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Discussion Starter #1
Greetings Ford 7.3 fans. I have a beautiful 2001 F250 7.3 in excellent condition. I baby the truck. I need some help to see if I can find some recommendations.

My OD started blinking and lost my cruise control. The truck runs and drive excellent with no issues with transmission shifting funny.

The codes thrown are the speed sensor. Changed rear speed sensor, sensor above the transmission, had a new ABS module installed as we thought that was the issue-nope.

Even replaced the PCM as well. No changes.

I am at a loss and pulling my hair out. I have no issues w my speedometer or shifting or anything but I still have this pain in the next thorn in my side that will not go away.

When I start the truck no symptoms. It only starts once I start driving. If a go real slow, like 5 mph the OD wont flash but once I start going the flashing starts.

A real pain in the rear. Any help would be great. And I've read through many of the posts about the flashing OD but haven't been able to find the solution.
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What code did you pull? Is it still flashing? Maybe clear the codes and see what comes back. What code reader are you using? Most generic devices won't read Powerstroke codes and even if they do, they might not get the transmission code.
 

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You changed the wrong sensor. The sensor that controls the shifting and the cruise control is located on top of the rear differential, not on the transmission.

The flashing OD light just means that there is a code set. There is no one fix for a flashing OD light.
 

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  • What code did you pull? Is it still flashing? Maybe clear the codes and see what comes back. What code reader are you using? Most generic devices won't read Powerstroke codes and even if they do, they might not get the transmission code.
The codes are P0500
P0541 P1501

These are the codes pulled from a Ford scanner or a scanner used at Ford.
You changed the wrong sensor. The sensor that controls the shifting and the cruise control is located on top of the rear differential, not on the transmission.

The flashing OD light just means that there is a code set. There is no one fix for a flashing OD light.

Hi Mark. The one above the differential was also changed as well.😞
 

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Then there is a problem with the wiring to the sensor on the rear axle.
 

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Well, two of those are VSS codes. I had a similar problem. No change with a new VSS. I went to change the pigtail and when I cut the wire, the copper had oxidized to a black powder. I had to cut out several feet before I found clean copper. Maybe you have a similar issue.


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The SECOND pinpoint test for the VSS codes instructs us to diagnose the speed sensor circuits. Both of them from the sensor to the PCM and the specification is "less than 5 ohms" which is BS Ford diagnostics. LOAD test both circuits measuring the voltage drop ti identify resistance in the circuits and perform a wiggle test to identify intermittent open or shorted circuit conditions.

 

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Discussion Starter #8
Thanks a ton for the great guidance. I will check those recommendations.
  • What code did you pull? Is it still flashing? Maybe clear the codes and see what comes back. What code reader are you using? Most generic devices won't read Powerstroke codes and even if they do, they might not get the transmission code.
The codes are P0500
P0541 P1501

These are the codes pulled from a Ford scanner or a scanner used at Ford.


The SECOND pinpoint test for the VSS codes instructs us to diagnose the speed sensor circuits. Both of them from the sensor to the PCM and the specification is "less than 5 ohms" which is BS Ford diagnostics. LOAD test both circuits measuring the voltage drop ti identify resistance in the circuits and perform a wiggle test to identify intermittent open or shorted circuit conditions.

 

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Discussion Starter #9
So according to the wiring diagrams the vss signal comes from diff sensor to abs module and from there goes to everyplace it needs to go. I checked the pins at pcm. Load tested the wire and checked the signal to pcm with a lab scope. Signal is good. Ford diagnosed it as a bad pcm. It was replaced at ford dealer. The od did not work before pcm was replaced. Now it works. Watching live data for vss. No signal. Its at another dealer now. They say tech assistance claims the alt can cause issues. To check it. Anybody seen that?
 

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I have seen where an alternator with a bad diode was putting an AC signal on top of the DC voltage. This AC component was interpreted by the PCM as a speed signal.
 

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I have seen where an alternator with a bad diode was putting an AC signal on top of the DC voltage. This AC component was interpreted by the PCM as a speed signal.

Thanks Mark. Will get the alternator tested. At this point I really hope that is it. I'm crossing my fingers.
 
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