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Hi everyone! I've got a 96 f250 with the 7.5 and am going crazy looking at my speedometer bounce and rattle like crazy. It does it at all speeds very rapidly and jumps
+/-20 mph. Thankfully it's atleast centered on the right speed.
Here's what I have done:
-replace VSS on rear diff and that didn't fix issue. I reinstalled original part.
- verified the computer is receiving the incorrect speeds using an OBD2 sensor. In other words, the computer is reflecting the jumping speeds.
I believe these two things mean it's a signal issue so either wiring or something in the rear diff. If the computer is getting messed up speeds, I don't think it can be my cluster. Are these assumptions wrong? Is changing my fluid (probably hasn't been done in a while) the next step?
Try pulling the sensor again, and see if you can tell if the shutter wheel's firmly attached to the diff.
Then yeah, start checking the wiring as it goes forward. If you have access to an oscilloscope, that will be
invaluable in checking what's going on with the signal from the sensor in the diff.
You will need to dig up the wiring diagrams for your truck as there are a couple possibilities with the way the signal is routed, I'm not sure on this vintage truck but it may either go from the rear diff to the RABS controller first then on to the PSOM, or there is a completely separate speed sensor on the trans or t-case that feeds the speedo.
The VSS signal from the rear axle goes to the RABS controller on it's way to the PSOM. We have seen the RABS controller pull down the VSS signal enough to cause the fluctuating speedometer and VSS code. Pull the connector to the controller located behind the glove box, then see if the problem persists.
If the problem is still there you most likely have a failing PSOM.
I have repaired 5 OBS’s in the last 8 months, each having the same issue.
3 only needed a new Tone Ring, 2 needed both a Tone Ring and the VSS that reads the tone ring(sensor got chewed up). If I were a betting man, I’d say that the tone ring is your issue causing the Speedo bounce. The lower the speed that the bounce begins, the worse the tone ring is damaged. (Pitting on the teeth or actual missing teeth)
of note, you don’t have to remove the ring gear to remove the tone ring but it does make it easier. However, you do need to remove the whole assembly from the rear dif to take it off.
Try pulling the sensor again, and see if you can tell if the shutter wheel's firmly attached to the diff.
Then yeah, start checking the wiring as it goes forward. If you have access to an oscilloscope, that will be
invaluable in checking what's going on with the signal from the sensor in the diff.
fwiw,
t
I'm not familiar with the shutter wheel, can you please give some info on that? I'm hoping it's wiring because I think at this point that would be the best case. The tone ring looked fine from the outside but I didn't check it completely. Unfortunately I didn't read the forum in time, but there was some metal in my gear oil so that could be the issue with some flat teeth or missing teeth.
I have repaired 5 OBS’s in the last 8 months, each having the same issue.
3 only needed a new Tone Ring, 2 needed both a Tone Ring and the VSS that reads the tone ring(sensor got chewed up). If I were a betting man, I’d say that the tone ring is your issue causing the Speedo bounce. The lower the speed that the bounce begins, the worse the tone ring is damaged. (Pitting on the teeth or actual missing teeth)
of note, you don’t have to remove the ring gear to remove the tone ring but it does make it easier. However, you do need to remove the whole assembly from the rear dif to take it off.
Hope that helps.
Well that ain't good news. I did see some metal in my gear oil so I think I might have some flat teeth in there. What it's worth, it didn't look too bad, but unfortunately I didn't read your post before. I changed my oil yesterday. The issue is still persisting and didn't get better. Starts at very low speeds, literally one mile an hour. I will say it bounces more at higher speeds for whatever that's worth. Hoping it's just wiring.
If the problem is just wiring, how would I test that? I thought about wiggling around wires while it's up on. Jack stands in gear, but I'm not super great with that and feel a little sketchy starting my truck on jack stands
I have repaired 5 OBS’s in the last 8 months, each having the same issue.
3 only needed a new Tone Ring, 2 needed both a Tone Ring and the VSS that reads the tone ring(sensor got chewed up). If I were a betting man, I’d say that the tone ring is your issue causing the Speedo bounce. The lower the speed that the bounce begins, the worse the tone ring is damaged. (Pitting on the teeth or actual missing teeth)
of note, you don’t have to remove the ring gear to remove the tone ring but it does make it easier. However, you do need to remove the whole assembly from the rear dif to take it off.
Hope that helps.
Here's a video I filmed yesterday while changing my rear fluid. I have no idea, but the tone ring looks at least halfway decent to me. I saw a video on YouTube and his looked much more damaged. I know that this is only half of the tone ring, but here's the video on YouTube
Sorry for the weird breathing, as you can imagine I was contorted in 15 different directions
I have repaired 5 OBS’s in the last 8 months, each having the same issue.
3 only needed a new Tone Ring, 2 needed both a Tone Ring and the VSS that reads the tone ring(sensor got chewed up). If I were a betting man, I’d say that the tone ring is your issue causing the Speedo bounce. The lower the speed that the bounce begins, the worse the tone ring is damaged. (Pitting on the teeth or actual missing teeth)
Out of curiosity, how exactly do the tone rings fail? I have been inside a couple 8.8 diffs to change bearings and seals, and on one of them I even removed the tone ring because it wasn't needed, but I still don't see how they fail. Do they get loose and fall off after a while? Do the teeth get rusty?
The speedo is bouncing a little bit in my '94 F150. I bought a spare PSOM as I want to convert my US truck to Km/hr anyway, but I was hoping it also cures my slightly bouncing needle.
The VSS signal from the rear axle goes to the RABS controller on it's way to the PSOM. We have seen the RABS controller pull down the VSS signal enough to cause the fluctuating speedometer and VSS code. Pull the connector to the controller located behind the glove box, then see if the problem persists.
If the problem is still there you most likely have a failing PSOM.
What do you mean by pull the connector? According to that diagram is looks like signal will still get through from VSS to the psom without connector. So just disconnect it all together?
What do you mean by pull the connector? According to that diagram is looks like signal will still get through from VSS to the psom without connector. So just disconnect it all together?
Disconnect/ unplug.
Yes, it will, which is why he said that the signal can be weakened when it goes through the RABS module. Unplugging the module will allow the PSOM to get a stronger signal. If the speedometer is better then you have found the problem.
