Speedo needle bounce Problem

[Popeye2347]

At about 62-68 mph my speedometer needle will bounce, not a lot mind you, but enough to set my 'OFF' light on the shift lever to flashing. I have replaced and adjusted the FIPL switch, cleaned then replaced the VSS on the rear axle, then cleaned the contacts with cleaner, [they seem normal tight on the pins], [found no fuzz, chips or shavings on it], and replaced the rear axle fluid.

The code reader shows a bad vss when it is acting up, but good when the light is not lit[as one would expect it to act.] I have shook and inspected the wiring and it all looks good, and it works good unless I travel ~65 for a distance, then with the needle bouncing [only a slight wiggle mind you] the OFF light will illuminate. The tach needle is rock steady. If cruise is on the truck will slightly surge if engaged ~65 mph. 60 or 70 is OK, just about 65 is where the problem manifests itself. I have used the search feature and after about wearing out the 'back' button, I can not find anything similar to my problem [imagine that!]

Other than the obvious of not travelling ~65 mph, what could be wrong? This is a class one PITA! See sig for year, etc.

 

[Agnem]

I realize not everybody has an ocilloscope like I do, but that's what you need. You need to take a look at the rear axle speed sensor signal as your driving. My bet is that either your amplitude is dropping off or your frequency is getting eratic. But you will need a scope to determine if the problem is at the source or if the signal is being corrupted somewhere along the way. A set of jack stands, and taking the rear wheels off will yeild the best testing conditions for the first attempt. If you can't see the problem then, you most likely have some kind of road/vibration/motion issue contributing to the cause.

 

[Popeye2347]

I realize not everybody has an ocilloscope like I do, but that's what you need. You need to take a look at the rear axle speed sensor signal as your driving. My bet is that either your amplitude is dropping off or your frequency is getting eratic. But you will need a scope to determine if the problem is at the source or if the signal is being corrupted somewhere along the way. A set of jack stands, and taking the rear wheels off will yield the best testing conditions for the first attempt. If you can't see the problem then, you most likely have some kind of road/vibration/motion issue contributing to the cause.

Hummm.....Good idea. Although I do not own an O'Scope I am no stranger to one, and may be able to borrow one. I could hook it to the VSS without much trouble, but the new one acts like the 1994 factory unit, so I doubt it is bad. Assuming that it is good, where else can I intercept the signal? I dislike probing thru the insulation in the middle of a run.

'Nother question, if the signal is erratic from the VSS, where then, given the vss itself is not bad, would be the problem?...a loose ring gear????

Thanks.

 

[Agnem]

The tone ring is on the ring gear, so it seems unlikely that it could be moving away from the sensor without having a lot of other problems, so my thinking is that once you reach a certain speed there may be some kind of electrical interference or added capacitance that is drawing off the signal. Again, you should be able to see that on a scope. Only problem is, you may need a good truck to compare against. Other thoughts... metal particles in suspension in the oil? Wind blowing a wire and breaking contact? RF interference from the alternator?