PSOM Reprogram Mileage Reading
Fleet Owner
Joined: May 2009
Posts: 21,437
Likes: 76
From: Gilbert, PA
It looks like he is sending a false signal so it thinks its being driven. That would work to add miles but not take them off. It would take a while to have it come back around and up to the correct mileage doing that. From what I understand it rolls over to zero again after 400k.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
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From: Easton,Ks
Where was the VSS located on your E350 Bus?
Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
It's on the rear axle.
I discovered the speedometer high end sweep was off after I changed out the speedometer it self(meter movement and back plate). After bench testing the original and replacement Speedo's, I found out about the meter deflection adjustments and finally figured out where they are saved in the EEPROM. One data set is for the low end of the scale and other is the high end scale.
I discovered the speedometer high end sweep was off after I changed out the speedometer it self(meter movement and back plate). After bench testing the original and replacement Speedo's, I found out about the meter deflection adjustments and finally figured out where they are saved in the EEPROM. One data set is for the low end of the scale and other is the high end scale.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
When you are saying " upper scale and lower scale needle calibration" are you talking about the speedometers that one goes to 80 mph and the other goes to 85 mph?
Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
The lower data set is for the 0 to 20 or 30MPH adjustment and the high end is for 50 and up MPH.
After I swapped the EEPROM into a better PSOM pcb. The 55mph and higher readings were not linear. After 60mph the needle really took off and read too fast.
After pulling the speedo head out(again) I bench tested side by side with same VSS input and finally found what was the cause.
When the speedometers are manufactured, they are calibrated for the meter movement from the factory. Most of the ones I messed with usually are so close that you may not even notice after EEPROM swap/reprogram.
I happened to get a meter movement that was better than my old meter movement and the settings caused the non linear issue I found. As a side note, the original speedo meter did show the opposite with the other EEPROM installed.
That did answer the question of what those data points were storing while figuring the data on the EEPROM. Now if I just can figure out the data parity encoding......
Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
A 1989 F-series does not have a PSOM. The Speed/Odometer is mechanical. 1992 and later trucks have a PSOM with the associated EEPROM that stores the odometer mileage.
Hotshot
Joined: Nov 2008
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From: Semper Fi tell I die!
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Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
Two functions:
1. Converts the raw VSS signal to something usable (8000 revolutions/mile) for the speedometer and odometer functions.
2. Records the current odometer mileage and has a digital display that is shown at the bottom of the speedometer.
1. Converts the raw VSS signal to something usable (8000 revolutions/mile) for the speedometer and odometer functions.
2. Records the current odometer mileage and has a digital display that is shown at the bottom of the speedometer.
Last edited by rla2005; Nov 28, 2018 at 05:17 PM. Reason: Correct revs/mile
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
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Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
Also #3 it sends speed data out to the ECU(engine run strategies and E4OD transmission shifting) and speed control as the 8000 pulse per mile output. Failure of this output will cause the E4OD transmission to shift improper.
Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
I recall its the +4(4214) and +5(CEA5) of the lower segment of data. I never fully figured which data does what on that. Got to figure out that parity encoding(back burner for now) so we can plug in data with out the "error" in the odo.
I got a "error 3" flashing now on my 95 E350. Odo and speedo work proper, wonder what that error means this time....
I got a "error 3" flashing now on my 95 E350. Odo and speedo work proper, wonder what that error means this time....
Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
Its 8000 pulse per mile like the older 8000 revs per mile of the speedometer cable days. Ford and Chrysler were that way, I think GM was the 9000 revs per mile.
I stand corrected
Also #3 it sends speed data out to the ECU(engine run strategies and E4OD transmission shifting) and speed control as the 8000 pulse per mile output. Failure of this output will cause the E4OD transmission to shift improper.
I stand corrected
Also #3 it sends speed data out to the ECU(engine run strategies and E4OD transmission shifting) and speed control as the 8000 pulse per mile output. Failure of this output will cause the E4OD transmission to shift improper.
