Speedometer Dilemma

Electronic speedometer clipped into a '98 TranSport, but the model is really irrelevant. The speedometer and odometer are inoperative. The easy call would be a VSS problem. However, the VSS will output a solid 2.5V minimum signal at less than 3 MPH. The PCM receives the signal and displays the vehicle speed properly. It records the mileage, and operates the transaxle and TCC correctly. However, the speedometer still fails to indicate. The owner has changed the speedo head with a used one with no change. The speedo electronics are presenting 5.01V to the PCM for switching. I checked VSS output signal at C210 and directly at PCM. The signal frequency/duty cycle is correct, but amplitude is only about 6mV regardless of whether the speedometer load is present or not. Further, the speedometer indicates speed and displays an odometer change if that signal is grounded at 6-10Hz.

PCM Speed Data:
http://72.19.213.157/files/PCMData.jpg


PCM - Speedometer signal sinking output waveform:
http://72.19.213.157/files/SpeedoInputWave.jpg

Can someone tell me what the problem might be if not the PCM sinking output? I'm all ears...

I'm just trying to justify replacing an otherwise fully functional PCM.

Does the speedo head plug into a flexible PCB? If so, have you checked all of the traces and solder points for continuity?

It has good contact. I can back-probe the PCM/IP connector (C210) or the PCM itself and cycle the signal to ground manually, and get the speedometer to register. It's just that the PCM won't do the switching for me, but it seems to be trying.

I'm not at all familiar with these PCMs, but I am a little bit with electronics. If it has a buffer with a pull-up or pull-down resistor on the input, perhaps either that buffer transistor or the resistor is bad, perhaps a diode.

AFAIK (at least from the way the older ECMs worked) the sinking input is an NPN with a float up resistor, emitter at ground and collector on the "load" device. Unfortunately, they used hybrid modules in the output section. Good luck finding anything like than in the NTN catalog.