Hey folks I have a 1996 f150 5.8 4x4 and awhile ago my speedometer started doin the waiver that happens to fords and it was like that for months then about a week ago we had about a foot of snow and slush and my speedo pegged itself to 85-90 mph and it crawled under the truck and noticed that the VSS electrical plug was damaged and I think it allowed moisture to get into the electrical plug so I replaced the plug and the speed sensor with a parts store oneand it dropped my speedo back down but then I got a 20-30 mph needle waiver so I put in a speed sensor off of another good Rearend and it fixed the waiver but it caused my speedo to be 10 mph off then it went to 20 off then 30 off and now my speedo reads 0. My Rearend is goin out so I don't know if that would cause it to do all that stuff.

My tach is also doing something funky.. After driving for awhile the needle will start to like stick for a little bit at a certain rpm then it would drop down the correct reading (like if it was at 2100 Rpms the shifted down to 1500 Rpms).

Im at a loss for where to look and it's driving me nuts!

Sorry for the long post!

 

Swapping in a VSS will not cause the speedometer to be "off". The VSS senses the pinion gear rotation via the tone ring bolted to the carrier. If the carrier berings are bad then you can and will get the wavering speedometer issue. Time to pull the rear differential cover.

 

Ok would it cause it to eventually read zero?

 

If it was bad, yes. But you would typically also get a code from insufficient or erratic VSS signal too. It may take several drive cycles to trigger the code. You stated you already know the rear end is going out so it's time to pull the rear differential cover, looks for signs of missing tone ring teeth, slop in the carrier bearings and pieces of gears, etc. All of those can and do affect the VSS operation.

You can verify the VSS signal at the RABS test port near the diagnostic data port on driver side inner fender. Perhaps you have more than one issue?

 

I'm pretty sure I have some bearings goin out cause it's got a growl.. I already have a replacement Rearend sittin here I just need to swap it in and slap some brakes on it. How would I test the port?

 

1983 Ford Bronco PSOM picture | SuperMotors.net

Instructions are written to check the signal at the PSOM, but the RABS test port is electrically exactly the same.