PSOM test procedure verification
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4th Gear
Joined: Apr 2023
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PSOM test procedure verification
Hey all. Having an issue w/ my '97 F250 HD. 104k on the original 460 and rebuilt E4OD.
Just recently I've encountered hard shifts and a non-functional speedometer. The digital odometer display shows correct mileage but will not 'roll' as the vehicle is being driven.
The speedometer needle will 'sweep' in test/program mode, but is non-functional when driving.
I've spent hours reading the forum and here's what I've diagnosed so far:
In trying to determine if the PSOM is bad (that's the direction I'm currently leaning), which pins would I test on the C252?
Just recently I've encountered hard shifts and a non-functional speedometer. The digital odometer display shows correct mileage but will not 'roll' as the vehicle is being driven.
The speedometer needle will 'sweep' in test/program mode, but is non-functional when driving.
I've spent hours reading the forum and here's what I've diagnosed so far:
- Unplugged RABS module (although I have drum brakes all around... so I highly doubt I have ABS?) but no change.
- Original VSS showed 1950 ohms. Replacement (which I later found was not installed far enough into the pumpkin) showed 1730 ohms. The original is in the vehicle right now.
- I've verified continuity from VSS sensor to the RABS
moduletest-port under the hood. - Verified a more/less linear AC voltage increase at the RABS
moduletest-port while driving... although 'idle' voltage is 0.5v AC. Which I find quite strange. - Confirmed continuity from VSS to pins 4 & 5 on the female (C252?) connector that plugs into the PSOM. (I front-probed this with the PSOM unplugged and also back-probed with PSOM attached).
- I'm using this picture for reference. I believe this 12-pin connector is C252, right? https://www.ford-trucks.com/forums/1...l#post20317135
- Confirmed key-off 12v between pins 1 & 2 on C252 (front-probe)
- Confirmed engine-running 12v between pins 3 & 2 on C252 (front-probe)
- Verified a more/less linear AC voltage increase at pins 4 & 5 on connector C252 while driving (both front and back probe both with and without PSOM plugged in).
In trying to determine if the PSOM is bad (that's the direction I'm currently leaning), which pins would I test on the C252?
- My brain says I *should* see voltage between pins 2 (ground) and 7 (PSOM output to PCM).
- Is it AC or DC? My gut says AC, but it also feeds to the PCM... so who knows?
- If testing between pins 2 and 7 is valid and I see 0v AC across those pins (while driving), then my PSOM is demonstrably bad. Right?
- If testing between pins 2 and 7 is not valid, then which pin combination should I be testing? AC or DC?
Last edited by daikerjohn; Apr 20, 2023 at 06:26 PM. Reason: Add items 8 to diasnosed items. RABS test-port clarification.
Hotshot
Joined: Jul 2020
Posts: 10,541
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From: Near Salt Lake City
That VSS should put out AC millivolts
You have no scan tool to check the VSS pid?
It will be reading low or no increasing voltage with speed IMO from what you have said
We used to run them on a hoist with the instrument cluster pulled out, and check the voltage there (at the cluster)
The ABS and PCM and PSOM all use the VSS signal and there was a splice in there somewhere if I remember right
You have no scan tool to check the VSS pid?
It will be reading low or no increasing voltage with speed IMO from what you have said
We used to run them on a hoist with the instrument cluster pulled out, and check the voltage there (at the cluster)
The ABS and PCM and PSOM all use the VSS signal and there was a splice in there somewhere if I remember right
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4th Gear
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Thanks for the reply.
My meter only has 100mv resolution, but it showing 0.5v AC (500 mv) is weird. I do have an analog meter my Dad lent me years ago for this express purpose though... I'll dig that out to see if the analog meter says anything more interesting. And trustworthy.
I have a EEC-IV scan tool and it shows no codes... just 111 for everything. There *was* a stored 452, but I suspect it was from when I'd had the aftermarket/parts store VSS (improperly) installed.
I re-installed the factory VSS (after a light cleaning) and did all my measurements. Scanned again and saw no codes *except* the 452... which I cleared.
I'm still hung-up on whether testing for AC voltage between pins 2 & 7 on the PSOM connector is useful. Is it? Are those the right pins?
My meter only has 100mv resolution, but it showing 0.5v AC (500 mv) is weird. I do have an analog meter my Dad lent me years ago for this express purpose though... I'll dig that out to see if the analog meter says anything more interesting. And trustworthy.
I have a EEC-IV scan tool and it shows no codes... just 111 for everything. There *was* a stored 452, but I suspect it was from when I'd had the aftermarket/parts store VSS (improperly) installed.
I re-installed the factory VSS (after a light cleaning) and did all my measurements. Scanned again and saw no codes *except* the 452... which I cleared.
I'm still hung-up on whether testing for AC voltage between pins 2 & 7 on the PSOM connector is useful. Is it? Are those the right pins?
Hotshot
Joined: Jul 2020
Posts: 10,541
Likes: 2,664
From: Near Salt Lake City
I only trust a factory evtm diagram
So, as far as which pins????
Maybe you can use wire color for reference but,
I would not just trust the wire colors on anything but a factory EVTM
007
So, as far as which pins????
Maybe you can use wire color for reference but,
I would not just trust the wire colors on anything but a factory EVTM
007
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
Attachment 277292
Attachment 277293
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Thread Starter
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4th Gear
Joined: Apr 2023
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Thanks @subford ! I was secretly hoping/knowing you'd chime in here!
Knowing a test across pins 2 & 7 is valid makes be more confident in my preliminary diagnoses of a bad PSOM.
One clarification though: at lower speeds (say 10 or 15mph), would the PSOM output show a lower voltage or a lower frequency? Or both? Unfortunately I don't have an oscilloscope so I'm left with an analog meter. I suppose it won't matter in my instance because 0v AC would be quite telling either way.
To add:
I had the realization overnight that my ABS light may actually be burned out. I also found video on how to pulls codes from the RABS module by shorting a connector below the steering wheel (
)
It also occurred that after I'd re-installed the factory VSS and verified connectivity and AC voltage at speed... I should have re-tested with the RABS module disconnected.
I've got a couple social engagements today but hopefully I can report back tonight with a final diagnoses.
Thanks for the guidance folks!
Knowing a test across pins 2 & 7 is valid makes be more confident in my preliminary diagnoses of a bad PSOM.
One clarification though: at lower speeds (say 10 or 15mph), would the PSOM output show a lower voltage or a lower frequency? Or both? Unfortunately I don't have an oscilloscope so I'm left with an analog meter. I suppose it won't matter in my instance because 0v AC would be quite telling either way.
To add:
I had the realization overnight that my ABS light may actually be burned out. I also found video on how to pulls codes from the RABS module by shorting a connector below the steering wheel (
It also occurred that after I'd re-installed the factory VSS and verified connectivity and AC voltage at speed... I should have re-tested with the RABS module disconnected.
I've got a couple social engagements today but hopefully I can report back tonight with a final diagnoses.
Thanks for the guidance folks!
Fleet Owner
Joined: Oct 2005
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From: Kentucky
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Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
I bench tested a PSOM to test it and set my generator so it would point to 30mph.
Here is an image of my frequency counter doing the test.
Attachment 277290
Attachment 277291
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Here is an image of my frequency counter doing the test.
Attachment 277290
Attachment 277291
/
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