My Dad is an electrical engineer, and he thinks that there could be a weak connection at the VSS or the VSS may be the issue. From what he was telling me it seems that both the speedometer and ABS module put a load on the VSS. He said that when they are both connected the load is more. So maybe the VSS or a connector is making a bad connection.

 

Well so far I have tried 2 salvage yard RABS modules and 1 remanufactured RABS module. I have one more salvage yard module that I am going to try.

 

Its weird, because the speedometer doesn't act the same way with any of the ABS modules plugged in. They all make the speedometer erratic in a different way.

I really would like to know why this is. Does anyone have any ideas what is wrong?

I'm wondering if this many modules can be bad.

 

Why did you replace the VSS?
Do you still have the old one or did it brake when you removed it?
I would go along with what your father said.
The signal generated by the VSS may be weak and is only good enough for one or the other but not both.

It seems like someone had the same problem a few years back and they shaved off some of the VSS to make it set deeper and that solved their problem. You could also have a problem with the tone ring or bearings in the rear end.
Also there is more than one VSS for these trucks and you can get the wrong one and have a problem. One is for the rear end and the other is for the rear of the transmission on the large trucks that do not have the RABS system.
I know one poster bought the wrong one and had a problem as they look the same but one sets deeper than the other.

You can check all of this out by putting a meter on the VSS (RABS) signal two wire plug at the drivers hood hinge.
To test put the trucks rear end up on jack stands and block the front wheels.
CAUTION:
Make sure multimeter is in the voltage function and not in the resistance function to avoid damage to multimeter.

Connect Digital Volt-Ohmmeter to speed + and speed - on the AC scale.

The voltage should increase smoothly and continuously from 0 to approximately 3.5 volts AC as vehicle speed increases from 0 to approximately 30 mph.

Try this with the RABS module plugged in and unplugged.
Your new PSOM may also be putting to big of a load on the signal also.

 

Thanks for the reply.

I'm almost positive I have the correct VSS. I replaced it with a Wells brand from Autozone back when this problem first came about 5 years ago. I don't have the original one. I do have a new Motorcraft one still in the box though. I do remember another one that I tried that did not look exactly the same and did not fit well at all. It actually went in to far and hit the excitor ring.

I'll do the test with jack stands with the multi meter this weekend.

Why would the Lightning PSOM be putting too big of a load on the signal? It did also do this with the original instrument cluster as well.

If I do decide to shave the VSS down, how much do you think I should take off? I bet not very much. If I do this I'll do it to the Wells one.

 

I would have no idea on this. As your dad said it may be bad wiring or the plug at the rear end. Or any place along the wiring that could have a voltage drop.

I do not know this either, the guy did not say how much he took off and I have not ran into this problem.

But you have to maintain the 3.5 volts AC voltage level on the wiring with both plugged in @ 30 MPH.

 

I tried the other salvage yard RABS module and it works better than any of the others. It has a very mild fluctuation or bobble at all speeds, so cruse control is still out of the question, but at higher speeds like 70 mph or faster it will overy so often drop 5 mph and then go back up. So apperently there is something about this module that my truck likes better than the others.

I am planning on doing the voltage test on Sunday.

 

While your at it , i would remove the rear cover & inspect the tone ring to make sure its on straght & not damaged....Lew

 

I put the rear up on jack stands yesterday evening and did the voltage test. Using a Fluke multimeter, it showed 1.7-1.8 AC volts with the RABS plugged in at 30 mph. With the RABS disconnected it showed 2.8-2.9 AC volts at 30 mph.

I guess that means that the VSS is not providing a stong enough signal.

Good point Lew. I inspected that a few months ago when I serviced the rear end. I could not find anything abnormal with it.

 

So I would guess its time to chase down wires and find where the voltage drop could be.

 

I tried a new Motorcraft VSS and that didn't help.

The tabs are broken on my VSS connector, so I have it held on with zip ties. I did get a new connector. I was thinking that it may not be making a strong connection there. I may change that out soon and see if that helps.

I'm open to any and all suggestions as to where to start looking now.

 

Ok just curious, with a speedo problem attempting to determine the fault with it, how did you determine 30mph while its sitting on the stands?

it showed 1.7-1.8 AC volts with the RABS plugged in at 30 mph. With the RABS disconnected it showed 2.8-2.9 AC volts at 30 mph.

Yea clearly you gotta voltage issue there but how can one reliably use the speedo as part of the testing equipment when that very device has issues that are at least from the start unclear.

disconnected it showed 2.8-2.9 AC volts at 30 mph.

Maybe spinning just shy of the 30mph mark? and the cause of the problem is the rabs circuit and nothing to do with the VSS. Without rabs connected and a true rear wheel speed of 30mph you might just have the 3.5v AC there.

Rabs is added on, wired in parallel with the VSS, not in series (well based on my diagram).

So its not interfering while disconnected, you connect it and right of the bat the voltage drops a 3rd.

How about the POSM, Are you disconnecting it during the test? no using it to figure rear wheel speed? Does the voltage change without and then with it as well?

 

His speedometer was working OK without the RABS module so am guessing he used the speedometer to get the 30 MPH and checking the voltage with with the RABS module unplugged.

But he could also use a frequency counter and 667 Hz would be 30 MPH.

I have bench checked the speedometer and it will read 30 MPH at 667 Hz and 3.5 volts AC voltage level.

 

Was that with both the rabs and PSOM connected?

If so ok I see what your saying then, without the added load of the rabs the VSS just doesn't have to produce as much voltage to read 30mph on the speedo.

Without it it will read 30mph at say 2.9 volts, an actual rear wheel speed of 30mph.

With it and the PSOM both connected it must produce 3.5 volts to make the speedo indicate that same 30mph reading.

 

I did the test with the PSOM plugged in and got 2.8-2.9, and then with the PSOM and RABS plugged in and got 1.7-1.8.

I do know that the speedometer is fairly accurate when the RABS is not plugged in. I have paced cars with digital speedometers that I could read several times. How would it be possible to verify speed with the PSOM disconnected?