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Old 06-16-2013, 08:50 PM
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Speedometer jumping

I just picked up a 95 7.3 and am starting to fix all the little things and one is the speedometer jumps at high speeds thus screwing with the cruise, My 95 Bronco did the same thing and reading some threads others have the same problem. Im new to the OBS so please if someone could explain how to fix this please
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Old 06-18-2013, 06:50 AM
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Bump anyone?
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Old 06-18-2013, 08:45 AM
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VSS - vehicle speed sensor - in the rear diff is the prime suspect. May also be the wiring to it, or the tone ring on the diff that it interfaces with. Any idea how old the diff fluid is?
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Old 06-18-2013, 08:53 AM
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Quote:
Originally Posted by madpogue View Post
VSS - vehicle speed sensor - in the rear diff is the prime suspect. May also be the wiring to it, or the tone ring on the diff that it interfaces with. Any idea how old the diff fluid is?
probably about as old as the truck. Im in the process of changing all the fluids. When im in there ill replace the vss and take a look at the wiring and tone ring(thats just a small gear that engages the differential gears right?)
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Old 06-18-2013, 10:18 AM
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And when the vehicle speed sensor doesn't work, you will end up with a new speedometer/odometer like I did. My problem was intermittent, and I thought the VSS fixed it, at least for a couple hours. The speedometer needle is powered by a little motor. On my truck, there must have been a short somewhere, because it worked a few times to smack the dash. It only worked a few times. You can go to a junkyard and pull one out fairly easily, lots of screws and three harnesses to disconnect on the gauge cluster, but easy enough to do. Go slow or you'll break the clear cover like I did. DOH!
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Old 06-18-2013, 10:37 AM
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Originally Posted by Woodturner Nate View Post
And when the vehicle speed sensor doesn't work, you will end up with a new speedometer/odometer like I did. My problem was intermittent, and I thought the VSS fixed it, at least for a couple hours. The speedometer needle is powered by a little motor. On my truck, there must have been a short somewhere, because it worked a few times to smack the dash. It only worked a few times. You can go to a junkyard and pull one out fairly easily, lots of screws and three harnesses to disconnect on the gauge cluster, but easy enough to do. Go slow or you'll break the clear cover like I did. DOH!
Well maybe ill do that first to see if that alleviates my problems since ill be at the junkyard probably next weekend.
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Old 06-18-2013, 10:45 AM
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Easiest way that fixes most is just pull the plug off the top of the rear clean it and check, then while your changing the diff fluid inspect the tone ring if your problem persists, then go into the harder stuff listed
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Old 06-18-2013, 10:53 AM
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Originally Posted by 351fifty View Post
Easiest way that fixes most is just pull the plug off the top of the rear clean it and check, then while your changing the diff fluid inspect the tone ring if your problem persists, then go into the harder stuff listed
Sounds good will do I should just PM you about my problems instead of mucking up the threads
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Old 06-18-2013, 02:00 PM
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The way the system works is there is a tone ring attached to the carrier in the rear, the VSS counts the teeth and sends a square wave signal to the front. The wire is split to the PSOM and RABS. The PSOM converts the signal to sign wave and sends it to your PCM which controls the trans if you have a auto.

The VSS is the most common cause however the PSOM is a close second. The good thing about the PSOM is you can get one from any F-150, F-250, F-350 or Bronco from a similar year and just swap that section of the cluster. If you have access to a oscilloscope, you can watch the signal and see where the issue is. In my case it ended up being the RABS unit which I just left unplugged.
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Old 09-17-2013, 06:28 AM
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Quote:
Originally Posted by 351fifty View Post
Easiest way that fixes most is just pull the plug off the top of the rear clean it and check, then while your changing the diff fluid inspect the tone ring if your problem persists, then go into the harder stuff listed
Well about 3 months later i finally got around to doing this and surprisingly it was actually my problem. The speedometer works great now. Still has a slight twitch at high speeds but not like going up and down 10mph at every bump so i will worry about the tone ring when i change diff fluid
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