I have a 1998 Winnebago Chieftain on a Freightliner chassis. ISB 275 with Allison 6 speed transmission. The speedometer now does not respond, the odometer is stalled but visible. The Tach works correctly. I checked at the data plug with the VMSPC and find the odometer shows about 200 miles less than the VMspc data. It must have quit about 200 miles ago. Since I use the GPS as my speedometer, I did not notice. Does anyone have a clue as to where to start troubleshooting? Evidently the sender is working correctly as well as the Data Module. Perhaps a connection somewhere?

I'll bet that yours is a similar setup to ours. If so, here is what I learned:
1. The sender is in the transmission. If you are not also having transmission errors (shift inhibit, etc.) it is good and working fine.
2. The road speed data passes from the TCM on pin 26 on the gray connector (according to Allison). I have a 1000 transmission (5 speed) so it might be different but I doubt it.
3. On my Freightliner chassis, the road speed data passes through VIM (Vehicle Interface Module). The connectors on the VIM are acknowledged by Freightliner to be too stiff. It was either the input or the output to the VIM that caused me to loose my speedometer and odometer.
4. The next step in the wiring of the road speed data is the engine ECM and I was getting ECM errors (Road speed data invalid) which confirmed signal loss prior to the ECM.

Freightliner sent me a hardcopy of my wiring diagram. There were mistakes on it in the area of the VIM. After getting those corrected, I was able to trace the path of the road speed data from the output of the TCM (pulled the connector off) to the output of the VIM (also the input wire to the ECM) with an ohm meter. Since I worked on it and tightened up the VIM connectors, my speedo/odometer has not failed.

You should be able to swap the plugs on the back of the tach and speedometer, at least I can. The data is common to all plugs and the instrument that needs the data uses it. Obviously if you did that and the speedometer started to work, you'd have a different problem than I did. I had VMSpc which is a wiring harness which can feed a laptop. It collects data off of the chassis buss. In my case, VMSpc wasn't seeing the road data either. VMSpc also collects error messages and that is how I knew about the ECM error. I was getting a "check engine" light on my dash but only intermittently. My speedo was cutting in and out.

Let me know what else I can do to help you isolate this. Between the dealer and I, we spent nearly $1,000 letting a local Freightliner shop troubleshoot my speedo problem - and they never came close to fixing it.

Charlie

Thanks for your reply. I talked to FCC Support and was pleased with the knowledgeable response. I was sent via email a schematic for my coach. I guess on my model in 98 the instrument panel determines speed independently of the engine/transmission. Anyway, I lifted up the bed, reached down and unplugged two connectors from the box on the wall and re plugged. I didn't think anything changed as the odometer still read the same. But now it works. The Instrument panel odometer is now about 200 miles less than the engine computer. The connections looked all ok but must have been a loose connection.