When I bench checked the speedometer and it will read 30 MPH at 667 Hz and 3.5 volts AC voltage level and the RABS module was not plugged in.
I may be able to load the circuit with a RABS module and do the test again but I would think the output voltage of the VSS should not change.

The information on the 30 MPH and checking the voltage is from the 1995 Ford F-series Shop Manual.

Here is what it says:

Pinpoint Tests

PINPOINT TEST G: SPEEDOMETER INOPERATIVE

G8 CHECK SPEED INPUT SIGNAL

NOTE:
Only wiring harness end of connector is to be probed.

Connect Rotunda Digital Volt-Ohmmeter 014-00407 or equivalent to Pin 4 (speed in +) and Pin 5 (speed in -).

Does the voltage increase smoothly and continuously from 0 to approximately 3.5 volts as vehicle speed increases from 0 to 48 km/h (0 to 30 mph)?
OR,

If available, a frequency counter may be connected to Pin 4 (Speed in +) and Pin 5 (Speed in -).

Does the displayed frequency of the signal increase smoothly and continuously from 0 to approximately 667 Hz at approximately 48 km/h (30 mph)?
OR,

If neither a voltmeter nor frequency counter is available, vehicle speed control may be used as a good indicator. If it works normally, then the speedometer module is at least receiving a speed input signal and the wiring and sensor can be assumed to be good.

Does the speed control work normally?

Yes
REPLACE the instrument cluster as described in this section.

No
SERVICE wiring. CHECK for open or shorted wiring. If no problems are found, REFER to «Section 06-09A» or «Section 06-09B».

================================================== ========================================

Section 06-09A: Brake, Rear, Anti-Lock Control

PINPOINT TEST U: REAR ABS AND SPEEDOMETER ARE NOT FUNCTIONING CORRECTLY

This procedure is intended to determine whether the cause for loss of or erratic Programmable Speedometer/Odometer Module (PSOM) function is due to the lack of a signal from the RABS Sensor.

There are two basic reasons that PSOM would not receive a signal from the RABS Sensor:

(1) The RABS sensor or the signal from the RABS sensor is faulty and either not being generated or is not reaching the anti-lock electronic control module (2C018) and/or PSOM. In cases where the concern is not intermittent and not reaching the anti-lock electronic control module, the rear ABS warning light will be on upon vehicle start up and a diagnostic trouble code should be obtained upon RABS II diagnostic trouble code retrieval. Where the concern is intermittent and the rear ABS warning light does NOT come on during vehicle start up, a diagnostic trouble code may still be stored in the anti-lock electronic control module (see the «Symptom Chart» ).

(2) The wiring or interconnections in the vehicle harness between the anti-lock electronic control module and the PSOM is faulty. This may lead to erratic PSOM function and is difficult to diagnose.

This procedure will only determine whether the RABS sensor is functioning based upon whether a signal is received at the PSOM. If it is determined that the RABS sensor is sending out a proper signal, refer to «Section 13-01A», PSOM System Diagnosis.

U1: DETERMINE STATE OF ABS WARNING LIGHT
U2: OBTAIN REAR ABS CODE
U3: VERIFY RABS SENSOR SPEED SIGNAL AT ANTI-LOCK ELECTRONIC CONTROL MODULE

================================================== ===========================================

U3 VERIFY RABS SENSOR SPEED SIGNAL AT ANTI-LOCK ELECTRONIC CONTROL MODULE


Key OFF.

Remove the harness connector from the anti-lock electronic control module.

Raise the vehicle so that all the wheels (1007) are clear of the ground. If the vehicle is a 4x4, verify the transfer case is in the 4x2 mode.

Connect a Hand-Held Automotive Meter, Rotunda 105-00053 or equivalent between harness connector Pin 10 (Circuit 523-R/PK) and Pin 3 (Circuit 519 -- LG/BK).

Set the meter to the frequency counter (Hz) setting.

Gradually spin up the rear wheels.

Does the frequency increase as the rear wheel speed increases?

Yes
Anti-lock electronic control module is receiving a proper signal. REFER to «Section 13-01A».

No
GO to Pinpoint Test K, step «K11» .

 

but I would think the output voltage of the VSS should not change.

I was working under the same assumption, the voltage remains at a constant at a given rear wheel speed regardless of load. like household AC current does for example, Amp's increase with additional loading but the voltage does not.

Guess I been lucky so far, never had to delve into this system to this degree and no, no electrical engineer here.

Ok let me ask this, the fact the speedo reads 30mph at voltages less then 3.5 volts, doesn't that tend to suggest the PSOM is faulty?

If it wasn't rather nasty out today I'd test mine see where its at, granted mine is adjusted some for larger tire size but should be in the neighborhood of 3.5v at 30mph.

A road test might shed some light? run with another vehicle at 30mph. Yea the other vehicles speedo might be a mph off at 30 but close enough

If the truck has a tach how many rpms running in drive at 30mph? use it as a base line for rear wheel speed while up on the stands.
If not, connect the meter where its in view of the driver, test the output of the VSS while running with the other vehicle. Test only the VSS, no PSOM and no RABS first, then add them in one at a time to see what effect each has on voltage reading.

Just strikes me as odd the speedo requires approximately 3.5volts to read 30mph, that is if I understand correctly, yours reads 30mph at vastly differing voltages, voltages well under that.

 

The reason I said the the voltage would not change is because on the bench test the sign wave generator was set to 3.5 VAC P to P @ 667 Hz.
If I plugged in another item (RABS module) the bench generator would hold the voltage the same.

I do not know about it mounted on the truck.

The VSS sensor is this: