speedometer surge

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Oct 14, 2006 | 05:57 PM

drag racer

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speedometer surge

We removed the 3.08 gears and installed new 3.73 gears.
On the test drive, after 60 mph the needle started to wiggle a little.
After 75 mph, it whipped back and forth.
I installed a new sensor in the rear housing and that made it worse yet.
I re-installed the old one, after cleaning everything.
Same thing--what's going on? Or what am I doing wrong?

Oct 15, 2006 | 07:57 PM

Popa Tim

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From: Central NY

possibly a psom going bad, dirty vss, or a damaged ring gear.

Oct 16, 2006 | 03:44 AM

nighthawk285

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From: La Grande, OR

what year is the truck?

Oct 16, 2006 | 08:40 PM

drag racer

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speedometer surge

Sorry for not giving all the information! 1996 F150 4x4 351W auto.
Speedometer appeared to be fine before we changed gears.
The brass gear in the rearend--teeth looked okay.
I don't have a clue what a PSOM or a VSS is?
I have the old sensor in there now and it works; but wiggles enough to bother the cruise control. When I drive with out cruise, the truck doesn't surge, just the speedometer?

Oct 16, 2006 | 08:50 PM

nighthawk285

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From: La Grande, OR

well on your year of truck, the speedometer signal is electric. I believe it is sent straight from the VSS (Vehicle Speed Sensor) in the top of the pumpkin of the rear end. If you changed the gears, the VSS is still programmed for the 3.08 gears, and now that the 3.73 gears are spinning faster, the VSS is getting it's signals crossed and sending the wrong speed to the speedometer. Try renting or borrowing a programmer tool from a parts store or a friend, plug it in, and it should let you reprogram your computer for the new gear change.

Oct 16, 2006 | 09:08 PM

drag racer

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I thought the sensor read axle speed not driveshaft speed? I didn't change tire size, so wouldn't the axle still be turning the same? Or DOES it read the driveshaft turning?

Oct 16, 2006 | 09:25 PM

Cheggie

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From: South Carolina

Kinda sounds like a bad PSOM.

Last edited by Cheggie; Oct 16, 2006 at 09:29 PM. Reason: technical error!

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Oct 17, 2006 | 01:55 AM

nighthawk285

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From: La Grande, OR

is it bad or does it just need to be reprogrammed?

Your PSOM is your Programmable Speedometer/Odometer Module. It is a circuit board inside of the cluster. It can be seperated from the cluster by taking it apart. I thought that you would have to reprogram it, because I was under the impression it needed new programming for both tire size changes AND gear ratios. But I do know that that if you need to reprogram the PSOM for mileage or changes in gear ratios/tire sizes, you only get six tries. After that, the microprocessor won't accept any further changes and you have to get a new one or take it to the local stealership to get the counts reset.

Check out these other threads they each have their own explanations
https://www.ford-trucks.com/forums/5...ogramming+PSOM

https://www.ford-trucks.com/forums/5...ogramming+PSOM

And here's the technical "How To" Brought to you by FTE
https://www.ford-trucks.com/article/...ND_BRONCO.html

Last edited by nighthawk285; Oct 17, 2006 at 02:03 AM.