i've been trying to chase down a couple problems i've been having with my 95 f150 for quite sometime to no avail.

when i first bought the truck a little over a year ago i noticed it shifted rather harshly. especially from 2-3 it seemed to start to shift and then its almost like the transmission went into limbo and when it finally caught 3rd it would slam into gear. this was concerning as the transmission was replaced 10k before i bought with a new jasper unit (wouldnt have been my first choice) so i pulled codes and got an error for VSS sensor. so i replaced the sensor on the mount of the rear dif with a OEM motorcraft. this seemed to soften up the shifts but it was still unreasonable in my opinion for a new trans.

i did notice after replacing the VSS that the speedo needle would flutter around 50-65. its not significant but that leads me to believe the PSOM may be shorting out? i havent replaced yet because doing research on here most people who have a bad PSOM exhibit an out of control speedo and mine just flutters ever so slightly.

ive even went as far as taking the truck to a (what i was told) reputable transmission shop in my area. after a quick diagnosis and speaking to jasper since the trans is still under warranty they decided to replace the solenoid block in the trans. this in no was fixed the problem and the shop told me since my truck wasnt throwing any codes related to the problem they didnt want to look any farther. they did say when they dropped the pan that it was spotless so i dont believe its a problem with transmission slipping.

over the last couple months its progressivly gotten worse. sometimes the trans will downshift/ shift into neutral out of the blue.

another problem ive been having is since i replaced the fuel pump in January upon start up when i put the trans into gear the truck will start to sputter and shut off. ive hooked up my fuel pressure gauge and im getting a constant almost 50lbs at the rail. it doesnt drop when it sputters.

truck is throwing no codes at the minute and im almost at my wits end. its a real solid truck for having 260k miles and i can tell the PO took good care of it i just dont know what else to look for. any advice or assistance would be greatly appreciated

 

A PSOM does not "short out". There can be many causes of a bouncing speedometer. Since you already replaced the VSS, rarely fixes the problem, I would suggest unplugging the RABS control module (located behind the glove box). This is in the signal path between the VSS and PSOM.

If the bouncing speedo continues I would suggest to pull out the PSOM and look for signs of broken solder joints and/or leaking capacitors.

If all of that checks out okay then it could be worn bearings in the rear axle.

 

Welcome to FTE.

I went down this path. Shifting issues & no transmission codes leading to a rebuild with limp mode slamming shifts afterward.

My ECU had leaking capacitors & after getting a replacement, it spit out several 452 VSS/PSOM codes. (replaced VSS, no change.)

After pulling several VSS sensors in the junk yard, I have to say if the bearings are worn in the rear, you'll see evidence of either iron filings build-up on the magnetic sensor end or full scale damage due to the tone ring eating into the sensor.

After trouble shooting the RABS circuits, (unplugging the cruise servo as well which stopped the codes & replacing same, no change) I wound up replacing the PSOM & my shift issues disappeared.

 

am i understanding correctly that the RABS module should be throwing the ABS light if its malfunctioning? or is just one of those sometimes and sometimes not type of deals? will getting to the module require removing the entire dash and moving back to allow access or can it be reached without full removal? as for the VSS the origional showed very little signs of iron shavings on the magnetic portion. certainly not enough to where i would be concerned of the bearings in the rear. ive had one of these rear ends fail in another 150 i had and the symptoms are worlds apart. maybe ill crack the rear dif cover and take a look anyway just for peace of mind if it comes to that.

 

The RABS module can be faulty, display no ABS light but it's impedance is low enough to drag down the VSS signal which causes the bouncing speedo needle.

The same thing can happen, very rare IMHO, with the Speed Control module. It's a simple process of elimination.

Remove the glove box door for easy access:

courtesy of Steve83

Or reach up from behind the dash.


That's a good sign the root cause is most likely beyond the rear axle.

 

unplugged the RABS module and took it for a drive. it didnt stop the fluttering speedo. shifting seemed a tad bit more responsive maybe? it didnt seem to get stuck in the limbo between 2-3 like i mentioned earlier. still shifted harshly however.

so next course of action would be removing the gauge bezel and PSOM and opening that up? unless there is some other way i can test the RABS module that im not aware of?

appreciate the help thus far!

 

I would try unplugging the Speed Control servo next. If that does not resolve the problem, it's time for a look see at the PSOM IMHO.

 

from what i'm reading the speed control servo is for cruise control correct? my truck doesn't have cruise. bare bones XL so i'm assuming my truck isn't equipped with said servo?. so that should leave the PSOM as the most likely culprit. guess ill try and find time in the next few days to pull it.

 

Speed Control is Ford's name for cruise control. If the truck dies not have it, there is no servo for it on the driver side inner fender.