I have a problem with my speedo and I think it might be related to my VSS being replaced but still, things are not working correct. Here's a short summary of what I think is relevant:

• Several months ago I noticed my ABS and Parking Brake light constantly on while driving. Couldn't pull ABS codes, nothing came out, never resolved that, just unplugged the ABS computer
• About a month ago I replaced the ABS computer with one I got from the pull-a-part yard. The ABS and Parking Brake light worked as usual for one week and then came back on.
• A week ago I had my truck in the shop for some brake work on the front and I asked them to see if they could identify the issue with the ABS and Parking Brake light (posting here didn't give any answers).
• The shop said it's my VSS going bad in my rear diff that was causing the ABS and Parking Brake light to go on like it was. So they replaced it. Well, that seemed to fix the lights being on when they shouldn't but now I see my speedo jumping around at 70 MPP. Intermittently, when I am driving 70, the needle will jump back to 60 or 65 and then back to 70. Slightly worse the faster I go.
• Since it was the weekend the shop that did the work was closed and I wanted to see if I could replace the VSS and fix the jumping speedo. Got a BWD part and installed it. That didn't fix the jumping speedo and that makes me wonder if maybe there is something wrong in my diff and my VSS can't get the signal it needs...not really sure. Seems strange that two new parts would be bad like that.
• About twice in the last week the ABS light and Parking Brake light were on at the same time but after I turn off the motor that seems to go away. I am not sure if this is actually related to my problem or not.

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Anyone have any opinions on this?I have pulled codes from the PCM and they are clear, so there doesn't seem to be anything the PCM can detect.

 

Sounds like you need a graphing meter or oscilloscope to display the actual waveform being produced by the VSS so you can see any irregularities if any in the signal.

That way you'd know if the problem originates from the VSS itself or if the PSOM or RABS module either one are imparting noise in the system instead.

Yea sounds like the VSS/tone ring but doing so you can isolate the system test it add in one at a time testing again and that way you'd know for sure and before tearing into anything what the source of the problem is.

The AC voltage should range from 0 volts at 0 mph to 3.5 volts at 30 MPH, frequency should increase from 0 Hz at 0mph to about 125 hz at 55 mph, however you'd be looking for clean signal more over accuracy to start with.

 

Maybe I'll try putting back the RABS computer I removed as one step.

I am not sure I understand about the tone ring...have only heard of that and not really sure what it is other than it's in the diff. Are you saying that may be bad causing a problem with the signal but the rest of the diff is fine?

 

Odds of it being directly related to a problem with the tone ring itself are small but possible.
It be more likely to be related to bad bearings in the diff allowing the ring to spin off axis and or its distance from the sensor constantly changing the "air gap" fluctuation in and out of spec.

Myself I'd think by that point you be at least to a degree aware of noises emanating from the rear end, grind clank bang etc something causing you to suspect a problem with the rear end and before it became speed sensor issue.

You swapped the VSS and if had large issues in the diff enough to create tone ring problems, the VSS because its a magnet it would have been covered with at least some substantial sized metal shavings. Don't seem like you mentioned finding anything of the sort at least not beyond what would be considered normal wear, would have surely questioned it at least more perhaps than brief mention.

That's the reason for using the graphing meter or oscilloscope (I'd think a graphing meter be easier to come by these days) it'll show the exact output of the VSS and without need start tearing into the rear end, not even take the cover off just unplug couple things and connect to the test port under the hood.

You've had enough problems with it at this point, and doesn't sound like anyone has done so yet, so its time to do so find out what the deal is. Is it a problem with the VSS and its circuits or no its something else causing the problem.

You could try a simple voltage test, using a multi meter monitor output but may not react fast enough to notice a problem if there is one. At least not using a digital meter, an analog meter maybe but even then have doubts, unless suffers fairly large blips in its output.

 

The sender they replaced in the rear axle could easily be bad, too. It happens.

EDIT
Read the next post. I agree with it. The tone wheel is messed up.

 

The toothed gear on the left side of the photo is the tone ring.


Look closely, you will see where there is a slight "wave" in it. That one has been tweaked....


Bill aka subford has posted this diagram many times of how the VSS and tone ring interact:

 

when I removed the VSS that was replaced by the shop, I didn't notice any debris on the end that would interfere with the signal. I did notice the oil needs to be replaced back there, the level was good but oil was dirty.

I don't notice any issues with my rear end like noises or vibrations.

 

You say the ABS light and the brake light is on at the same time. I do not see anything in test C that relates to the VSS sensor. Not saying the speedometer jumping is not but most of the time the waving needle is a bad PSOM or so I hear.

YELLOW REAR ABS LIGHT ON AND RED BRAKE LIGHT ON -- TEST C
C1: LOW BRAKE FLUID
C2: MASTER CYLINDER FLOAT
C3: DIODE/RESISTOR ELEMENT
C4: DIODE/RESISTOR ELEMENT PARKING BRAKE AND/OR DIESEL LOW VACUUM SWITCH REMOVED
C5: ANTI-LOCK VALVE SWITCH
C6: MASTER CYLINDER SWITCH
C7: BRAKE LIGHT WIRING

 

what would make a run twice as fast as what you are really doing?

 

speedometer sorry

 

Not C1 or C2
Not sure about C3 & C4, would need more info. Where is this located, where do I test? Or is it in the valve on the frame rail?
Is C5 the valve on the frame rail? If so, I have not replaced that yet.
Not C6, when I remove the connector from the Master Cylinder one of the lights comes on but not both
Not C7 either, I did have an issue with my brake lights but only the ABS light came on, not both. And this has been fixed - switched back to regular brake light bulbs from the LEDs that were giving me a problem.

 

C3 DIODE/RESISTOR ELEMENT
Check for proper functioning of the diode/resistor element.
Turn ignition key to the on position.
Check parking brake and release if applied.

Both the REAR ABS and BRAKE warning lamps go off
REPLACE RABS diode/resistor element.

Both the REAR ABS and BRAKE warning lamps stay on
GO to Step C4.

C4 DIODE/RESISTOR ELEMENT PARKING BRAKE AND/OR DIESEL LOW VACUUM SWITCH REMOVED
Continue to check for proper functioning of the diode/resistor element.
Remove the parking brake switch and the diesel low vacuum switch, if so equipped.
Both the REAR ABS and BRAKE warning lamps go off
REPLACE RABS diode/resistor element.

Both the REAR ABS and BRAKE warning lamps stay on
GO to Step C5.

C5 ANTI-LOCK VALVE SWITCH
Obtain the flashout code as described in Diagnosis and Testing in this Section.
Flashout code is obtained
REFER to Flashout Codes Charts in this section.

REAR ABS and BRAKE warning lamps stay on steady
GO to Step C6.

 

Brake Warning Resistor/Diode Assembly 14A601 located LH rear of engine compartment, taped in harness near C202.
RABS Valve Assembly 2B373 located on LH frame rail, near front of transmission.

 

Am I the only one that thinks there is an issue with the tone ring itself? It looks bent to me.

The old musical instrument, a HAMMOND keyboard organ uses tone wheels similar to this to make music. I've worked on one before.

 

With the information from you and rla2005 (Thanks, it's the most detail I've seen so far), I think it's either this diode or it's gotta be the valve assembly. I do have one in my harness, be a pain to check that. I'd rather pay for the valve.