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Discussion Starter #1 (Edited)
*Intermittent* bouncing speedometer and strange shifting

Okay, so from my title you are going to say "Well, duh, it is your RABS sensor on the rear axle" right? I wish it were that simple!!!

I bought this 97 F350 crew cab automatic 7.3 powerstroke truck the beggining of this year. Shortly after buying it, I discovered that once in a while when I would drive it, the speedometer would be bouncing by about 10 miles per hour, and the tranny would not shift right. And then after driving it for a while, the speedo would go smooth, and it would shift perfect for the whole rest of the day.

So after searching on this website, I discovered the cause of that is usually a bad connection at the speed sensor on the rear axle, or a bad RABS sensor. So......... I replaced the sensor. Worked great for a couple days, then started acting up again. So I replaced the plug and about a foot of the wiring going to the sensor, and soldered and heat shrunk the connections. Worked great for a couple days, then started acting up again. And it does not do it every day, but when it does, it ALWAYS does it in the morning or the first time driving it in a day. That's one thing that's got me confused. It NEVER starts doing it after I have been driving it for a while. And if it is doing it, and then all of a sudden stops doing it, it will NEVER start again until the next morning, or a few days later when I first drive it again.

So it has me completely stumped! It seems as if maybe the moisture that condenses on some component over night causes a short, and then after I drive it for a while, the moisture dries up, and it starts working fine again. But my problem is, I don't know what else to check. What else besides that sensor on the back axle controls the speedometer, and therefore the shifting of the transmission? Is there another sensor that is likely bad that could cause an intermittent problem like that? The transmission ONLY acts up when the speedometer is bouncing around. So that seems to steer me away from the TPS like some people suggest, UNLESS the TPS also controls the speedometer which I don't really think it does.

Anyway, any help would be greatly appreciated!! I have now sold the truck to my single female cousin, and really need to get this problem fixed for her. It only does it once in a while, so she can drive it most of the time no problem. HELP Please!!!! :dunno:
 

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My 96 PS did the same thing plus the O.D. light flashed, after replacing the speed sensor twice, having the tranny rebuilt I found out it was the speedo cluster. Hope this helps.
 

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Discussion Starter #3
My 96 PS did the same thing plus the O.D. light flashed, after replacing the speed sensor twice, having the tranny rebuilt I found out it was the speedo cluster. Hope this helps.
So was your problem intermittent as well? The speedo cluster screwing up actually affected the way the transmission shifted? The O.D. light does not ever flash on mine. And I'm pretty sure the light still works. But I guess that still could be the problem.
 

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Yes, it only did it once in a while, thats why it was so hard to find, It would start doing it & I could stop, cut it off, crank it up again & it would be fine, take it to a shop & they couldn't get it to act up. Finnally took it to the local dealership & he kept it about a month & finnally got it to do it, said he had saw one truck do it before & it was the speedo cluster sending false readings to the tranny, replaced it & hasn't did it in over 4 years.
 

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Bad PSOM. PSOM = programmable speedometer/odometer module, its a little computer module that is integral to the speedometer. It receives the speed signal from the VSS and sends it on to the PCM.
Easy test- swap clusters from a truck with a known good one. If the other truck isn't the same year you may have a few random lights come on but don't worry about it. If the speedo doesn't bounce anymore, you found the problem.
The cure? Another cluster assembly (or have your PSOM rebuilt... expensive). Bad part is the PSOM also stores mileage, so whatever miles the donor rig had will now show on your truck. If the replacement cluster isn't from the same year rig, you'll want to disassemble it and swap the speedo into your cluster so that you don't have random lights on all the time- there are minor changes in the wiring and printed circuit board on the back of the cluster among the years.
 

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same here

i have the same issue with my PSD and am in the process of removing the speedo and sending out to get repaired, i spoke to a fellow by the name of allen from arspeedometer repairs in OK and was quoted $150.00 including shipping,here is the web address:
Speedometer & Gauge Repair - Home
 

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i have the same issue with my PSD and am in the process of removing the speedo and sending out to get repaired, i spoke to a fellow by the name of allen from arspeedometer repairs in OK and was quoted $150.00 including shipping,here is the web address:
Speedometer & Gauge Repair - Home
It was MUCH more expensive a few years back when I checked into it!
 

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bump...

i think i have the same issue here. i just got done repairing a coolant leak. about 3 miles into the test drive my speedo pegs 90, odometer is out. it now shifts hard into 2nd, and when going to reverse, it REALLY wants to go, i have to lay into the brake hardcore to slow and stop. does this sound like the same part? speedo is pegged even when off. all other guages work fine...
 

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bump...

i think i have the same issue here. i just got done repairing a coolant leak. about 3 miles into the test drive my speedo pegs 90, odometer is out. it now shifts hard into 2nd, and when going to reverse, it REALLY wants to go, i have to lay into the brake hardcore to slow and stop. does this sound like the same part? speedo is pegged even when off. all other guages work fine...
I don't know about your speedo, mine wasn't pegged, it would start bouncing at around 55 mph and would swing 10mph back and forward then OD light would begin flashing,i still haven't sent it out to get fixed yet, is your OD light flashing , get a scan tool and check for errors/codes,might be something simple, then again.....
 

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i spoke with Allen at arspeedometer, he doesnt think its the PSOM, he leans toward the VSS or the wiring for the VSS. now i need to find out what i need, i know about the VSS on the rear, but rockauto has several listed for my truck, rear, tranny, ecu, etc.
 

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check the rear end for metal shavings.. they can mess up the sensor..
 

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will do, thanks! so the only VSS that will apply here is the one in the rear end?
 

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i spoke with Allen at arspeedometer, he doesnt think its the PSOM, he leans toward the VSS or the wiring for the VSS. now i need to find out what i need, i know about the VSS on the rear, but rockauto has several listed for my truck, rear, tranny, ecu, etc.
There is only one, the one on the diff. Ask for a RABS sensor so you get the right one.
 

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so they are the same thing?
 

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thanks, i hope this is the fix, i really dont want to send it to a mechanic.
 
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