I'm gonna post a real poser for y'all... I recently had the pinion & carrier bearings replaced while towing thru CA after a severe whine appeared in the rearend. Since I had 292k on the truck, I expected other wear & was not surprised to find the ring & pinion both worn down. Since the shop didn't have a replacement gear set, I asked about the survivability of the parts for an additional 1000 miles to get home to the local shop. The answer was "Yes", so up the road I go. The howling was still there, so I babied it north, stopping every 100 miles or so to check the case temp. Took truck into local shop on return & found all the new bearings had been wasted by heat and the gears looked like they'd been attacked by 60-grit sandpaper! Shop replaces gears & all the bearings, including axle brngs. I head out for another run & get about 500 miles when the speedo starts swinging +/- 1 mph. I have to quit using the cruise. Purchased a new VSS after talking w/ shop mechanic. Went to install it & found the front of the RE housing covered with fresh gear lube coming out of the VSS hole. The old VSS was covered with metal filings, so I pop the cover, drain & clean case then refill the box. Noticed the tops of all the teeth on the VSS trigger gear were slightly ground down, maybe 0.010". Factory spec on VSS clearance is 0.005 - 0.045 inches. A couple of fluild changes later & I still get the same swing in the speedo.

My question is... Do y'all think that the VSS trigger gear can cause this symptom? Ford shop says "no way". I'm inclined to disagree.

 

Yes it can.

I've seen it happen many times. It's more than likely that the old VSS is simply getting tired. When you consider the miles on your truck, and the amount of readings the VSS takes per revolution of the carrier, it should be no surprise that it's getting weak.

Another possibility is the cog ring too. The new rearend settings may have thrown it a hair out of the reach of the old VSS.

From what you mention, I'd also reccomend taking a look at your axel vent tube for restrictions. With your amount of miles I'd bet that it's clogged up and helping to push the axel oil out your seals or other places.

 

Thx. Will check the vent as soon as it gets back from the shop. Ford says they'll replace the trigger gear (cog ring???) for free, since the symptoms appeared after they did the overhaul. Guess I'll see from sure when I pick it up. BTW, the new VSS didn't fix the problem.

 

The VSS is a hall effect transistor, it detects when a magnetic field moves close to it and sends the signal to the computer. The metal filings can have either confused the vss, or even demagnetized some of the teeth, if they are not magnetized, the VSS can not read them (almost as if teeth were missing on the pinion, the ring would make less turns). If you have access to an oscilloscope you can check the signal from the VSS and see when it varies, it might even be just one tooth demagnetized or missing. Metal filings on the teeth will also confuse the vss. Oscilloscopes you can connect to any laptop are sold on ebay for like $25, and they can be useful for many other diagnostics too.

 

I forgot to mention that most shops, and I believe Ford, reccomend that the cog ring be replaced when the rearend is serviced for the very reasons that alalor mentioned above.

This might be something to also consider if your rearend shop reused the old ring.

 

Magnetizing a gear inside a case full of oil seems like a very bad idea to me; it would collect all the stray iron particles. The VSS itself appears to contain the magnet. My limited understanding of inductive proximity switches leads me to believe that each passing tooth of the ring causes a change in the field induced by the magnet, then the switch amplifies the change, creating a proportional AC signal. I'll have to go dig my o'scope out of storage to determine what the signal looks like.

 

Your right BigTow......the sensor generates the magnetic field internally.....and the teeth cutting through that magnetic field, causes the pulses that the PCM counts to determine speed etc..

 

the vss generates a "sine wave" signal or ac voltage. it can be checked with an ociliscope or dvom. and there is a spec for the resistance value across the terminals on the sensor. the reluctor teeth are NOT magnatized, and there is a clearnace spec for the distance between the sensor and the reluctor wheel. if the old sensor was damaged by hitting the wheel it will have to be replaced, for sure. have a good day !

 

I thought it worked like the speed sensor in boats. I am sorry for giving wrong information.

 

Got the beast back from the Ford garage last night. They're telling me that the PSOM (Programmable Speedometer/Odometer Module) is the culprit. This after they replaced the exciter ring (for free!!!). I went for a short test drive and, yes, the speedo still wiggles. But, subjectively speaking, the cruise seems to be more stable. Guess I'll change the PSOM after I make a few more $$$.