where does scan tool recieve vehicle speed from

[integrx18]

im stumped on a p0500 code that I posted a thread yesterday. my scan tool will not read any speed while driving down the road. it always says 0mph does this signal come from the ecu or the gem module. I replaced the gem module and wodering if this could be the root of the problem because all else checks out and speedo works. I would imagine the ecu would tell the scan tool but im unsure.

 

[RonD]

The scan tool only reads ECU(PCM)

In 1997 the electronic speed signal comes from the VSS(vehicle speed sensor) located in the old speedometer cable hole in the trans tail shaft or transfer case if 4WD.
1995-1997 speed correction was done by changing the speedo gear, same as when a cable was used so no electronic conversion was needed.

That electronic signal goes to speedo/odo, cruise and ECU independently, there is no interconnection between them except they share that signal.

So your likely issue is a bad connection at the ECU or the splice where signal splits, if cruise and speedo work

In 1998 the ABS VSS on rear axle was used, at that time the speed needed to be electronically corrected for axle ratio and tires size, if truck has 2 wheel ABS the signal went to GEM and was corrected then sent out to ECU, cruise and speedo.
If 4WABS then signal went to 4WABS module where it was corrected and sent to ECU, cruise and speedo

 

[integrx18]

thanks

thanks for the reply on that end. I thought that was the way that it worked but wasn't sure. so that being the case I tapped into the wire from the sensors black grey wire to the pcm pin 58 while leaving the wire connected to the speedo and gem.... no cruise on this truck. still don't get any sort of speed reading from the scan tool. I even went as far as clipping the pink and orange wire and running a straight ground to pin 33 on the ecu seeing as that is the ground wire for the sensor while running a separate wire from black grey to pin 58 and still no reading. I would think that would bypass everything and give me some speed reading.... ill keep on tinkering but not having luck.

 

[RonD]

I answered in your other post about this.

But pull off the PCM/ECU connector, disconnect battery first.

Could be wiring is OK but pin/connector for PCM 33 or 58 is bad
Or PCM circuit inside is bad, not sure how/if you can test that

 

[integrx18]

thanks for your help ron.... it must be the circuit in the pcm that is bad... just odd seeing everything else in the truck works. I took it out and don't see any bad spots but you never know... well maybe ill try a pcm if I can find one reasonable

 

[RonD]

Just as one more test, since your's is a the old speedo cable setup.

Pull the sensor and gear out of trans, put a drill on on it and get it spinning then use Volt Meter set to AC volts and see what the reading is at the sensor and then at the wires at the PCM with sensor plugged in of course.
Depends on RPMs of course but .5 to 1 volt should be seen, like testing a Crank position sensor

 

[integrx18]

the vss isn't the cable set up but with the truck jacked up in drive with wheels spinning the voltage ac goes from 0 and keeps climbing up to 3 volts and higher with speed.... is that wrong? as speed increases the voltage keeps climbing.... I assumed that would be how it works....I tried a new ecu and still the same issue im ready to find some dynamite

 

[RonD]

1997 has TWO VSS sensors
The one on the rear axle is only used for ABS

The one on the transmission tail shaft is for PCM, speedo, and cruise
It is on the old speedo cable gear.

In 1998 Ford used rear axle VSS for everything.

So there is no VSS on the trans?
Drivers side tailshaft, or 4WD transfer case output shaft

 

[integrx18]

hey Ron I appreciate all the help.... so after installing new ecu I still had no reading on my scan tool for vehicle speed.... I tried a second scan tool with still no luck. I just said screw it this doesn't make any sense. I put the original ecu back in and decided to clear the code and drive the truck down the road for the heck of it. Still no vss value on my scan tool but drove the truck for about 15 min and all monitors have set except for catalyst. This is unreal. Yesterday the code would come on after about 3-5 min of driving after resetting. After that I just kept testing the vehicle on jackstands and figured that when the problem was solved that I would finally get a speed reading on my scan tool. Anyways I don't understand why I cant get any speed value on the scan tool while driving but as of tonight there is no code. ill cross my fingers it wont come back. very strange situation. every obd2 vehicle I ever had a scanner plugged into gave a speed value while driving.

 

[RonD]

Yes you should see speed on live scan, if moving.

hopefully code will stay off until testing is done