On my 94, at speed (starts at around 50 mph) the speedometer needle will jump around about in about a 8 mph span even though vehicle speed doesn't change. I thought it was the speed sensor on the rear axle, so I got a new one, and now its worse. It made me aggravated. Now it jumps around earlier and a wider speed range. What else could be the issue?!

 

Take a minute and look at the other posts. There are craploads of the same posts, with alot of answers...or, do a search...

 

Do a search for VSS or PSOM or RABS.

 

That's pretty sad when both of you have had a similar issue, had people answer you questions, but when you are asked, you say search. I'm sure if I would have asked "What size tires fit?" Or "what muffler sounds good?" There would have been 15-20 responses by now.

I had a feeling it was the PSOM, because I have had the diff cover off and I know the tone ring is clean and free of chips. I just wanted to see if I was on the right track.

 

inevitably you are right... when you ask for a diagnosis you'll be told to search because the answers are already on the forums some where... but when you ask for a opinion anyone and everyone can reply. Texas guys is right though... I'd look for PSOM forums and they will have all the info you need.

 

I didn't mean for that to sound rude, but this comes up and has been discussed alot. Everyones issue is the not same so its easier and probably faster for you do do a search. What fixed someone elses may not fix yours. If you read through the posts you may see something that relates to what you have already done to yours.

By the way, mine is still not entirely fixed. Its better than its been in years, but it still isn't 100 percent right.

 

Which sensor did you get and did it look like the one you removed? There is another VSS sensor that looks similar but does not fit as well. I actaully tried that one several years ago and it hit the tone ring in the diff. I changed it out and was going to go for a test drive and heard popping as a backed up. I knew what was happening, so I put the old one back in. I should have known better, but I just figured its what all the replacement ones looked like. I ended up exchanging it and telling them that it didn't even fit the application it said it was for.

This is the one that sucks! It says actaul OE part, my butt.

http://shop.advanceautoparts.com/web...S_906585892___

One of these is the one I have. I didn't even know there was 2 of them. I wonder what the difference is?

http://shop.advanceautoparts.com/web...S_667260973___

http://shop.advanceautoparts.com/web...S_521683437___

 

The sensor I got looks like the ones in the last two links. It made my problem way worse. So right now I have the stock one back in there. And was going to go get a PSOM and see if that helps. With the stock sensor it doesn't act up tell about 70 mph. When I put the new one in, it acted up starting at 25 mph. Everything in the diff looks good, and there are no codes in the computer.

 

I'm really surprised that the new VSS made it worse. Mine usually doesn't act up until about 70-75. Even then it sometimes wont do it for a while and then it will do it several times in a row.

I have also replace my PSOM when I did the Lightning instrument cluster swap. That didn't change things much if any either.

Somone suggested unplugging the RABS and see if that fixes it. It was also mentioned that the tone right could be the issue, but I would think it would have to be damaged for that to be the case.

 

Well my tone ring looks good. I had the diff apart not long ago to put my locker in it, and I checked ther condition of that. I guess I'll have to unplug the RABS one and see if that helpss

 

I'm still having issues with mine, but it's much better.

I ended up unplugging the RABS, and that made it 95% better. I unplugged the speed control module under the hood last week, didn't seem to make much of a difference.

Before I did all this, I replaced the VSS with one that looks like the bottom two that Tex posted. It did nothing to help the problem.

My tone ring is clean and undamaged, rear end oil is new. Speedo varies a little bit (less than one mph) under 60. Over 60 mph, it bounces maybe 5 mph, but only sometimes.

It's kinda weird how those two sensors Advance lists look the same, but have different part numbers. The only difference I noticed is that one is listed as a wheel speed sensor, and one is listed as a vehicle speed sensor.... Odd.

So Dave, here's the steps I took, and what seemed to make things better, or at least liveable for the moment:

-Cleaned original VSS, then replaced. No difference.

-Checked wiring from VSS up to the front of the truck, all looked OK.

-Unplugged the RABS module behind the glove box.... Bingo!

-Unplugged the speed control module under the hood. It may be my imagination, but it seems just a little bit smoother.

-Swapped in a different PSOM, operated the same way as my original. This leads me to think I have problems elsewhere.

Steps I'm planning on taking next:

-Get a new VSS directly from Ford. They're about $25. I'm wondering if the aftermarket ones are off on length just enough to throw things off. Curiosity is killing me, and it's not a lot of dough...

-If that doesn't fix it, I'll get the rear wheels off the ground, hook up my DVOM to the RABS test connector under the hood, and check for 6v AC (yes, AC) with the speedo reading 30 mph. The RABS test connector is under the hood, driver's side, right near the EEC test connector. It's the round one.


I hope this helps Dave. I know I got a little frustrated when I was searching for answers on this issue, it seems that a lot of times once someone solves a problem, they don't post back, so we never find out what their issue really was.

 

I'm not near my repair manual right now, so could u tell me, RABS control , is that the first black box behind the glovebox lid? I'll try that on my way home from work.

 

Thanks for the help guys, I did read a lot of stuff before I posted this question, and thewre was conflicting info. I could have swan I read some where about a replacement sensor or something that fixed the one mph needle jump that a lot of these pickups have. But I can't for the life of me find it again.

 

Yes, it's the first one you'll see when you drop the glovebox lid. I'm still running with mine unplugged, no ABS light, and no other side effects. Make sure you let us know how you make out with it!

 

There is a ton of conflicting and incomplete info scattered throughout multiple threads. It does get confusing...

I don't remember reading anything about replacement sensor issues, however I've wondered if this is one of those cases that only an OEM sensor will do the job on. I do, however, remember seeing a pic by subford that had the length of the sensor, from the bottom of the flange to the end of the sensor, at 1 1/16". I would imagine this length is VERY important to a smooth reading.