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I just learned about the PSOM. I have a 96 5.8 with the E40D. Hard shifting at 30ish, 45ish, and after replacing the VSS in the rear diff, the speedo is wild above about 50mph but the shifting is less hard (but still too hard). I even tried a second VSS and that was the same as the first new one (but better shifting and worse speedo than the original).
Someone mentioned the PSOM. I'm trying to figure out if this is a likely culprit considering the above issues, and considering that the speedo part of the problem only started after I replaced the VSS.
I'm pretty confident it's not the u-joints, and I've got clean fluid clean filter and greased driveshafts (that was no fun).
UPDATE:
So another thread and this one converged on the ABS module. I removed it, and the shifting was fixed the first, third, and fourth time I drove it. The second time, the shifting was much better but still too hard. In all cases the speedo now behaves.
Why would it work 3/4 times? Does the PSOM have to relearn how to shift? Or do I have an intermittent problem?
Last edited by wannabuyaford; Sep 7, 2023 at 09:38 PM.
Reason: Stuff Changed
Could be the PSOM or the ABS module dragging down the signal from the vss. You can unplug the abs module temporarily to see if your PSOM behaved itself. Could also be crappy aftermarket vss. Have you tried a ford or motorcraft units. If you have a oscilloscope (or a multimeter), you can watch the vss signal from under your hood by using the VSS test connector that ford built into every vehicle.
Could be the PSOM or the ABS module dragging down the signal from the vss. You can unplug the abs module temporarily to see if your PSOM behaved itself. Could also be crappy aftermarket vss. Have you tried a ford or motorcraft units. If you have a oscilloscope (or a multimeter), you can watch the vss signal from under your hood by using the VSS test connector that ford built into every vehicle.
Thanks! This is tantalizing information, can you complete the thoughts?
I took your comment and did some searching but I am not finding ABS module info for the 96 era, and I don't know where this "VSS test connector that ford build into every vehicle" is. Even if I did, I wouldn't know what a passing test would look like. Can you share?
I see your photo, and the wires are disappearing under the hood. It could be that where they are going is the same test connector that I use to pull the computer codes with a test light. If so, I know where it is but not how to connect a test lead or what to look for.
I think this would be a great two things to do, and hope you are willing to tell me what exactly you mean. As I said, a YT and general search came up dry for this era. Thank you.
Could be the PSOM or the ABS module dragging down the signal from the vss. You can unplug the abs module temporarily to see if your PSOM behaved itself. Could also be crappy aftermarket vss. Have you tried a ford or motorcraft units. If you have a oscilloscope (or a multimeter), you can watch the vss signal from under your hood by using the VSS test connector that ford built into every vehicle.
Could I pull an ABS fuse just to run the same test?
Thanks! This is tantalizing information, can you complete the thoughts?
I took your comment and did some searching but I am not finding ABS module info for the 96 era, and I don't know where this "VSS test connector that ford build into every vehicle" is. Even if I did, I wouldn't know what a passing test would look like. Can you share?
I see your photo, and the wires are disappearing under the hood. It could be that where they are going is the same test connector that I use to pull the computer codes with a test light. If so, I know where it is but not how to connect a test lead or what to look for.
I think this would be a great two things to do, and hope you are willing to tell me what exactly you mean. As I said, a YT and general search came up dry for this era. Thank you.
This is the vss test connector under the hood. I believe only obd-1 trucks have them, but I might be wrong. You have a 1996 f250, so if it's a federal truck, it's obd-1, if it's a California truck, it's obd-2. You can also check the resistance of your vss sensor coil. It should be around 1k ohms.
Could I pull an ABS fuse just to run the same test?
No. The ABS module might be damaged and might be literally shorting out the vss wires, muting the signal. You gotta disconnect it for the test. The ABS module is behind the glove box. It's a black square.
Could I pull an ABS fuse just to run the same test?
No. Electrically the RABS control module may be loading down the VSS signal. Removing the fuse is not a substitute. You have to remove the connector to the RABS module located behind the glovebox.
This is the vss test connector under the hood. I believe only obd-1 trucks have them, but I might be wrong. You have a 1996 f250, so if it's a federal truck, it's obd-1, if it's a California truck, it's obd-2. You can also check the resistance of your vss sensor coil. It should be around 1k ohms.
I'm pretty sure it's neither of those but to be honest I'm still not sure after all this time if it's ODBI or II. I have never read a good method to find out. But it's not from california I don't think.
Anyway thank you for the photo, but you still haven't told me what to test for. If I find that piece, and put the multimeter on, what reading do I want and what should I do to to car to perform the test?
Also you mentioned another component, the VSS coil. I can check its resistance, sure, but can you tell me where it is? Or point me to a reference so I can look these things up? I appreciate your time but practically speaking, half-information gets one no farther than no information. I'm happy to do the research if there is a good reference guide for this stuff but I haven't found one. If you know of one, please share.
No. Electrically the RABS control module may be loading down the VSS signal. Removing the fuse is not a substitute. You have to remove the connector to the RABS module located behind the glovebox.
So that means disconnecting the black connector that I circled, yes? I don't know how you got this photo. In mine, there are layers of stuff on top (really below) that box, so it's hard to see and access but I do see the black connector and can make a go at it. It's easier to change a tranny filter than play with these hidden electronics.
When I did, the speedo normalized and the shifting got better, then rough after it was at operating temperature, then better again, and I just drive 30 miles with very smooth shifting.
I'm having trouble understanding what caused it to be smooth - but also then rough and then smooth again. Like, should I go back to worrying about the PSOM, or call it good?
Why would it have been rough that one time if it's good?
Why would it be smooth since then if it's not?
Is it possible that the PSOM had to "relearn" how to shift correctly, now that the faulty ABS is out and what I assume was a faulty VSS was replaced? Like, now it's getting good signal, it had to shift poorly until it could relearn how to shift correctly?
If that's a real thing that happens, then I think I'm good! If not, I still have to look into this stuff.
1994 f150 5.0 E40D Throwing a 452 code. No speed sensor to PCM. Speedometer works fine. Runs ok but very harsh shift. It has a new transmission and torque converter. It had a bad tranny when I bought it, so the first thing was to have it rebuilt. It still shifted very hard from day one. No check engine light. New speed sensor in differential. Cruise control doesn't work. I was thinking that the PSOM might have an issue?
Found and corrected an "open" PSOM but still shows no speed sensor input to PCM
It is a good chance the PSOM has a leaking capacitor on it causing your intermittent trans shifting issues. As these electronics are reaching almost 30 years old, they are failing. A junkyard PSOM may have same issues, so its a chance to take. I am now on the 5th PSOM board in my E350 now. The capacitor acid does a number on the multy-layered PCBs
There are many many threads here about the PSOM problems with good diagrams and photos to help out.
