Okay I have a 1995 F150 with a 4r70w and the od light starts blinking at 25-30mph. I checked the voltage at the rwabs connector under the hood and found 1.7 to 2 volts present around the 30 mph mark. So I unplugged the rwabs and the voltage didn't change, I unplugged the psom again with no change, so I went to the Ford dealership and bought a new vss and still the light starts flashing at 25-30 mph. So I took the rear cover off the diff and measured the sensor gap and found it right around .050 in which according to some specs is good and some it is over the max. I checked all the teeth on the sensor wheel and couldn't find any damage. The truck shifts fine with no issues and the speedo does not bounce until you get up to 65mph then it goes crazy and bounces from like 40mph to 80 mph. Also once it shifts into overdrive it seems to have a hard time holding overdrive and down shifts into 3rd with any load that is applied. The only code I am getting is dtc 452. I am at a loss right now and don't know where to go. I think it may be the psom but not sure. I have also seen where the speed sensor in the back of the transmission can cause problems. Anybody got any ideas?

 

Jack up truck put it on jack stands put truck in neutral rock wheels back and forth, watch pinion for play if your pinion is moving up and down or side to side your carrier is moving as well causing the vss to read and not to read.

 

Okay so I jacked the truck and checked for play and couldn't find any play. So I began checking the wires and found that the connector at the sensor had varring amounts of resistance depending on how I moved the wires. So I replaced the connector with a new one. Before I connected it to the truck and checked the voltage out of the sensor with nothing else in the circuit. I got between 3.38v and 3.5 at 30mph. Please keep in mind that I cannot keep it at exactly 30mph due to the uneven roads and that my cruise isn't working. So I connected the new connector to the truck and went on a test drive and still the light started flashing at 25-30mph. So I started over and checked the voltage to find it no longer bouncing around and sticking to 2 to 2.1 volts at 30mph. So I disconnected the rwabs and now my voltage is the same as my voltage from checking just the sensor at 3.38 to 3.5 volts. So I know I have a problem with the rwabs module. Now with the rwabs unplugged the speedo no longer bounces at all but the light still starts blinking at 25-30 mph. Still the only fault I am getting is 452 even with the rwabs unplugged. So now my question is where do I go now? Is it setting the fault still because I have the rwabs unplugged or is it cause I still have problems elsewhere?

 

According to my info Code 452 is a stored code in Continuous Memory (CM). Clear the codes then go for a drive.

 

I cleared them with my code reader and it comes back every time at the same vehicle speed. Always starts flashing at 25-30 mph with the same code

 

Therefore you still have a problem.

 

I know I still have a problem. The question I had is where to go to check not to state the obvious. If I had fixed the truck by repairing the wire and removing the rwabs I would not still be asking questions. Is it possible to swap in a different psom and see if the light stops blinking or are they so specific that they cannot be swapped from one truck to another?

 

Sounds like you are trying to trouble shoot a 4R70w WITH e4od information.
The PSOM and the rwabs sensor (VSS) has nothing to do with the transmission.
You can disconnect the PSOM and the rear end sensor and the transmission will shift just fine if it has no other problems. I had the PSOM die in my 1995 F150 with a 4R70W transmission and I drove it just fine for a week until I got around to putting the mileage in another PSOM and installing it in the truck.
The 4R70W transmission shifts off the Output Shaft sensor (OSS) and not the VSS.
You might have a bad OSS.

As for the 452 error code you might have a bad Speed Control Module, RABS module, PSOM wiring or problems in the rear end as in bad bearings.
You can try unplugging the RABS and the Speed Control modules and see what happens.

I am not sure how a PCM will shift the transmission if it has conflicting speed information from the two sensors. But you can leave the PSOM unplugged and see how the transmission shifts.

 

Thank you for the new information and that explains why my transmission is shifting just fine with no problem. And now that I fixed the wiring issue and found a bad rwabs and I am still getting a fault for the vss signal I am just trying to get any other diagnosis tips to check the last two components. I am leading towards the psom. Honestly I am trying to get my cruise control to work and to keep the od light from blinking. Next weekend I will use an osliscope to look at the wave pattern from the rear end. I just want to know can parts be swapped and if I have the psom rebuilt do they use the new updated part that can ignore minor problems in the wave form?

 

You can sub any PSOM from 1993-1997 in for a test but the sub PSOM will have the truck it came out of tire size and mileage.I think any PSOM after 1994 will have the needle wave fixed.

I have no idea what they do when they rebuild a PSOM. But I think they just replace the caps and maybe some board repair.

I get a replacement PSOM from a pick-n-pull salvage yard and copy all the information out of my old PSOM boards EEPROM to the new PSOM EEPROM. That way I keep my old mileage and tire setup and reset it to 6 changes left.

 

I was finally able to get the fault to quit setting by changing the psom out and at the same time I installed a cluster with tach so killed two birds with one stone. I also got lucky and the new speedometer is dead on so I didn't have to calibrate it. Thank you everyone for the help. Now on to fixing the front tank fuel pump.

 

Thanks for the reply back with the resolution.

 

I'm having an issue with speedo,odometer,radio,the inside nightlight and rough shifting this all happened at once I was leaning toward the vss or a blown fuse I've replaces the vss and that did nothing anybody think they know what it might be????

 

if your dome light fuse blew,then so did the PSOM memory.remove the dome light assembly and check for screws shorting to the cab.if so,install shorter screws,then replace fuse.