PSOM speedometer surge
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5th Wheeling
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PSOM speedometer surge
1996 Ford F150 4x4 5.8L w 132,000 on the engine and chassis.
I am having some issues with the speedomter surging (10-15mph) to a lower speed when using cruise control over 73MPH. I am having some needle bounce when not using cruise, but no surging. Any speed under 73 (using cruise) i don't have any surges but do have some needle bounce which i presume to be normal as its within 2mph and that seems to be in spec.
I just changed the diff fluid today and cleaned out the differential real good, no big chunks of metal, to be quite honest there was hardly any metal flakes either, tone ring seemed to be fine, no teeth missing or dents that were visible to the eye. I inspected the RABS sensor and there is no sign of damage. I also ohms tested the sensor itself and it read 1,110 ohms, I believe that is within spec.
I should also note I do not have any stored engine or ABS codes, although interestingly enough when i disconnect the RABS module (behind the passenger glove box) my surging disappears and my needle bounce reduces significantly. Why would this be and does this mean that the RABS module is directly related to my issue. Would it be worth my time to invest in a new RABS sensor to see if the problem goes away even though the ohms are within spec and there is no physical damage? Would replaceing the RABS module be a reasonable move. Or is this a sign of a faulty PSOM?
Any info/help would be greatly appreicated. I drive highway quite often 75mph+ so using cruise as of late has not been a viable option and i'd like to get that option back to its normal use. I do understand i could just drive the vehicle with the RABS module disconnected however, where i live during the winter RABS sure does come in handy so i would still enjoy the use of it.
I am having some issues with the speedomter surging (10-15mph) to a lower speed when using cruise control over 73MPH. I am having some needle bounce when not using cruise, but no surging. Any speed under 73 (using cruise) i don't have any surges but do have some needle bounce which i presume to be normal as its within 2mph and that seems to be in spec.
I just changed the diff fluid today and cleaned out the differential real good, no big chunks of metal, to be quite honest there was hardly any metal flakes either, tone ring seemed to be fine, no teeth missing or dents that were visible to the eye. I inspected the RABS sensor and there is no sign of damage. I also ohms tested the sensor itself and it read 1,110 ohms, I believe that is within spec.
I should also note I do not have any stored engine or ABS codes, although interestingly enough when i disconnect the RABS module (behind the passenger glove box) my surging disappears and my needle bounce reduces significantly. Why would this be and does this mean that the RABS module is directly related to my issue. Would it be worth my time to invest in a new RABS sensor to see if the problem goes away even though the ohms are within spec and there is no physical damage? Would replaceing the RABS module be a reasonable move. Or is this a sign of a faulty PSOM?
Any info/help would be greatly appreicated. I drive highway quite often 75mph+ so using cruise as of late has not been a viable option and i'd like to get that option back to its normal use. I do understand i could just drive the vehicle with the RABS module disconnected however, where i live during the winter RABS sure does come in handy so i would still enjoy the use of it.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
Sounds like a weak VSS signal.
When you unplug the RABS you are taking part of the load off the VSS signal.
This could be from a bad RABS sensor (VSS) or its gap is wide between the tone ring and the sensor.
Did you wipe off the sensor when you had it out?
When you unplug the RABS you are taking part of the load off the VSS signal.
This could be from a bad RABS sensor (VSS) or its gap is wide between the tone ring and the sensor.
Did you wipe off the sensor when you had it out?
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5th Wheeling
Joined: Mar 2017
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I tried getting a feeler guage in between the sensor and tone ring but there wasnt enough room to get it in there, unfortunately i didnt have an extra long pair.
So in theory if it turns out NOT to be the VSS sensor, should i suapect a faulty RABS module or a issue with the PSOM itself? Thanks for the response!
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
We have had a weak signal getting to the two modules before.
Some have just unplugged the RABS module and drove that way after replacing the VSS two or three times.
It could be that one or the other module is loading the signal too much or maybe even wiring or plugs between the modules and the VSS.
Both modules have about the same input part wiring.
So have never seen a good fix for this yet and I have never ran into the problem with any truck I have worked with.
The module input parts wiring:
Attachment 283102
/
Some have just unplugged the RABS module and drove that way after replacing the VSS two or three times.
It could be that one or the other module is loading the signal too much or maybe even wiring or plugs between the modules and the VSS.
Both modules have about the same input part wiring.
So have never seen a good fix for this yet and I have never ran into the problem with any truck I have worked with.
The module input parts wiring:
Attachment 283102
/
Thread Starter
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5th Wheeling
Joined: Mar 2017
Posts: 42
Likes: 0
We have had a weak signal getting to the two modules before.
