Quote:
Originally Posted by Jason7121
Far as I can tell I have a single computer speed signal going to the sn74z speedo in terminal. Switches 1-5 off on the sn74z on the back of speedo switches 5,6,7,8 open. I used the marked mile method and according to GPS it's right on the money. According to my notes I called Classic instruments and that's what they told me to do. Gm uses 16,000 ppm for speedo.
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Thanks, Jason. From what I can find online, I think several things are going on.
1. There's an orange wire coming out of the GMPP harness marked SPEEDOUT that's coming from the TCU. It is what I currently have connected to the SPEEDO IN terminal on the SN74Z with switch settings as you described.
2. There's a brown wire (circuit 818) that passes thru the 12 pin bulkhead connector that is supposed to be Vehicle Speed - Out. The I-sheet states it is a non-scaled output for use with auto-scaling speedometers and will not function unless a vehicle speed sensor is connected to the ECM thru the VSS wire in the harness. This is the wire the GMPP help desk tech referenced in our conversation Friday when discussing the inoperative speedo. However, as far as I can tell, there's no non-terminated wire in the harness that ties back to it.
3. Again, as best I can tell, the plug marked VEH SPD is meant to accept a sine wave that varies between 0-5V. The I-sheet states it's optional and not needed for engine operation... which is true to a point. But, apparently, without it there may be idle and stalling problems per
this article. While it may ultimately be a driveability, fuel mileage issue, I don't think it's relevant to the speedo not working.
So with all that, do you recall the wire color coming out of your harness that goes to the speedo in on the SN74Z? If your setup is like mine, the harness wire is too short to make the trip and had to be spliced.