Computer vs speedometer speed difference
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Junior User
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Computer vs speedometer speed difference
Ok so I swapped out a speedometer on my 95 PSD for a lightning speedometer. In doing this I put my PSOM on the lightning speedo. The issue I am having is the computer reads the correct speed (checked VIA GPS) but the speedo reads fast. Its a percentage issue as its less of a difference at low speeds and really far off at high speeds
I tried the lightning PSOM just to see but no change.
The only thing I can think of is the computer is acting like there is normal speedo and moving the needle to far? Shy of setting cruise at 50mph and seeing what the lightning speedo says and holding up my stock speedo and adjusting it to 50 and seeing if the needle is in the same relative spot I dont know how to test that theory.
If I need calibrated what can be done? would changing the revolutions per mile on the PSOM change anything in my favor?
I tried the lightning PSOM just to see but no change.
The only thing I can think of is the computer is acting like there is normal speedo and moving the needle to far? Shy of setting cruise at 50mph and seeing what the lightning speedo says and holding up my stock speedo and adjusting it to 50 and seeing if the needle is in the same relative spot I dont know how to test that theory.
If I need calibrated what can be done? would changing the revolutions per mile on the PSOM change anything in my favor?
Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
How do you know what speed the computer is reading?
The computer can not control the pointer on the speedometer.
The PSOM just converts the information from the VSS to the old stile information and sends this to the speed control and the computer.
It also keeps track of the mileage and send this to the odometer read out.
It also operates the speedometer head that has the speed pointer attached to it.
The conversion factor will change how fast the PSOM thinks that you are going and give these findings to the computer, speed control, the drivers for the speedometer servo stepping motor for the pointer and the odometer update in the EEPROM.
The computer can not control the pointer on the speedometer.
The PSOM just converts the information from the VSS to the old stile information and sends this to the speed control and the computer.
It also keeps track of the mileage and send this to the odometer read out.
It also operates the speedometer head that has the speed pointer attached to it.
The conversion factor will change how fast the PSOM thinks that you are going and give these findings to the computer, speed control, the drivers for the speedometer servo stepping motor for the pointer and the odometer update in the EEPROM.
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
That is the nature of these speedometers so you have to choose a speed to make it dead on(60mph for example) and live with some +/- error above and below that.
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I get that there is going to be some difference across the board as electrical current does diminish between its sending source and the end user.
This issue mimics that of the wrong PSOM setting for tire size yet the computer sees the corrected speed. I need to check it on my other truck and see what the change is. I may try reprogramming the revs per mile in the PSOM and see what that does
This issue mimics that of the wrong PSOM setting for tire size yet the computer sees the corrected speed. I need to check it on my other truck and see what the change is. I may try reprogramming the revs per mile in the PSOM and see what that does
Senior User
Joined: Nov 2011
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From: Owensboro, KY
The only two options I see here are to run the Lightning PSOM, or live with a wonky speedometer. The Lightning speedometers have a different scale that reads to what, 100 or 120 mph? The original PSOM of the 80 mph speedometer can never be calibrated to read right with the Lightning speedometer face, and even if it were to come close, your odometer would be waaaay off on mileage, which defeats the purpose of using the original PSOM anyway I would think.
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Cargo Master
Joined: Mar 2007
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From: Arcadia, Fla
Reprogram the Lighting PSOM for your truck. I think the Lighting's were the 8.8 axles and had the smaller tone wheel. Your PSD truck is going to have the larger tone wheel in your axle.
Like Ranger80 said, the non-lighting PSOM will not drive the speedo needle correctly using a Lighting face. The needle sweep angles are different.
I am still puzzled as how the ECU is getting correct MPH when it is fed by the PSOM.........
Like Ranger80 said, the non-lighting PSOM will not drive the speedo needle correctly using a Lighting face. The needle sweep angles are different.
I am still puzzled as how the ECU is getting correct MPH when it is fed by the PSOM.........
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Junior User
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I am puzzled also. Im about 1 step away from yanking it out and selling it. Its a novelty piece and not worth the pain. Plus I feel like I am going faster when I can max the 85mph speedo in a hurry
Fleet Owner
Joined: May 2009
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From: Gilbert, PA
If you want to part with it, let me know
Senior User
Joined: Nov 2011
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From: Owensboro, KY
Were you by chance using your original PSOM with the Lightning face at this point? That would explain the computer seeing the correct speed.
From what I can see, you're just gonna have to use the Lightning PSOM, calibrated with your tone ring and tire size. There's a chart of conversion factors around here somewhere. If you want the odometer to read correctly, assuming the Lightning PSOM had less miles than your original, you can trick it by using a stereo for example, using the electrical pulses to make it read the same mileage as your old one. Don't remember exactly what was involved with that method, but somebody who knows by experience can hopefully offer some insight.
From what I can see, you're just gonna have to use the Lightning PSOM, calibrated with your tone ring and tire size. There's a chart of conversion factors around here somewhere. If you want the odometer to read correctly, assuming the Lightning PSOM had less miles than your original, you can trick it by using a stereo for example, using the electrical pulses to make it read the same mileage as your old one. Don't remember exactly what was involved with that method, but somebody who knows by experience can hopefully offer some insight.
Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
Note all PSOM are the same. It does not make any difference if they came from a Van or Truck. It does not make any difference what the speedometer heads numbers go to the PSOM is the same.
A photo of a PSOM:
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A photo of a PSOM:
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Senior User
Joined: Nov 2011
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From: Owensboro, KY
I was under the impression that although they are physically identical, the Lightning PSOM used different conversion values, is this not correct?
Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
The Ford shop manual for testing calls for all of the trucks in all of the years in our year era to read 30 mph @ 667 Hz.
Every PSOM head combo I have bench tested at 667 Hz have read 30 mph.
Now the Lightning has a different speedometer head and its servo motor may be calibrated to indicate a little different as the Lightning head speedometer numbers do go higher.
Every PSOM head combo I have bench tested at 667 Hz have read 30 mph.
Now the Lightning has a different speedometer head and its servo motor may be calibrated to indicate a little different as the Lightning head speedometer numbers do go higher.
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