Hey guys, really needing some help here! I'm trying to get my 96 F250 back road worthy. It sat for a few years, and in that time was used to jump a VERY dead battery with 2 sets of cables at once, so not sure what caused my problems, but none the less, my odometer is blank (will still light up with the lights on) and my speedometer doesn't move at all. Right before the truck was parked, my dad put a new vss in, and the speedometer was a little jumpy, but it still worked and showed the milage. I've checked all the fuses. I've made sure the 3rd brake light still works. I made sure the vss cable hadn't gotten unplugged. I've pulled the cluster so I can wiggle all of the connections. I've checked that I get power to pins 1 and 3 on the psom plug. They both get fluctuating power, that is most consistent in the 7-8 volt range. I've tried doing the speedometer self test of holding down the trip meter button on you turn the key to the on possition, but I can't get that to make the speedometer needle move. My abs light is not on, and it does like up for the first couple seconds of starting the truck, so it's not just burned out. I assume that rules out it being the rabs controller? Any ideas of things to check? If I just need to have the psom rebuilt, that's not a problem, but I just am hesitant that is in fact my issue since there isn't any corrosion on it, and I can't visibly see a melted spot or anything...
Thanks in advance for any help!

 

Remove the instrument cluster
Check the VSS output at the instrument cluster connector
You could test it at the rear axle first and also check the connector back there for broken wires
You are measuring AC millivolts for the signal
The voltage will go up as the speed increases

 

The PSOM has a capacitor that leaks like every capacitor that was made in that era. Look closely at the board.

 

Check voltage between psom pin 1 and 2 with the key off. You should have a solid 12v. Check voltage between pin 3 and 2 with the key-on. You should have a solid 12v. If you dont, you need to trace those wires to the inside fuse-box and find out why you dont have 12v. I suspect you have some bad fuse contacts or a melted fuse holder.

 

Hey guys, thanks for all if your help! I really appreciate it!
I am not getting a solid 12v at either of those two spots, so i will try to do some digging there tomorrow. It sure would be cool if I didn't need the psom rebuilt!

 

If both pin 1 & 3 don't show 12 v, I would suspect a bad ground. Test 1 & 3 with the ground probe on a piece of the metal frame. Of course check both fuses.

 

Ok guys, not sure what was different about this time, but I'm getting a solid 13-14 volts at both pin 1 with the key off and pin 3 with the key on using the provided ground of pin 2.
Am I correct in thinking that I am at the point of sending the psom off to be rebuilt, because even if the vss DID have problems (loose connection, bad sensor) it would still do the needle sweep test (turn ignition to on possition as your holding town the trip meter reset button down, and then let up)?