Some have just unplugged the RABS module and drove that way after replacing the VSS two or three times.
It could be that one or the other module is loading the signal too much or maybe even wiring or plugs between the modules and the VSS.
Both modules have about the same input part wiring.
So have never seen a good fix for this yet and I have never ran into the problem with any truck I have worked with.
The module input parts wiring:
/
Some have just unplugged the RABS module and drove that way after replacing the VSS two or three times.
It could be that one or the other module is loading the signal too much or maybe even wiring or plugs between the modules and the VSS.
Both modules have about the same input part wiring.
So have never seen a good fix for this yet and I have never ran into the problem with any truck I have worked with.
The module input parts wiring:
/
I would like to think that since my truck is the newest/last version of the OBS that it would have came woth the updated PSOM.
I do like the use of the RABS when it works, i know certain people dont find it useful but i personally have never had an issue with it, until seemingly now with the needle waver.
I plan on looking for a mew RABS module and testing that out.
Thread Starter
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5th Wheeling
Joined: Mar 2017
Posts: 42
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We have had a weak signal getting to the two modules before.
Some have just unplugged the RABS module and drove that way after replacing the VSS two or three times.
It could be that one or the other module is loading the signal too much or maybe even wiring or plugs between the modules and the VSS.
Both modules have about the same input part wiring.
So have never seen a good fix for this yet and I have never ran into the problem with any truck I have worked with.
The module input parts wiring:
/
Some have just unplugged the RABS module and drove that way after replacing the VSS two or three times.
It could be that one or the other module is loading the signal too much or maybe even wiring or plugs between the modules and the VSS.
Both modules have about the same input part wiring.
So have never seen a good fix for this yet and I have never ran into the problem with any truck I have worked with.
The module input parts wiring:
/
From what Ford says there was an update to the PSOM called a PSOM3. Being that my truck is the last/newest of the OBS models, i would think that my truck would have already had the updated PSOM. Do you know off hand if Ford still makes/sells those units.
Contrary to the thoughts of many that i see, i do enjoy having functioning RABS, i have never had any issues with it and would definately prefer to have it up and available instead of driving around with it disconnected. But i may not have a choice as far as it sounds. I guess im sorta of between a rock and hard place since cruise is a neccessity for me and having functioning RABS is nice as well.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
I have heard that the PSOM3 board has a green board for ID.
I would think your truck would have the updated PSOM3 board.
I would guess that your problem is in the rear end and this can be checked by looking at the VSS wave form with an O-scope.
/
I would think your truck would have the updated PSOM3 board.
I would guess that your problem is in the rear end and this can be checked by looking at the VSS wave form with an O-scope.
/
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5th Wheeling
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Thanks for the diagrams, any chance you have the full wiring diagram for testing the psom signal? It looks like that wiring diagram you gave me is missing some info. I'll definately look into the rear end, also my truck has a red board on the back if that whats you are talking about?
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Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
Some images and diagrams:
Attachment 277289
Attachment 277293
The RABS/VSS test connector:
Attachment 277905
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Attachment 277289
Attachment 277293
The RABS/VSS test connector:
Attachment 277905
/
Thread Starter
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5th Wheeling
Joined: Mar 2017
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Looking at my engine bay and comapring to the picture you posted. I have neither the EEC self-test plug, im guessing because my truck runs EEC-V, and i also dont have that RABS plug, would my RABS plug be located anywhere else?
So there is an update to this issue, I am now finding out that regardless if I have cruise enabled or not I get a pretty erractic speedo needle when going 75 mph or more, as soon as i get to around 70 my needle simmers down.
So there is an update to this issue, I am now finding out that regardless if I have cruise enabled or not I get a pretty erractic speedo needle when going 75 mph or more, as soon as i get to around 70 my needle simmers down.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
My Ford Shop manual information show the C143 RABS Data Link Connector (PLUG) in that location for the 1996 5.8L engine.
Maybe you have a 5.0L engine or a 5.8L Under 8500 GVW or a California setup (OBD2 ??) ?
Maybe you have a 5.0L engine or a 5.8L Under 8500 GVW or a California setup (OBD2 ??) ?
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5th Wheeling
Joined: Mar 2017
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Any who it is OBD2 i guess i should have mentioned that. Unsure of any scan tools that can read the ABS via OBD2 in our trucks. The only way i know of diagnosing ABS codes is grounding the Blk/Org wire and getting a flash out code. I have no codes via ABS nor check engine.
After looking at your wiring diagram it looks like the RABS data link connector is near or at least in between the RABS module and the PSOM. Or is that wiring diagram referring to the OBD2 port as the RABS data link connector?
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