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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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Discussion Starter #4

  • 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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Discussion Starter #11
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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Discussion Starter #13
Still trying to figure out my flashing OD overdrive. One thing to note to some of you pros that when I first start the truck there is no OD flashing light. When I put it in drive all is well. If I drive slow the light still doesn't flash. It doesn't start flashing until the truck hits usually around 15-20mph.

Again the 2 VSS coded are
codes are P0500 and P1501.

The strange thing is the sensor is fine and has been tested and replaced. And looks like the VSS signal goes all the way to the PCM.

The PCM was replaced but I still have the same issue. It would seem extremely rare that if it was the PCM that was the issue, that the replacement PCM would coincidentally have the same problem. Seems highly unlikely.

The alternator was checked and that wasn't it and nothing funny with the transmission shifting at any speed.

Does anyone have any additional thoughts or experience of what this could be? I'm desperate to get this fixed.

Thanks in advance!

(The P0541 code I believe is from me upgrading from the stock air filter system that had a plug in it to the SB Air filter system and the plug has no where to plug into anymore.)
 

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It is extremely unlikely that the PCM was bad in the first place. They do not fail very often.

I think we probably can eliminate the VSS itself as that has been replaced. That leaves a problem with the tone ring in the differential, or an intermittent open or short in the wiring between the VSS and the PCM.
 

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Discussion Starter #15
Thanks Mark. Makes sense. I have been searching in forums all over trying to find out what else it could be. I see your very valuable contributions in sharing your knowledge which is invaluable. I sincerely thank you. I sure hope that is the problem.

I wish I had a direct line to you on this issue but having you respond to this post really is appreciated.

I will see how to quickly get that checked.
 

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Discussion Starter #16 (Edited)
So if I put the back axle off the ground, put it in drive and go above the speed my OD light always comes on at giving me the P0500 and P1501 codes, with the back wheels moving off the ground but my front tires on the ground not moving my OD light does not come on.

Can the front wheel speed sensors cause this whole problem?

I don't have any ABS sensor codes but it is strange that if the front wheels are not turning while the rear axle is jacked up and wheels turning that I get no OD light issue?

According to the wiring diagram the rear VSS speed sensor and the front 2 wheel speed sensors all route to the ABS module.

Then one wire (black/grey) circuit 679 runs to the PCM.
 

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The front wheel speed sensors cannot make the OD OFF light flash.
 

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Discussion Starter #18
Ok. So any thoughts as to why when the rear is suspended and in drive going a speed well over the speed that always trips the OD flashing would not flash?

Banging my head against the wall.
 

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Nope. No idea.
 
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