On our newly (as in this morning) acquired '96 F350 PSD, the speedometer works fine. Then every 2 or 3 miles it will drop from 70 to 50 and then come right back. The faster you go, the more often it happens. It would be all right except that it kicks the cruise control off every time it jumps.

Ideas?

 

Somebody??

 

Either the VSS tone ring has a small chip (it's located inside the rear differential), or you have a loose connection in the PSOM, does any other needle jump when the speedometer does? it no other needle jump then check the connectoins in the spedometer dial. If none of these seem to be the cause you can try replacing the VSS itself, but those usually don't fail.

An image of the tone ring, it's located below the differential gear in this oddly-positioned image:


You can notice how some of the teeth in my tone ring are chipped (my needle does jump a lot), look at the left side of the pic.

If the ring shows any damage, check the differential gear for broken teeth and the differential and its oil for debris.

Uploaded with ImageShack.us

 

Was your speedometer like this?

Then you have bad wiring on C Channel frame from PCM to Rear axle. It got me 1 week of trace try find out what going and couldn't drive out 3 gear due speedometer control E40D trans.

 

No it's not like the video. It is perfectly steady. The one every 2 or so miles it drops 20 to 30 MPH and hops right back up and is smooth as glass. The time between jumps is always consistent.

 

Check to make sure fuse #8, IIRC, in the underdash fusebox is nice and secure in the fuse holder.

I have a "tap in" connector on my #8 fuse in my '94 and the fuse was loose (not making a secure connection) and I spent months trying to find where my "random" speedometer problem was, hence it threw my e4od into fits as well.

Just a thought and something simple to check.

Bob

 

Just to summarize and add my own two cents in, the problem is one of:

- VSS in the rear axle. The sensor doesn't actually fail but fine metal particles can collect on the sensor, blocking its function.
- chipped tooth on the tone ring inside the differential
- wiring between the VSS and the PSOM in the dash
- the PSOM itself

If the behavior is consistent, what is the frequency like? How often and does anything affect it?

 

My 94 f150 will do the same thing, only after you get to about 70mph. Then it will bounce between 70mph & 50mph, go under 70mph and its fine..

scot