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Discussion Starter #1
Any idea how the speedo could be off by so much ?
The differential ratios are correct to the truck and so are the tire size. This thing is way off. Never seen that before. Bad sensor ?
Oddly the cruise works and no codes. This was a govt owned fleet maintained truck so doubt there was any aftermarket PCMs etc.
 

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Is it a straight 15mph at all speeds? If so maybe the needle rotated on the shaft somehow.
 

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Discussion Starter #3 (Edited)
ok, So I re calibrated the VSSB/Drac according to the procedure here and no difference. Also changed the sensor on the TC tail shaft. No difference.
What the heck ? I am out of ideas.
 

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ok, So I re calibrated the VSSB/Drac according to the procedure here and no difference. Also changed the sensor on the TC tail shaft. No difference.
What the heck ? I am out of ideas.
Is it -15 or +15?
 

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Discussion Starter #5
It says 80 mph when I am actually going 65 so it's reading more than I am going.
 

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tire size?
 

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The vssb will be a percentage so at 65 it shows 80. At 32 will show 40. If it's a straight even 15 over at all speeds...... Never seen that before. Let us know when you fix it.
 

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All of a sudden or since you have had it?
I'd think someone installed 410 or higher when you're setup for 373...
 

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Discussion Starter #9
Stock 245's and it was an ex govt vehichle. Unlikely anyone changed both gsar sets. Someone screwed with the vssb already because it was loose and yes, been off since I got it.
 

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Discussion Starter #10
Does anyone know if Carcode can see the pulses coming from the VSS ?
 

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Ace, mine came with 235/85 16
 

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Discussion Starter #12
I decoded the vin and the 245's were stock. Ok now I really don't know WTF.
Put a Snap On Solus on it today. The VSS is reading correct MPH but the speedo is not and when we hooked up to the abs for shits and giggles it was all over the place. Both front wheels were reading all over the place even up to 215 mph just sitting and both front wheels had Short ciruit errors. Also there was a stored 501 vss code but it was old.
This is why I hate these trucks. I'm gonna dump it and by another square body.
 

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Well, when you shut the key off does it return to "0"? Is it always 15mph out? As in if you're going 5, it says 20?

Another easy thing if you have access to a scan tool, look at the reading on that.
 

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Yes returns to zero and reads zero on scan tool. Scan tool reads correct mph while driving. Abs is all over the place.
 

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Yes returns to zero and reads zero on scan tool. Scan tool reads correct mph while driving. Abs is all over the place.
Well as long as in the PCM the speed is correct, it's most likely the cluster. Abs has nothing to do with the speedo. Cluster pulls speed from the same wire as the PCM input, so PCM input is the same as cluster input. Unless the splice happens to be bad, but that's highly unlikely, since it's a square wave signal, it should work or not work.
 

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Discussion Starter #16
I thought both the abs and speedo get thier signal from the VSSB which Im starting to think may be defecfive
 

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Old P0501 Vehicle Speed Sensor Range/Performance

Following from Section 7A-17A "A/T Diagnosis"
Transmission Input/Output Speed Sensor Assembly
(edited version)
The output speed sensor may be thought of as replacing the governor. Also note that output speed may differ from speedometer readings because the output speed sensor reading is not corrected for minor variations in axle ratio or tire size.
The PCM/TCM uses speed information from these sensors to determine the following:
- Engine is running
- Vehicle speed
- Calculate gear ratio
- Calculate TCC Slip
- Calculate turbine speed

If the input speed sensor is not operational at start up and reads 0, this will cause the output sensor to also read 0. There is no direct code associated with an input sensors fault, however, an inaccurate input speed sensor could cause other codes to set especially those associated with ratio's or slip.



Following from Section 8C Instrument Panel and Gages
Electrostatic Discharge (ESD) Notice
Many solid state electrical components can be damaged by ESD. Some will dislay a label but many will not.

Electronic Speedometer
The electronic speedometer drive replaces the mechanical drive for all C/K applications. Integrated circuits control the air core speedometer and stepper motor odometer. The speedometer and odometer receive a signal from the vehicle speed sensor calibrator module that is generated by the vehicle speed sensor. For diagnostic information, see Section 8A-81.

Vehicle Speed Sensor
The vehicle speed sensor (VSS) is a permanitemagnet signal genrator located on the transmission or transfer case output shaft housing. The vehicle speed sensor sends an anolog signal proportional to the propellar shaft speed. This signal goes to the vehicle speed sensor calibration module. For diagnostic information, see Section 8A-33.

Vehicle Speed Sensor Calibrator Module
The vehicle speed sensor (VSS) calibrator module is a solid state device that is used to change the signal from the speed sensor to a digital signal. The VSS calibrator module will change the signal from the speed sensor to a signal containing 4000 pulses per mile (PPM) for the instrument cluster. The vehicle speed sensor calibrator module is matched to the final drive and tire size of each vehicle. It must be replaced with the proper module to match the final drive and tire size of that vehicle. If final drive or tire size is changed for any reason, the module must also be changed to continue to produce an accurate speedometer/odometer reading. The incorrect module will also affect the anti-lock brakes, vehicle control module (VCM), and the cruise control module. For diagnostic information, see Section 8A-33.


IDK.. wild guess.. Have you check'd the VSS buffer? DTC 24 (VSS Circuit Low- output speed signal) will set if the A/T output speed sensor is faulty and the PCM/TCM will substitute a default value (for 4:10 perhaps?).
Other possible default codes: 72- VSS Circuit Loss (output speed signal), 74- Trans Input Speed Sensor Circuit.

AFAIK, the VSS buffer via LT BLU/BLK (824) wire is what actually provides the speedometer the correct (re-calculated) final signal. In other words, the speedometer signal does not come from the PCM so they may very well be different..
 

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I thought both the abs and speedo get thier signal from the VSSB which Im starting to think may be defecfive
They do, but separate circuits, the speedo splices off the PCM line, abs has its own. I can get you a Mitchell or alldata diagram after work. Just on mobile right now
 

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Discussion Starter #20
I tried two different VSSB's I got t a junkyard and even re-calibrated one of them. No change at all...
 
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