I've been struggling with a bouncing speedometer.

The speedometer drops suddenly from driving speed to nearly zero, and then bounces back almost immediately. It can be sporadic, happening just a few times in a trip. Or it can happen continuously for up to 20 minutes, bouncing exactly every 10 seconds. If the bounces are not continuous, they still occur in multiple of 10 seconds, like at 20, 30, or 50 seconds from the last bounce.

In the last couple months, the bounce is accompanied with a relay-like click in my dash, just to the right of the radio. But there is not always a click, even now. When it does make a click, the dome light goes out. No click, no dome light goes out.

If I am driving at 30-35 mph, the transmission downshifts to accommodate the speed "change", which revs the engine, but then immediately upshifts when the speedo bounces back. HIgher speeds than that, the transmission doesn't react to the bounce.

I know the speedo is electronic, and I know there is a speed sensor is on the rear transaxle. But I don't think that's the problem.

I have noticed that the brakes lock up a little when braking at moderate speeds. A little tire squeal. So it seems to be affecting the ABS.

I don't get any engine warning lights, and there's nothing on the computer when I read it, other than the speed "change". .As far as the computer is concerned, everything is normal.

 

Welcome to the forum

Good to include the year and engine model with questions
2000 Ranger?

Since speed and ABS seem to both be effected, change the rear axle ABS sensor to see if that solves the problem(only used in 1998-2000 Rangers for speed)
You can check the 2 wire connector at the ABS sensor for corrosion or loose wires

This type of VR sensor generates its own voltage, AC volts, so no test for the vehicle wires when unplugged
You can test the sensor's OHMs but since its an intermittent issue I would guess it will test as OK or codes would have been set
Rear Axle ABS sensor should be 800-1600 ohms, 0.8-1.6k ohms