My speedometer was bobbing slightly for the last few years. I researched psom's and speed sensors and looked for noise sources. I replaced the sensor but was never convinced enough to replace the psom. My recently rebuilt transmission was still stuttering sometimes when it shifted. My rear end had to be rebuilt during the same period with no changes in the speedo behavior. I was pretty sure the signal was missing pulses from the sensor, causing the speedometer to bob, the cruise control to bob, and the transmission to shift funny.

I know how proximity probes work (speed sensor), but it took a lot of being fed up to decide to modify it. I pulled it out and spun the differential to check all the teeth in the tone ring. That all felt good so I used a bolt to measure the distance from the housing face to the tone ring, and compared it to the length of the sensor. The gap was about 0.100 inches, which is more than I'm used to seeing on industrial equipment of about 0.020. I don't know how much runout is allowed in there, but I wanted to take it down a little.

I used a wood rasp to shave about 0.020 off of the plastic mounting face of the probe, lowering it toward the tone gear. That seems to have cleared up the bobbing and most of my late/early shifting problems. The only idea I have come up with is that the tone ring has become magnetized over its 240k life.

Whether or not it was the right thing to do, it works for now and is an interesting experiment.

 

That is very interesting! It definitely sounds plausible. (The tone ring becoming magnetized)
How bout this theory: Is it possible that the magnetization of the tone ring can mess with the electric pulses to the PSOM; then somehow causing problems at the PSOM as well? Specifically, changing the amount of electricity to the PSOM, and causing failure of one or more of the components of the circuit board? There just seems to be so many PSOM failures! I hardly know anything about circuit boards; but if you do; any idea what specifically could possibly fail on the circuit board?

 

Thats interesting. Thanks for sharing.

 

I've never done anything to the psom other than clean the connector on the back of the cluster, and it seems to be working great now. I don't think it is damaged, other than being old.

I think I can see a tiny bit of bobble sometimes, but it's hard to watch that closely and drive. I would have liked to tighten it further, but I remember when the rear bearing wore out and would wobble a good bit. Probably didn't affect the tone ring, but I was surprised at how far the bearing could wobble without completely self destructing (probably 0.100 side to side).

On large diesels, we could run the probe down until it touched the gear and then back it out and lock it down.

I often think that I have the wrong model of speed probe, but I have replaced it before with no change in behavior. I have received new parts that were bad before, but this thing has been so consistent that it is hard to say it degraded and stopped.

Watching the speedometer, you would think there was a tooth (or more) missing from the tone ring. Either the tone ring is bobbing around (I don't know how), or there is some sort of spot on it that prevented a proper signal generation. I did try to look for metal between the teeth, but never found any.