Hey I'm still trying to figure my problem out, but will start a new post as Xclimation seems to be GTG with his truck. Good to see how much help is available here, though.

 

This speedometer deal is driving me nuts. My speedometer just wiggles and my transmission won't shift. I have replaced the VSS,Ohmed all the wires for shorts and opens and tested fuse voltage to the PSOM. I have disconnected the ABS controller and Cruise control to take them out the picture. I jacked truck up and have only 3vac at the abs test connector. Does it require 6vac. I have been as high as 3500 rpm and all i get is 3vac.

 

I guess that voltage would depend on the speed of the rear end ring gear.
I bench tested a speedometer the other day and it read right at 30 MPH at 3.5 VAC & 667 Hz.
My guess is that you have a bad PSOM.

Here is a clip from the 1995 Ford Shop Manual:
NOTE:
Only wiring harness end of connector is to be probed.
Connect Rotunda Digital Volt-Ohmmeter 014-00407 or equivalent to Pin 4 (speed in +) and Pin 5 (speed in -).

Does the voltage increase smoothly and continuously from 0 to approximately 3.5 volts as vehicle speed increases from 0 to 48 km/h (0 to 30 mph)?

OR,

If available, a frequency counter may be connected to Pin 4 (Speed in +) and Pin 5 (Speed in -).

Does the displayed frequency of the signal increase smoothly and continuously from 0 to approximately 667 Hz at approximately 48 km/h (30 mph)?

OR,

If neither a voltmeter nor frequency counter is available, vehicle speed control may be used as a good indicator. If it works normally, then the speedometer module is at least receiving a speed input signal and the wiring and sensor can be assumed to be good.

Does the speed control work normally?

 

I have put 4 different speedometers in and they all do exactly the same. I have no way of knowing when I am around 30mph. It seems to me I am over that. Any way to put some of signal on VSS wires to see if speedometer changes. I ohmed wires for grounds and continuity but i quess it could be picking up some interference or something.

 

I retested and realized I am not getting 3vac I am getting .3 vac. I quess i am just picking up noise. funny thing is it does seem to be in relation to wheel speed.

 

If I run two wires directly to VSS and test across what should I read with Speed at 30mph. Would it be the same as what should be at the ABS test connector.

 

Well that only leaves the wiring/plugs to the rear end, the VSS sensor and its gap to the tone ring or the tone ring/ring gear bearings.

 

Yes you should get the 3.5 VAC with nothing hooked to the wires at 30 MPH.
The VSS sensor is a A/C voltage generator.

To more specific it is avariable reluctance sensor (VRS):

 

I cut the wires and ran new wires from the connector forward. Hooked staraight to my meter i still just get .25 to .30vac. it does increase and decrease with speed. How often is a speedometer problem in the rear end? I already replaced the VSS. The parts guy showed two different types and I got the one that looked like the one i took out.

 

I see it on here about two times a year that the problem is in the rear end (about due for another one). Most of the time the problem is with the tone ring.

Just about everybody changes the VSS sensor but I do not recall that ever fixing any of problems. Most of the time it is the PSOM and then the tone ring least of all the RABS module in that order.
I have not seen the Speed Control module do it yet but have hear it could.

 

Yeah I noticed the other day it bounce once around 55, now it does it intermittently but only at higher speeds. So the problem is officially not fixed yet. I live around an area that is mainly 45 mph at the max for miles, so it wasn't until I took a little longer trip that the problem resurfaced. And I'm prepared to just swap one out if I can find a decent one.

 

Subscribing

 

dont mean to resurrect dead here, but i am having similar problems, no speedo or oddo at all, have power to post 1 on the psom, have ground in 2, but no power to 3(with key on or off) and fuse 18 is good.(not sure if theres power threw it or not) so were does the power come from exactly. also truck wont start from key, replaced nss and still no go. any idea for a plan of attack?

 

I take it this is for the 1997 F250.
The power for fuse 18 comes from fuse 10 in the engine compartment fuse box then through the Ignition Switch and then to fuse 18.

 

do you know the colors in, and out of the switch? i cant see behind my fuse panel are they both white/pink?