Hello-
Started my truck this morning to find that my speedometer and digital odometer stopped working. Auto transmission started shifteing really hard too. Checked the fuses and transmission fluid level, everything looked ok. From the quick search I did here, consensus seems to be that either the PROM or the VSS is responsible for these things. Is there anyway to test the PROM before I get another one? There are no junkyards anywhere around me, so I have to try to find one on ebay, etc. When I get home tonight, I will check all of the electrical connections. This just happened; drove home on friday and everything was fine, drove to work today, and problem was there this morning. This is a 93 F-150 with the 5.0.
Thanks!

 

I don't think there's much difference in the '92 and '93 as far as the speedo and odo are concerned (same signal for both). Don't bother looking on the tranny for a speedo pickup. The speedo signal comes from the rear axle, and is the same signal used for the RABSI (Rear Anti-Lock Brake System, version I). The information I have says RABSII was introduced on '94 models. Is your RABS light coming on? Check the wiring coming from the rear axle, particulary around the dif. See if something came unplugged back there. RABS light might not come on if the speed signal is missing altogether, and certainly won't come on if the RABS light is blown (tongue firmly in cheek). Weird shifting would certainly be a good indication that the speed sensor is kaput, or at the very least, disconnected. I have a manny tranny, but noticed the first time I was under the truck there was no speed sensor in the tranny tail section. Unlike 4 wheel ABS, RABS only looks for a sudden change in axle speed (ring gear) while brakes are applied, as would be expected when you stick the nose in the dirt because some little old lady pulled out in front of you. Fronts will still lock up and flat-spot, but backs will pulse and keep the back where it should be, behind the front.

 

pull codes www.fordfuelinjection.com shows how. you probably have a bad mlp solenoid , but also unplug your vss and clean it good

 

My ABS light does not come on, and I know the light works because it came on when I pulled that fuse. My cruise control does not work either. My check engine light does not work period. (yes I know, this truck has alot of electrical issues). I'm going to look underneath really close tonight, and clean the vss.

 

Here is how your 1993 is wired.

 

Sounds like you have lost your PSOM Board. They are all the same from 1992 up through 1995 and maybe 1996. It just unplugs out of one cluster and plugs into your cluster. Whatever mileage is on it will come along with it and you will have to reprogram it for your tire size. You only get to reprogram it about four times.

 

Huh. It's the oddest thing. Yesterday morning when this happened, I checked every single fuse that had to do with anything speed and transmission related. I was driving home last night, thinking about the problem. I pulled out the fuse cover, and went to turn on the map light, and lo and behold, that didn't work either. So, I check the maplight fuse, and it is blown. Replace that, and now all of my problems are cured. If I recall, it was fuse position number 8, and I don't think that there are any transmission/speedometer functions on that fuse.

However, I'm just glad that everything is working again, and am grateful to all of the help and advice. I think I will pull out my VSS and clean it this weekend, just to be on the safe side.

By the way, just out of curiousity, will a PSOM board from a same year 250 or 350 work in a 150?

 

Yes, as I understand all PSOM are the same. I just might have look up all the part numbers for all the years and see if they are the same.
Here is a photo of a PSOM out of a 95.

 

Fuse 8 is also Speedometer Memory.

 

The PSOM modules out of any F-series or bronco, from 92-96 are the same. 97 F250/350/450 also.

If you want to test your PSOM you can easily do so. At the very bottom of the dashboard, behind the glove box, the black plastic dash support has a long slot in it which the wiring harness across the width of the vehicle is stuffed into. If you stick your head in the passenger foot area, you can look up and see the clips. Gently remove the clips, pull the harness out of the slot it's in, and unravel the tape from about the middle of the glovebox and unravel to the left (driver side). Somewhere in about a foot of distance, there will be a single black spade connector, that is labeled "PSOM".

Attach a volt meter between that lead, and a good ground, and let the truck roll very slowly. Below 2-3 mph is best. Your volt meter's needle should oscillate or swing around a bit, indicating a good signal from the PSOM to the EEC. That's what that wire is for.

If you have a steady voltage if the truck is moving, then the PSOM module is good but the sensor on the rear axle is bad, disconnected, or so dirty it can't follow the ring that goes around the differential.

If you get no voltage no matter what, then the PSOM module is bad. Or the black connector on the back of the cluster is loose or hanging off. Or the cluster isn't getting power, etc.