The transmission yoke that the mechanic speaks of is the part that attaches the universal joint (at the front of the driveshaft) to the transmission output shaft. On the outside diameter of the yoke is a "tone wheel" which looks like evenly spaced teeth on the yoke. The VSS (Vehicle Speed Sensor) is mounted with it's probe in very close proximity to the tone wheel. As the trans output shaft (with yoke & tone wheel attached) rotates, the VSS creates a pulse each time one of those teeth on the tone wheel passes by it. These pulses are "read" by the ECM which sends a signal to the speedometer based on how many pulses are occurring in a given amount of time, causing the speedometer to display your MPH (or KPH).

Since the VSS is in such close proximity to the tone wheel on the yoke, a loose yoke can cause the tone wheel to actually strike the VSS probe, destroying the VSS. When that happens, the ECM puts your engine into "limp mode" since it is no longer receiving a signal from the VSS.

.

 

Thanks. I understand better the workings of a speedometer now. I've been doing research on Internet and compiled a short "cheat sheet" that I"m going to give to the mechanic tomorrow. Here's what I have so far:

"DEAD SPEEDOMETER & ODOMETER
Possible causes:

- wiring connectors, incl. those on the firewall (loose? corroded?)
- bad ground
- speed sensor and/or sensor plug
- transmission yoke and rear tranny cover
- ECM chassis harness
- frayed wiring/shorts somewhere between the VCC and ECM
- loose fuses/moisture in the fuse box

NOTE:
1) The tranny had some work done on it in January 2009 and was taken out and then put back in.
2) The windshield was replaced in July 2009.
3) I sprayed all wires/connectors with the Termin8or after the speedo died the first time about a month ago."

If anyone can think of any more causes, please let me know - I'll add them to the list for the benefit of the International Trucks mechanic. Thx!

 

i'm back from the dealer. they took the speed sensor out and found a very small crack in it, which wan't evident when the sensor was inside its socket. replaced the sensor. the speedometer is as good as new