had an issue where the speedometer starts increasing speed and then just pegs out. When this happens the odometer increases the rate at which a mile is registered and the engine ECM will still control the vehicle speed if the cruise control is set and there is no apparent difference in transmission shift points. I have read through several of the previous speedometer posts and I’m not sure anyone of them correlate, but I'm willing to be wrong.

The first time this happened I thought it related to the speed sensor located on the transmission. That first time and every subsequent time until just a week ago I would remove the electrical plug from the sensor and use either an electrical connector or other dry film cleaner. Spraying the cleaner onto the plug end and into the sensor receptacle, when dry, reinstalling the plug and the speedometer would work correctly. This effort never failed to work and did not need to be repeated for days, weeks or months.

During these passing years I thought it must have been some kind of moisture on/in the plug or sensor receptacle, however now I’m questioning that logic. The other day after driving a little over 300 miles we were a short distance from shutting down for the night when the speedometer acted up. I let it be until after we stopped for the night and then cleaned it. Everything was fine the next day for just over 300 mile and like the previous day, we were just short of stopping, so again I did nothing until we shut down. Both days were sunny and relatively warm, mid to upper 80’s so the heat coming off the road surface would have been warmer than that and combining with the operational heat got me thinking.

My thoughts were that it was strange this happened two consecutive days under close to identical conditions, so maybe something else might be in play here, heat. That evening I did nothing and can happily report the speedometer worked perfectly the next day.

So my questions are; has anyone else had this issue and what was done to correct it? Can a speed sensor fail in this manner and if so how do you replace it? Are any special tools, procedures or measurements needed to replace it? Is this a DIY kind of a procedure or do I need to take it to a shop?

We have a 2000 Fleetwood Bounder 39Z with the Cummins ISB and the Allison Transmission Model MT 643.

Any and all ideas and suggestion as to what is going on with the speedometer/sensor are welcome. If my thought, this issue is somehow related to heat, is in error please explain where I’m wrong. If you have an idea why this issue is happening, please include the logic to support your thoughts.

Thank You
Tom and Barb
2000 Fleetwood Bounder 39Z

[Skip426]

I'm not sure about your transmission , but I'd think that a faulty sensor would also give some kind of shifting issues or a check trans light.

I have this pdf from Freightliner on testing the gauges .

[tnrvers]

Skip426,

Thank you for your reply and I also believe a bad sensor would cause shifting issue which we haven't experienced. Also, thanks for the pdf file I'll down load it and take a look.

Good news on this issue, a very knowledgeable gentleman from our Bounder forum had the exact, in every way, issue as we. After much chasing and investigating, it turned out to the the noise level being transmitted down the 12 volt line from the alternator and as the heat within the engine compartment increased, up went the electrical noise, and triggered the speedometer malfunction. It sound way out there but I've had noise issued on electrical line in my early life and totally aware of strange things the noise can cause.

Thanks again for your input.
Tom and Barb
2000 Fleetwood Bounder 39z