Speedo reads 120+ at 5mph?

[shocontinental]

1998 Ranger 3.0 manual 4x4. I bought it with the speedometer not working.

A quick search found that the speed/ABS sensor on the rear axle wasn’t plugged in, I plugged it in and took it for a test drive. The speedo would start normally up to 5-10mph and then suddenly shoot the the maximum ~140mph accompanied by rough engine running. I put in a new motorcraft speed sensor and it’s exactly the same.

I searched but can’t seem to find anyone with the same experience. Any ideas?

 

[Curious Hound]

It will be interesting to see what the pros say. It almost sounds like there is something wrong with the tone ring for the VSS causing too many pulses per revolution. If that same signal is also sent to the ECU, then the engine thinks you are going fast and is trying to match spark to that erroneous apparent speed. But don't take my word for it.

Is the truck stock? OEM axle and gears? Original engine? ECU? etc.

 

[shocontinental]

Everything looks stock. The rear end has some newer looking RTV on the diff cover but it’s still sporting an open diff like the axle and door tags say. It shows 341k miles but on the short 1 mile test drive around the block it added 9 miles (to go along with the maxed out speedo) to the odo so the miles may be lower but who knows how long it was driven with the sensor unplugged.

 

[Curious Hound]

As I said, I'm not an expert on that year. My '93 gets all it's speed sensing and odometer drive via the cable from the transfer case. The sensor on the differential is only for ABS on mine. But I have read that things like that have changed on newer trucks. If the odometer changed the way you say it did, then look at the gear at the transfer case.