Sorry about that. I was looking at the photo on page two then skipped down to page four which is for the HT 746 but the connections are in the same location. Page 6 shows a cutaway of the HT 748 but it appears to show the location of speedometer gear drive to be similar to external connections shown for the 741 and the 746.
It looks like those connections are on top or am I looking at the picture wrong?
You are correct. Those photos do not show connects under the transmission.
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.
I believe on page one of this thread, Larry indicated that his transmission check light had come on.
If the check light came on about the time the gauge quit then there probably is a problem with the sensor or its wiring. This can be confirmed if there is a transmission check switch which you hold in the "on" position with the ignition on. The check transmission light Wil flash a code. If it is code 22, then the ECU is saying it has a problem getting reliable output shaft speed.
You can check for a wiring problem by measuring the resistance between the two contacts in the top row of the J1B connector in its right corner. The J1B connector is is the upper right one when facing the connector end of the ECU. The resistance be 115-165 ohms including the sensor.
According to my manual the sensor is not adjustable on on-highway transmissions.
Larry, on Greenie there were two mag pickups at the out put of the tranny. Mounted outboard and picking up their signal from a toothed wheel on the tailshaft. since Greenie was an 1989 and had a 741, i thought Newell might have used the same setup.
One MPU was for the speedo, and the other was used by the cruise control for the Silver 92. Somebody has mentioned the gap previously and testing is not difficult but you'd need a meter that reads frequency. Not common for MPUs to fail but possible.
Happy 80th
Thanks for the updates, I talked to my service center yesterday and they recommend that I take it to the Allison service center in Ft. Wayne, about 60 miles from here. It is due for the recommended 250k service, which I am told requires new fluids, filter and pan gaskets. (maybe sensors?) I'm a little concerned about driving it that far with the Ck Trans light on but I drove it to the service center last week with no problems (about 20 miles round trip) Any thoughts?
Larry,
There is always the black vinyl electrical tape repair. Just put a piece of tape over the light!