The VSS does not receive any signal. It produces a square wave signal but does not receive anything.

The PCM is saying it didn't see a sufficient signal from the VSS. It can be the sensor, the wiring, the ABS, or the PSOM.

 

here is how that VSS signal gets to the PCM ...




This is From the sensor in the Differential to the PSOM ^^^^^^^^^^




This is where it Comes out of the PSOM to go to the PCM ^^^^^^^^^^^^ EDIT: my bad on the Green Label I wrote 697.. it should be 679 sorry

this Last image is the PSOM Connector .......





I'm not sure I can give any better instruction on how to test this other than to test it myself

AND as Mark says the signal on Pin #3 of the PCM is a Square Wave signal that changes in Frequency and is best seen using an Oscilloscope, so you can at best really only check the wiring to make sure all connections are good, IF your meter can read Pulses (has a Freq Counter Function) then you can detect the signal ...
You can set your meter to the AC scale and possibly see the AC component of the pulsing signal but this is only an indicator that the signal is there and I doubt you would ever see the signal drop.

 

I read somewhere to disconnect battery ground before checking continuity in the PSOM/PCM. True? I may have caused this problem because I checked grounds on wiper governor without disconnecting ground.
Can I disconnect VSS, PSOM, PCM and just "ring" the wires between same?
Can you tell me where connector C202 and C252 are located? You maybe already told me but I'm not sure. Is one the big one on the firewall by the steering column?
Will I be able to ring through RABS?
I am told there is a connection behind the cluster that has several wires in it before it runs to the frame ground. I'm told that is sometimes a problem. I should be able to check continuity between PSOM and frame ground to verify it's continuity. (disconnect connection at PSOM first?)

 

It is always Prudent to disconnect the batteries when working on the electrical system unless needed for voltage checks, BUT Modules and connectors should not be connected or disconnected while the battery is connected as it CAN cause damage to components. <<=== this is Generally always the first warning you see in most all service manuals ... it is often ignored and most times there are no issues BUT the ONE time that it does happen you end up Kicking yerself, as it could have been prevented

yes you can, IN Fact this is a good way to do things.... and you do not need the Batteries connected to ring out wires.

yes those are the firewall connectors Well C202 and C205 are the firewall connectors and also the PCM connector is on the firewall under the Brake booster and you will need to remove half a dozen screws that hold the Plastic fender well to the fender and frame to have good access to the PCM connector... it's just easier this way.
And Connector C252 is the actual Connector on the PSOM itself..... you can search Youtube for 93 F250 PCM replacement there are several videos that show you what fender well screws to remove.


The RABS module is behind the Glove box .. mine is a silver metal can... I would disconnect it while ringing out wires.

you will have the Cluster removed for ease of Maintenance so you shouldn't have trouble following the Harnesses .

 

Ok the black connector on the PSOM is C252

the area in the Red Circle is a MOV a Diode and a Capacitor ..the Red Arrow is showing another Capacitor....... I normally replace these Capacitors on any of these older trucks when I find myself digging into the Cluster, they leak and cause issues.

The Integrated Circuit marked by the Green Arrow is IC1 it is what actually takes the VSS sensor signal and conditions it to a Square Wave for the PCM...

 

so with what has been discussed IMHO IF the wires all Ring out Good you will need to check IF there is a Squarewave signal on circuit 679 as shown on my previous Schematics in Post # 17 the middle image.

IF the squarewave signal is not Present then there is an Issue with the PSOM... IF the squarewave IS present then the issue is the PCM.

Hopefully it is just bad wire connection.

Corrosion X is what I use on automotive and Marine Electrical connectors, it's not cheap stuff but then what is anymore

EDIT: here is a video of getting to the PCM and some of the issues with the PCM.. while this is not a Diesel ... it's the Same Difference


BTW Lowes has that 5.5 mm socket... I went thru Hell looking thru the hundreds of sockets I have and found everything except the 5.5 LMAO

 

Tom ... where are you at with your troubleshooting ???

I haven't seen a post lately... is there more I can do to help ???

IF you are stuck on something let me know, I'll try help you ....

I can't help IF I don't get feedback......


Anyway you are not forgotten

 

lonewolf. Sorry no reply lately. I gave up and brought to transmission shop. He had repaired electrical issues in the past so I figured to try him again. He replaced PSOM that he had rebuilt a few years ago, found a bad connection to same. Repaired connection and got clear codes. Test drive, no flashing O/D but still not shifting smooth. Adjusted FIPL to obtain smooth shift. Test drive, all well. Later in day he test drove again and back to same problem (flashing O/D, hard shift) He wanted to know what he should be reading at 679 at the PCM? Is that the digital signal that comes from PSOM? I hated turning this over to someone else but I couldn't physically continue with the troubleshoot. I have no idea what t-shooting equipment he has.
Thank you for hanging in there

 

OK I found post from Mark saying pin 3 is square wave sign from PSOM

 

yes 679 is the signal from the PSOM

well, looks like it was not a Problem with the PCM since your Mechanic changed that. so that sure makes me think the issue is wiring from the PCM to the PSOM or the PSOM itself.

working on these beasts can be taxing for us Old Farts... I know .... it takes me long time to get things done now.

 

Correction: He did not change the PCM

 

OOPs My bad... ya I see he changed out PSOM ..... I must have Misread that.... so IF all that wiring is good and the Square Wave signal is good it could Be Dropping inside the PCM.

 

Good news. My mechanic just called. A old guy he knows (older than us) has all the tech equipment needed to properly t-shoot this problem. Says he hasn't used it in 35 years. All that stuff getting old. I'm hoping. I'll let you know

 

Well, that was a bust. The guy with the testing equipment never showed up. We have never verified the signal from the PSOM to the PCM. He thinks it is the PCM. I am about to order one on ebay. There is only 1 available. For some reason, the one I pulled out is F4TF12B565AA which is listed for a 94 with a 7.3 in it. I don't know why that is.Do you know of another PCM that is compatible? The only one listed is used, not rebuilt.
If you or anyone on the Forum knows of anyone who has the measuring equipment for this problem that lives anywhere near the Florida Space Coast (Brevard County) I would love to see if they would be willing to look at my truck.
I appreciate you hanging in there and assisting me

 

Here is the listing of the latest and Greatest PCM's for our trucks ...https://www.ford-trucks.com/forums/1...3-f250-xl.html