10-17-2017, 08:24 AM
According to my Allison manual, the sensor in Larry's picture is indeed a speed sensor. It sends its signal to the ECU and is used by the ECU to determine if the output rotational speed is as expected for the gear the transmission is in and the input rpm's from the engine. If the output rpm is below the expected the "check transmission" light is turned on.
The ECU uses that output shaft rpm signal input to create an output signal for use by a speedometer. That signal is a switched 0.5v-3.5v on-off signal with a frequency consistent, though I cannot say identical, to the transmission output shafts signal.
Since the transmission has not thrown a check light, the ECU finds no problem with the output shafts speed sensor. The problem could be the ECU failing to provide the signal to the speedometer, a open circuit between the ECU and the speedometer, or a failed speedometer. I can't find any reference that the ECU tests for any of this.
I just got home and got the manual. I will admit my brain function is diminished due to the Amtrak trip between Essex, MT and home being delayed 3 times in MT. Once due to the preceeding train being stuck on a hill due to insufficient power, once due to high winds, and once from a derailment ahead that was probably from empty grain hopper cars being blown off the tracks into the path of a passing train of empty tank cars. Total of 40+ cars off the tracks and parts scattered everywhere. Thankfully crews are OK.
The ECU uses that output shaft rpm signal input to create an output signal for use by a speedometer. That signal is a switched 0.5v-3.5v on-off signal with a frequency consistent, though I cannot say identical, to the transmission output shafts signal.
Since the transmission has not thrown a check light, the ECU finds no problem with the output shafts speed sensor. The problem could be the ECU failing to provide the signal to the speedometer, a open circuit between the ECU and the speedometer, or a failed speedometer. I can't find any reference that the ECU tests for any of this.
I just got home and got the manual. I will admit my brain function is diminished due to the Amtrak trip between Essex, MT and home being delayed 3 times in MT. Once due to the preceeding train being stuck on a hill due to insufficient power, once due to high winds, and once from a derailment ahead that was probably from empty grain hopper cars being blown off the tracks into the path of a passing train of empty tank cars. Total of 40+ cars off the tracks and parts scattered everywhere. Thankfully crews are OK.
Jon Kabbe
1993 coach 337 with Civic towed