When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I have a 96 f150 4.9 it started shifting rough and at the same time the speedometer went out I replaced the fuse (17 I believe) and it immediately blew again. I have looked up a ground wire diagram and holy smokes there are countless ones anyone know which I need to chase?
if its blowing fuses,it would be a dead short,power to ground,lack on ground should make it function wrong,try the wire running to rear diff looking for a pinched wire,
A short in the VSS wiring will not likely cause Fuse 18 to blow. Fuse 17 has nothing to do with the PSOM/VSS circuit.Fuse 18 also supplies power to the electronic shift control (elec. shift transfer case), Airbag Diagnostic Monitor, Day/Night mirror, and the shift lock (automatic trans. only).
Any of the above items and their related wiring can cause Fuse 18 to open. Could be a failed component on the PSOM itself. It's a process of elimination at this point.
if its blowing fuses,it would be a dead short,power to ground,lack on ground should make it function wrong,try the wire running to rear diff looking for a pinched wire,
Check diff and didn’t find any faulty/crimped/burnt wires
A short in the VSS wiring will not likely cause Fuse 18 to blow. Fuse 17 has nothing to do with the PSOM/VSS circuit.Fuse 18 also supplies power to the electronic shift control (elec. shift transfer case), Airbag Diagnostic Monitor, Day/Night mirror, and the shift lock (automatic trans. only).
Any of the above items and their related wiring can cause Fuse 18 to open. Could be a failed component on the PSOM itself. It's a process of elimination at this point.
VSS/PSOM diagram:
Checked for faulty grounds and crimped wires. Spoke with my mechanic and they said it could be the speed sensor causing the issues but I don’t have a CEL so it really is process of elimination
You previously stated the speedometer/odometer was not working and you found a blown fuse. You thought it was Fuse 17, but that one has nothing to do with the PSOM, speedometer/odometer or VSS. I suspect the blown fuse was Fuse 18.
If my theory is true then that is the cause of your shifting issues and it's a matter of time before you will probably see the O/D lamp in the shifter start to flash. The indicates the computer senses a problem related to the transmission. Yes that includes a PSOM that has no power.
You previously stated the speedometer/odometer was not working and you found a blown fuse. You thought it was Fuse 17, but that one has nothing to do with the PSOM, speedometer/odometer or VSS. I suspect the blown fuse was Fuse 18.
If my theory is true then that is the cause of your shifting issues and it's a matter of time before you will probably see the O/D lamp in the shifter start to flash. The indicates the computer senses a problem related to the transmission. Yes that includes a PSOM that has no power.
You are correct it was 18 I replaced it and it immediately blew again. The o/d has always blinked for the several years I’ve owned it I thought that was its normal state 😬 Sorry to be so ignorant but I’m not sure what PSOM is an acronym for
nevermind I have found what PSOM is. I have no other problems with the cluster save the digital odometer going in and out.
Last edited by 500k96F150; May 10, 2019 at 06:56 PM.
Reason: Question answered
It converts the raw signal from the VSS located in the rear axle to something the computer, speedometer/odometer and speed control can use. It is powered by Fuse 8, retains trip odometer info (always Hot), and Fuse 18 (Hot when the key is in the Run position). The PSOM is located behind the instrument cluster.
As previous stated Fuse 18 supplies power to several other circuits. You need to figure out which one is pulling too much current and blowing the fuse. Without the PSOM doing it's job the computer will control the transmission with default/limp home values which usually includes late/hard shifts because the computer jacks the line pressure up to save the internal clutches.
