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post #1 of 15 Old 11-27-2013, 02:40 PM Thread Starter
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Shaking speedometer

Well, I have a small problem with my 97 F250. When I drive at almost any speed but especially higher speeds (55+mph), the speedometer shakes/moves back and forth. If I'm driving 60 mph it will shake between 58 mph and 62 mph. I think it has something to do with the wide 33" Dick Cepec radials that are on the truck but I have no idea. I don't have an extra set of rims and tires to put on it to test. Any ideas? Thanks and have a Happy Thanksgiving

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post #2 of 15 Old 11-28-2013, 06:51 PM Thread Starter
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Pulled the speed sensor...looks pretty rough. I think this is the problem so if any of you have this problem, check your speed sensor first

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post #3 of 15 Old 07-28-2014, 10:48 PM
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Mine still jumps after I replaced the sensor so either the gear is bad or it's behind the gauge idk yet
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post #4 of 15 Old 07-29-2014, 04:59 AM Thread Starter
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Quote:
Originally Posted by Knger View Post
Mine still jumps after I replaced the sensor so either the gear is bad or it's behind the gauge idk yet
Forgot all about this thread. It wasnt the speed sensor and since it only does it for me in OD, im guessing it is a bad gear.

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post #5 of 15 Old 07-29-2014, 02:21 PM
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Except there's no "gear". Speed sensor works like a bike computer; basically a magnet switch. Every time one of the notches on the reluctor (tone ring) passes the magnet, the switch closes, completing a circuit. The PSOM just counts how many times that happens in a given period of time, and translates that to a speed and distance, to drive the speedo and odo, and to send the speed signal to the PCM. That it's only happening in OD is pure coincidence, or perhaps it's actually only showing up when you're going fast enough to be in OD. The VSS is on the diff, and only "knows" how fast the diff is turning; it's not "aware" of anything going on with the transmission. (It works the same, after all, on manual and automatic trucks.) Hypothetically, you could put a truck together with no engine or transmission, just a rear diff and the electrical system and the dash, and push it down a hill, and with the key in RUN, the speedo would work.

One slim possibility is that OD is doing something electrically, that is somehow causing the PSOM not to pick up the VSS signal correctly. But there's no "data" feedback from the PCM to the PSOM, so it would have to be more like some sort of stray voltage or field issue.
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post #6 of 15 Old 07-30-2014, 06:17 AM Thread Starter
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I think that you are correct about it being an electrical or voltage issue but i have downgraded to 285 tires since and its not nearly as bad as it was before but still shakes

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post #7 of 15 Old 07-30-2014, 11:14 AM
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Yeah I could imagine how larger tires could "amplify" the issue. But it shouldn't bounce/shake at all. Dunno if it would shine any light, but you could put the back end up on stands and "drive" it up to speed and see what it does. Even try it with the wheels on / off, to see if the rotational mass of the wheels/tires is a factor. But my money would be on a damaged tone ring, or contaminants in it, or contaminated diff fluid or some such.
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post #8 of 15 Old 07-30-2014, 01:19 PM
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Mine did this was a dirty connection at the sensor only did it hwy speeds


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post #9 of 15 Old 04-22-2015, 02:34 PM
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Don't mean to revive an old thread but it seems better than making a new one. September of 2014 my speedometer was jumping around as well at 60mph so I pulled it and cleaned it. If it did help, it didn't do much. So a couple months ago (can't remember when but maybe February ) I installed a new one from ford, my old sensor was smooth, the new one had a bunch of notches on it like splines. The problem still continued after installation so is there anything I should do or check? I saw something about possible low voltage and the ring and magnets. 1996 F350 7.3 4.10 gears 5speed 4door longbox. 315/75/16's
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post #10 of 15 Old 04-22-2015, 08:33 PM
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No problem reviving an old thread, just means you used the search feature and can add more info for future searches.

Double check the wiring, I have a jumpy speedo at around 70+ but I need to get back there and check everything. I changed the sensor a couple years ago when it started to jump all over and had erratic shifting.
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