03 speedometer reads double

[50trimomni]

My ranger developed a problem last summer after the alternator went out and we jump started it. It's a 03 4.0 4x4 manual transmission. After fixing the alternator and putting a new battery in the speedometer and odometer read double what they should. In my scanner I get a code for speed sensor circuit in the 4x4 control module but only once I start moving. In the live data for the pcm the speed is correct but in the modules it outputs to its basically double. I've replaced the speed sensor with a new 1, and replaced the gem, cluster, 4x4, and cruise control modules with no change. I'm starting to think it's the ecu itself as its a reman unit but the only thing wrong is the speedometer. Any help would be greatly appreciated as its starting to be annoying. Thanks in advance

 

[RonD]

Welcome to TRS

In 2003(2001 and up) Rangers the speed signal for the cluster comes from the PCM(ECU)
PCM gets its signal from the OSS(output shaft speed) sensor on the rear part of transmission, manual or auto.
But thats a 25,000ppm signal so the PCM converts it to the 8,000ppm signal Ford has used since the mid-1980's, and sends that out to cluster on 1 wire, speedo/odo share that signal
So the speed signal from PCM is off if BOTH speedo and odo are effected, not a cluster issue

If the OBD2 reader shows what you believe is the correct speed signal then PCM is doing the conversion OK
So OSS is working like it should

But because its a 4x4 there is a software in the PCM so it can correct for speed in 4LOW
i.e. PCM has no driveshaft or wheel speed reference, just transmission shaft speed before transfer case
But I would expect that to cut the speed in half not double it

Try shifting into 4LOW and drive forward a bit to see what speedometer does, its best to be on gravel or dirt when in 4LOW vs dry pavement

Then shift back to 2WD


Its possible it is the PCM but would expect OBD2 to be off as well

The OSS is a VR(variable reluctance) sensor, which means it creates and outputs its own AC voltage
The voltage is not used, the Sine Wave from the AC voltage is used, the up and down pulses
And only 1/2 the pulses are used, AC voltage changes from +2v to -2v for example
The circuit is grounded so only the +2v peaks are used, and thats what is used for speed signal, the Pulse Per Mile(PPM)
So it is theoretically possible to get doubled pulses, but not an electrical engineer, so maybe its not, lol