Speedometer decreases at 60mph

[rk396]

I have a 1996 K3500 6.5l Diesel regular cab/full box pickup truck. When i reach 60 mph the speedometer starts to drop in speed (reading, i've have someone follow me, and the actual speed of the truck does not decrease any where near what my speedo is saying). once this happens the truck then shifts into 3rd, i'm assuming to try and keep up with the demand from the accelerator.

what i've done so far:

checked the wiring from both VSS on the tranny. i did this by jumpering the wires at the sensor, and checking for continuity at the computer. both sensor wiring looked good, showed about 5-6 ohm of resistance

checked both sensors: I mounted a metal gear in a lathe, and moved the sensor close to it. checked AC volts and HZ from the sensor. Both were consistent and appeared to be linear with lathe RPM. the first sensor showed a constant AC voltage, but an erratic HZ at high RPMs, I check again, and it wen't back to constant readings.

Hooked a scanner to the vehicle. unfortunately it looks like the 96 doesn't give a lot of access to PID's. I'm using a CP9185 scanner. If there's a better one i'm open to suggestions.

Cruise control works just fine (below 60mph). No codes of any kind are thrown.

I hate the idea of just throwing $$ at it i.e. new sensors etc.. I checked the gear on the x-fer case and it appears to be in place.

I'm hoping someone else has seen something similar, and can shed some thoughts on the problem.

thanks

[Schurkey]

I had speedo problems, but the symptoms were entirely different. My speedo would simply quit at random times, for random lengths of time. When the speedo quit, the odometer, cruise control, torque converter clutch, and ABS also quit working.

In my case, the circuit board was faulty. The speedo itself was good, the VSS and wiring was good, but the instrument cluster board was intermittent. The instrument cluster board takes the analog VSS signal, digitizes it, and divides it to a lower-frequency pulse that the speedo, odo, cruise, ABS, and ECM use for various purposes. This was on an '88, '96 may be different.