The only reason I mentioned the PSOM is the mention of bouncing speedo issues.

Sometimes you got to fix one fault to see another.

 

 

Replace all of the capacitors in that computer. PSOM is known to have bad solder joints. Sometimes components falls off. Other times the bad joint is at the connector header.

 

 

Odd question, after the odo and speedo went blank, does your dome light work? Had a buddy's truck blow the dome fuse and it started shifting odd

 

My 9yr old and I run RC trucks. This summer he'd climb up and sit on the cab. TWICE after he'd do that the PSOM/dome light fuse was blown. He doesn't get up there anymore.I haven't taken the headliner down to see the pinch point or whatevers going on.

 

My first involvement with an OBS was 95-96 F250 powerstroke I drove for a carwash builder & service co .

The drums I frequently hauled would bust the high mount brake & bed light & fill with water when testing out the washes.

It'd kill the OD (on/off light flashing) & put it in limp regularly till I started carrying whatever fuse it was blowing after the 3rd trip to the dealer.

 

Does this have a power seat? Long time ago my power seat wiring was damaged, and shorted if the seat was moved to certain places. This blew a fuse, and took out speedometer function. My memory says the truck stayed in 1st, but I never made it a block before fixing it.

 

All my fuses are good. The only other electrical problems I have noticed are 1)the door switch has never worked to turn on the dome light. Dome light switch works. 2) when the speedo goes out, I noticed the head light switch gets shifty. The headlights will go off until I fiddle with the **** and even still, doesn't come on right. This is the first I have noticed this as this is the first time I have driven it at night. Not sure if there may be a connection between the headlights and the VSS circuit.

 

Headlights are a coincidence. The Positive lead was loose.
I'm really at a loss. I've traced the wires from the VSS in the differential all the way to the PSOM and I get 6.7 volts at those pins on the PSOM connector (this is with the truck running and stationary. I'm not sure what it would be if I jacked up the rear and read voltage then). The PSOM has good voltage with the ignition in the off and run positions. I have tested every single fuse on the truck with my meter and they all tone.

Does anyone know what the voltage reading should be going into the PSOM from the VSS? 6.7v seems low but if I'm thinking of the way the PSOM would read the VSS, it should max out the speedo at 12v. So maybe it's alright. This is the 2nd replacement VSS. This one is a Ford part.

What gets me is that if I leave the truck alone for a few days. Come back and start it. The odo lights up. then after a couple minutes, shuts off...

IDK what to do next besides limp it to the dealer and tell them to fix the darned thing.

 

So has the computer been repaired yet? Codes run afterward?

Haven't seen any mention & if it hasn't, you're wasting time & effort.

 

 

should have just swapped in a manual.

 

452 is your vss. You can always try cleaning it. Sandy

 

The PSOM uses two fuses. Fuse 18 is to keep the trip odometer memory. Fuse 8 supplies power only when the ignition key is in the Run position.

VSS circuit diagram:

courtesy of Subford


The VSS output is a self generated sine/square wave (AC):
A basic diagram
Attachment 279845
courtesy of Subford

Your DC measurement of 6.7 volts when the truck is stationary is odd, but I have never metered the circuit with the meter set to DC. Output voltage is 0 VAC with the truck stationary.

Some info on how to test the VSS output/PSOM input:

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)?


You are losing power to the PSOM or you have another bad one. Hook up a real meter to the power lead from Fuse 8. Verify there is power whenever the key is in the Run position. If not, there is the major reason for your Code 452. If power is still there at Fuse 8 feed/Key in Run position then measure power on the PSOM itself. Perhaps you have a bad connection or connector.

Last place to ever take your truck. Most of them have no idea how the circuit works.