Speedo bouncing at 65
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New User
Joined: Jun 2004
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Speedo bouncing at 65
I'm wondering what would cause the speedometer to bounce up and down when diving at a constant speed (right around 65mph)??? It bounces 2 to 3 mph.
1996 Ford F350 Crewcab PSD
1996 Ford F350 Crewcab PSD
Last edited by Desieldummy; Jul 8, 2004 at 10:54 AM. Reason: Not enough info
Fleet Mechanic
Joined: Feb 2004
Posts: 1,816
Likes: 3
From: River Valley AR
If I am not mistaken the signal for the speedometer comes from the rear differential. There is a vehicle speed sensor that plugs into the computer wiring harness. If this sensor goes out it could cause speedometer problems and anti-lock brake problems. That particular sensor has been known to fail before, but I wouldn't call it a high-failure item. Hope that helps.
New User
Joined: Feb 2003
Posts: 21
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From: manhattan, ks
I don't think the sensor on the rear diff is for the speedometer. I can unplug mine and the speedometer still works. I do get a antilock brake fail indicator on the dash but speedometer works fine. No idea where speedometer is picked up but would like to know. Any other ideas????
Freshman User
Joined: Jul 2004
Posts: 31
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From: Lenexa, Ks
I am quite sure the speedometer "reads" off the transmission output shaft. The shaft is geared and on the side of the trans near the rear is a housing that holds a speedometer gear. A cable will run from there to the dash.
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Fleet Mechanic
Joined: Feb 2004
Posts: 1,816
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From: River Valley AR
OK then-
If it's not the sensor on top of the differential then it has to be on the tranny. I just replaced my 97 E4OD and it's in the bed of my truck right now. There is no mechanical cable coming off of the tranny. The only other possibility is on the passenger's side of the transmission. There is a heat shield mounted there that hides a large electrical connector. It's about even with the back of the oil pan. I seem to recall reading something about a speed sensor being part of that plug but it might have been a turbine speed sensor for the torque convertor. Also, my truck is a 4x4, so maybe the reading is taken from the transfer case on my truck.
I hate being wrong so I'm going to do some searching and I will post my results back here. Maybe I can find someone who knows what they are talking about.
If it's not the sensor on top of the differential then it has to be on the tranny. I just replaced my 97 E4OD and it's in the bed of my truck right now. There is no mechanical cable coming off of the tranny. The only other possibility is on the passenger's side of the transmission. There is a heat shield mounted there that hides a large electrical connector. It's about even with the back of the oil pan. I seem to recall reading something about a speed sensor being part of that plug but it might have been a turbine speed sensor for the torque convertor. Also, my truck is a 4x4, so maybe the reading is taken from the transfer case on my truck.
I hate being wrong so I'm going to do some searching and I will post my results back here. Maybe I can find someone who knows what they are talking about.
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Joined: May 2001
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From: Cabot, Arkansas
The VSS in the rear diff. is where the speedo gets it's reading. There is a tone ring mounted on the differential carrier and the teeth on that ring passing the magnet in the sensor is what the vehicle's speed is calculated from.
New User
Joined: Feb 2003
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From: manhattan, ks
Lots of suggestions on the speed sensor being in the rear diff. HOW do you explain that the speedometer still works when that sensor is unplugged. I have a 6 speed manual and have looked all over it and find a sensor for the back up lights and 4X4 indicator but nothing else, not even one on the transfer case.
