The RABs controller is a module located behind the glove box:

 

That is the point I was trying to point out.
The way the VSS signal gets to the PSOM is completely different from the way his truck is.
His VSS source is the rear end sensor and the source for the VSS in the diagram you posted is the 4 wheel brake module.

 

I think I got the PSOM right

 

rla,
OK, I disconnected the RABS module and the odometer still not working.
So, the way I understand the chain of events I need a new PSOM is that correct?
Thanks for your input.
Roger

 

Did you run this test as rla posted above?

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

Connect Digital Volt-Ohmmeter 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.

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Can you further define your statement "odometer still not working"? Are the digits on the odometer visible? If not then you very well could have a bad PSOM but it would not make the ABS light turn on. Fuse 8 supplies voltage to the PSOM at all times. Fuse 18 supplies voltage when the key is in the Run position only. Both fuses are located in the fuse panel under the dash. At least that is what I find in my 1992 Ford F-series EVTM CD.

This is a very good reason why you need to measure the VSS signal as noted before.

 

rla2005
I think I miss quoted what is has been done so far.
The total miles that is on the truck is the odometer and it is readable with the key on. The speedometer does not move and transmission is unpredictable.
When I disconnected the RABS module the abs light will not come on. I have checked all the fuses under the dash and all the fuses under the hood. all is well there.
I have not checked the voltage on pin#4 and 5 yet. I am not sure where to find the plug that has pin#4 & 5. At some point I need to re-think what is happening here.

 

You should be able to use this RABS plug for your voltage readings.
There are only two wires and it is an AC signal so you can put your probes on either wire.

 

The underhood does not look anything like the picture for the RABS plug, By the looks of the picture this is underhood on the passenger side? If so I have nothing but air conditioning and heating boxes, blowers etc.
I re-checked voltage at fuse #8 and #18 (Key on) 12.4 VDC.
One other clue? When I first turn on ignition the speedometer jumps about a 1/4" indicating a momentary pulse.
The ABS light is on, but the speedometer does not move when moving and transmission will not shift from 1st to 2nd. I have replaced the DSS on th rear axle. If I replace the RABS module and the PSOM, what else is there to check or replace.?

 

The photo is on the drivers side and a 1992 should have the plug at that location from looking at the 1992 wiring diagrams.

Just the wiring and the DSS gap on the rear end.
You might try unplugging the Speed Control Amplifier located behind lower center of I/P.
You have been leaving the RABS module unplugged while working this problem, have you not?

Speed Control Amplifier 9D844, Behind lower center of I/P (F4):



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SUBFORD,
I re-attached the RABS connector, I can unplug it if this will help isolate this problem.
I have located the RABS plug under the hood. If I understand this right I should put the truck on stands and run the drive train to approx 30 mph and the AC voltage should rise from 0-3.5 volts,correct? Assuming there is no voltage then a broken wire or the the DSS sending ring in the differential is bad, correct?
What should happen if I disconnect the speed control amplifier? Is there a test for this component?

 

I would leave it unplugged until you find the problem.
Yes

I would unplug the RABS module and the PSOM and then recheck for voltage. If no voltage then I would say what you said is true,

The PSOM has an out put going to the speed control amplifier. I do not know if the speed control amplifier can cause the PSOM to cause your problem or not. Just trying to cover all of the bases. I would not think it could.

 

SUBFORD,
Do the wires from the DSS go directly to the RABS connector? If so I could do a continuity check on those wires to a least eliminate that potential of a broken wire.
The DSS switch checks 1845 ohms and running a steel object passed the end it goes to 0 ohms momentarily.

 

The twisted pair leave the sensor and go up the drivers frame rail and to the plug on the firewall C205. From there on the inside of the cab they run across the back of the IP to the RABS module. They have a brake out before the RABS module near T/O to C202 of the main harness S248.
From the splice S248 the the wires also goes to the DSS test plug at the hood hinge going through the firewall at C202.

EDIT:
Note that the DSS sensor, RABS module, PSOM and the DSS test plug are all wired in parallel.



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Subford,
I disconnected the RABS module and have tested for AC voltage and DC voltage at the RABS test plug next to the hood hinge. No voltage. Can I assume that means a broken wire or the DSS not functioning? Should I have continuity between the test plug and the wires to the DSS ?
Also, does that mean that the problem is not with the PSOM or the RABS module?
Thanks for your help. I think the nightmare is getting closer to being solved, maybe.