Speedometer Needle Bounces

[thebovees]

'94 FZJ80: The speedometer needle has started bouncing significantly making so I cannot use the cruise control. Wondering if it is the sensor or, maybe, the corrector I added to compensate for the larger tires.

 

[DivByZero]

Well the cruise control is independent of the cluster, so the one thing you can say for sure is it's not the cluster. Either the corrector or the sensor could be a culprit. Dodgy wiring is also possible. The speed signal is just a square wave. As the gear turns inside the transfer case, it switches the signal from off to on and off again. The pulse rate determines the speed. The needle "bouncing" is reflecting what's going on with the signal itself, which is that it's getting stuck either on or off for periods of time. An oscilloscope is the right way to "read" the signal and know exactly what's going on, but you don't need one for this. It's a simple circuit.

I'd suggest removing your corrector. See what happens. If it's fixed, look carefully at it, see if it's a connection problem or if the corrector is junk. If it still doesn't work without the corrector, leave it out and keep on chasing. A multimeter will be required from this point. I'd verify wiring first, then look closely at the sensor. They can fail, was a frequent problem with Hilux models of the same era, although the speedo sticking on 0 was the more common symptom there.

 

[thebovees]

Well the cruise control is independent of the cluster, so the one thing you can say for sure is it's not the cluster. Either the corrector or the sensor could be a culprit. Dodgy wiring is also possible. The speed signal is just a square wave. As the gear turns inside the transfer case, it switches the signal from off to on and off again. The pulse rate determines the speed. The needle "bouncing" is reflecting what's going on with the signal itself, which is that it's getting stuck either on or off for periods of time. An oscilloscope is the right way to "read" the signal and know exactly what's going on, but you don't need one for this. It's a simple circuit.

I'd suggest removing your corrector. See what happens. If it's fixed, look carefully at it, see if it's a connection problem or if the corrector is junk. If it still doesn't work without the corrector, leave it out and keep on chasing. A multimeter will be required from this point. I'd verify wiring first, then look closely at the sensor. They can fail, was a frequent problem with Hilux models of the same era, although the speedo sticking on 0 was the more common symptom there.

I checked the wiring diagrams and found that the speedo transducer goes directly to the cluster and the speedometer then to the engine control, the transmission control and the cruise control together.
I removed the corrector and while the bouncing seemed to be a little less the speedometer is still bouncing and sometimes swings all the way to max and I can see the odometer increasing, all while stopped in one position. Since I have the corrector removed I have access to the wires and will attempt to use a multimeter.

 

[thebovees]

Speedometer bounce is gone. Replaced the speed sensor and not a bounce in a 15 mile test drive. And, a very important side benefit, the transmission hesitation I was also experiencing went away as well. Talked with my Toyota mechanic who indicated that speed sensor signal gets to the transmission controller and can effect the shifting. What caught me with that was that yesterday in the Walmart parking lot the speedometer, at a stop, was showing 100 MPH and I could not get it into reverse. Finally the speedometer dropped to 0 and I could get it to shift into reverse.
So, I was expecting to have to swap out the transmission shift solenoids and now that is off the table. Now, with 473,000 miles I can expect a few more.

 

[thebovees]

I took the bad speed sensor apart and found an electronic circuit with the word "HALL". Assuming that it was a reference to the "Hall Effect" which detects a magnetic field. So, I took the device that spins with the connection to the transmission and put a small compass near it and found that the compass needle moved as the spinner was moved near it. This verified that the sensor uses the Hall effect to produce the pulses that are sent to the speedometer.