First time post but long time peruser of this forum.
Here we go: 2005 F250 lariat, 6.0 diesel. 4x4.
SYMPTOM: Speedometer typically works normal for 10-30 minutes then starts dropping in speed, cruise control operates directly based on indicated speed and engine/ trans goes erratic so I have to immediately turn that off. Speedometer will drop from 70 to 55ish even though speed is maintained. At that point it can hover around the 40-60 mark or higher, or just drop down to nothing. If I go to neutral and key off then restart engine and continue on, speedometer will go back to normal, sometimes for a few minutes, sometimes for a few seconds. It seems completely random as to when it goes into failure mode, sometimes after 5-10 miles, sometimes not at all during a 50-60 mile drive. But most often It starts to occur around that 10-20 mile range. However, I can influence it in some ways, If the the indicated speed is floating around 15-20 mph less than actual speed, and i let all the off on the throttle, it will usually go to 0 mph. Get back into it, sometimes it comes back, sometimes not. If the a/c fan is on full blast and the speedometer starts to hover 15-20 less than actual speed and I turn off the a/c, it will typically go to zero.
DIAGNOSTICS PERFORMED:
I have done some fairly extensive diagnostics and thought for sure I had nailed it down to a bad PCM. Customer wanted me to throw the parts cannon at it at first to solve some other known issues. The issues described just do not sound like wiring to me. I can be on completely smooth road when the failure occurs and on rough road with the issue not active. I have replaced ALL 3 wheel speed sensors and have tested wiring from them all the way up to abs module. Replaced abs module with unit from donor vehicle. No codes read through abs module other than vin mismatch. I do not believe that is a factor because the issue was at hand before swapping module. When reading live data through abs module via scan tool, vehicle speed remains accurate even during failure mode and so do all 3 wheel speed sensors. When viewing live data through PCM, vehicle speed shown on scan tool matches identically with speedometer during failure mode.

ALTHOUGH WRONG, My thought was ABS reads good speed through wheel speed sensors, sends likely good signal to PCM (if it reads good at scan tool i would think it should read good at pcm) PCM failed internally and sends erroneous reading to speedometer.

REPAIRS PERFORMED: I have replaced the abs module with a donor, all new wheel speed sensors and verified good wiring, NEW (rebuilt) INSTRUMENT CLUSTER from circuit board medics, NEW (reman) PCM from flagship one with brand new TS+ tuner w/ custom tune.

MY THOUGHTS ON WHERE TO GO FROM HERE: I would like to have a complete wiring diagram of the can bus network. I understand that may be asking a bit much but I really want to know everything that talks on the same network as the PCM and speedometer. I am betting I have a bad module somewhere, maybe A/C or 4x4 that is muddying the waters just enough to **** of the PCM. Maybe it is a bad wire from abs to pcm but that is why I want wiring diagrams to help with that next step of the diag process. If anyone has access to those and is willing to pass them on to me, I sure would be grateful.

Sorry for the book, I sure would appreciate any help or insight available. I am betting it is a CAN issue but I have no experience diagnosing can on ford. I am primarily a JOHN DEERE technician and I know how those work just fine. Probably am going to unplug all unnecessary controllers: trailer brake, a/c, 4x4, stereo, etc. to see if the issue goes away, if it does I will plug in each module one by one until the issue returns.

UPDATE: I have unplugged the a/c module, the trailer brake module, and swapped out the 4x4 module with a known good from a donor vehicle. All to no effect. Issue persists as did before.
Side note: trailer control module rapidly blinks on and off when trailer is plugged in. Repeatedly dings on the dash "trailer disconnected". I have replaced both trailer side and vehicle side connectors. No effect.
Another side note: 4x4 does not work. no lights on dash when switch position is changed, switch is properly backlit and has also been replaced. No effect.

THANKS GUYS!!!!!

 

Can’t help without knowing what truck you have, a lot of changes between 99-16

 

According to his profile, he has a 2005 F-250.

 

Sorry, my mistake, 2005 F250 lariat.

 

On 05-07 trucks VSS is sent out from the ABS module on a single wire that goes to the PCM, 4x4 module, radio and wiper module. I would unplug everything but the pcm, go for a drive, if you have either a frequency meter or a scope I would hook that up to the VSS line and monitor it for drop outs or other weird behaviour.

This is not a CAN bus problem

 

Finally paid JustAnswer to talk with a ford certified tech. He gave me wiring diagrams and encouraged me to load test all positives and grounds for abs. I found a bad ground near the driver headlight. 150 miles so far and issue seems resolved. Another 2-300 miles before I am confident. Will update if anything further is found.