I bought this 1992 F-350 DRW w/ 7.3L IDI and E40D in 2022. Carfax said I am at least the 6th owner. The odometer LCD is illegible and from the owner’s claim and Carfax information, I’m reasonably confident the mileage is high 18x,xxx. The owner said he had the transmission rebuilt. Within a short time, I learned there was a transmission and ABS problem. The Rear ABS light would turn on and off occasionally. Reverse felt weak, sluggish and frequently when I would come to a stop, it felt like it never downshifted, stuck in 3rd gear without power. I could hit the accelerator and feel it clunk into a lower gear and take off. So I took it to a local recommended shop who found burnt trans fluid, “excessive material in pan and filter” and 3 valve body bolts in the pan. They also found continuous memory codes 29 (speed sensor), 49 (1-2 shift error), 59 (2-3 shift error), 62 (converter clutch slip 3/2 or 4/3 shift fault), 69 (3-4 shift fault). I had them rebuild and upgrade the transmission. They replaced the rear diff VSS, MLPS at the trans body, and the brake light switch.

In a week or two, I started feeling a similar problem. After coming to a stop, it felt like it wasn’t in first. It was intermittent; on some drives it shifted normal, others weren’t. It was also doing something new: occasionally it felt like there was an additional lower gear, like more torque at a stop. I lift off the pedal a bit and the truck lunges. Also, occasionally, while stopped in drive, the engine was so bogged down, it seemed near stall. I took the truck back to the shop who found nothing wrong on test drive. I spent 3 hours driving around the area until it had a symptom (lunging at stop) and got their technician inside to experience it. They pulled the pan and found a barely sticky 1-2 shift valve, addressed it and gave it back. The owner told me to watch for CODE 29, VSS Signal.

The truck continued to do the same things. I finally bought a code reader which showed CODE 29 (VSS) and 49 (1-2 shift error) in the continuous memory. I pulled the VSS out and it was a clean Motorcraft part, no reason to suspect it wasn’t the new part I paid them to put in. My Innova manual says a Code 29 is also “insufficient input from PSOM”.

After reading about the PSOM in forums, I swapped in the spare from a 1992 F-150 cluster I had originally bought only for its donor odometer LCD. That PSOM shows 70,xxx miles. Now after many start/stop/on/off drives over a few days, it has NOT repeated the shift symptoms. It sometimes still logs a Code 29 and at least once it logged a Code 49 1-2 shift error, however now I cannot tell anything is wrong like I could with the previous PSOM plugged in.

The original PSOM speedometer needle never bounced erratically, its always quite smooth. I have never got an o/d OFF light flashing at the stick and the bulb/switch work correctly. The ABS light has not come back on since rebuild/repairs.

My next step is to drive with the PSOM disconnected and observe any symptoms. If none, then I’d reconnect the original PSOM. If the problem persists, I would plan to have the PSOM repaired.

What do you think about this issue? What would you do next? Can the PSOM be responsible for this? Any thoughts or recommendations out there?