Question about speedometer
6/26/21, 08:35 PM
#1
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Thread Starter
Question about speedometer
So I finally got to drive my car tonight after about 3 months of being on jackstands. Transmission was shipped off to Calimer for a stage 3 build, also did a carbon fiber driveshaft, short throw shifter, etc... I noticed that my speedometer was pretty far off, was reading around 40mph when I was actually doing 55-60mph. The speedometer was accurate prior to the work. Calimer doesn't change any gear ratios in the car, and I know that there is a sensor for the out put shaft. If that sensor had gone bad, would the speedometer read incorrectly, or would it just not read at all?
6/27/21, 01:25 PM
#2
Legacy TMS Member
Yes.
Hope that helps!
What do you mean, it doesn't? I'm absolutely right! 
The output shaft sensor reads a reluctor/tone ring (whatever you wanna call it) on the output shaft and a computer somewhere then figures out how fast you're going based on pre-selected parameters. Which is why a tire size change results in a faster speedo, and that would have to be corrected in the computer.
So. If the sensor's bad, it could just not read anymore, or it may be intermittent. The only way to check it is either swap out the rather expensive part and see if that fixes it or get an oscilloscope hooked up to it directly and start spinning the driveshaft to get readings that way, counting expected pulse rates and seeing if there's gaps or weakness or whatever in the signal. A proper scan tool may can read the thing directly to you that way as well, much like a wheel sensor can be seen and diagnosed. Either way, you need fancy tools or just a new part and see what happens.
Also, there's a possibility the reluctor/tone ring is messed up inside the transmission (which would highly (siphon with a large amount of negative pressure)), or the sensor is possibly misaligned for whatever reason. And there's always wiring, either chafed/shorted wires or ground issues.
But... that's just the sensor. Mayhaps it's the computer is being fussy, or maybe you changed some other part like differential gears or tire sizes (but they'd have to be pretty tall sidewalls in comparison for 15-20MPH slower reading, I admit...) Or maybe they swapped out the sensor and it's got extra resistance or whatever... IDK, electronics get weird fast...
Again, I do hope that helps and good luck!
Hope that helps!
What do you mean, it doesn't? I'm absolutely right! The output shaft sensor reads a reluctor/tone ring (whatever you wanna call it) on the output shaft and a computer somewhere then figures out how fast you're going based on pre-selected parameters. Which is why a tire size change results in a faster speedo, and that would have to be corrected in the computer.
So. If the sensor's bad, it could just not read anymore, or it may be intermittent. The only way to check it is either swap out the rather expensive part and see if that fixes it or get an oscilloscope hooked up to it directly and start spinning the driveshaft to get readings that way, counting expected pulse rates and seeing if there's gaps or weakness or whatever in the signal. A proper scan tool may can read the thing directly to you that way as well, much like a wheel sensor can be seen and diagnosed. Either way, you need fancy tools or just a new part and see what happens.
Also, there's a possibility the reluctor/tone ring is messed up inside the transmission (which would highly (siphon with a large amount of negative pressure)), or the sensor is possibly misaligned for whatever reason. And there's always wiring, either chafed/shorted wires or ground issues.
But... that's just the sensor. Mayhaps it's the computer is being fussy, or maybe you changed some other part like differential gears or tire sizes (but they'd have to be pretty tall sidewalls in comparison for 15-20MPH slower reading, I admit...) Or maybe they swapped out the sensor and it's got extra resistance or whatever... IDK, electronics get weird fast...
Again, I do hope that helps and good luck!
Last edited by houtex; 6/27/21 at 01:32 PM.
6/27/21, 02:13 PM
#3
GT Member
Thread Starter
Yes.
Hope that helps!
What do you mean, it doesn't? I'm absolutely right!
The output shaft sensor reads a reluctor/tone ring (whatever you wanna call it) on the output shaft and a computer somewhere then figures out how fast you're going based on pre-selected parameters. Which is why a tire size change results in a faster speedo, and that would have to be corrected in the computer.
So. If the sensor's bad, it could just not read anymore, or it may be intermittent. The only way to check it is either swap out the rather expensive part and see if that fixes it or get an oscilloscope hooked up to it directly and start spinning the driveshaft to get readings that way, counting expected pulse rates and seeing if there's gaps or weakness or whatever in the signal. A proper scan tool may can read the thing directly to you that way as well, much like a wheel sensor can be seen and diagnosed. Either way, you need fancy tools or just a new part and see what happens.
Also, there's a possibility the reluctor/tone ring is messed up inside the transmission (which would highly (siphon with a large amount of negative pressure)), or the sensor is possibly misaligned for whatever reason. And there's always wiring, either chafed/shorted wires or ground issues.
But... that's just the sensor. Mayhaps it's the computer is being fussy, or maybe you changed some other part like differential gears or tire sizes (but they'd have to be pretty tall sidewalls in comparison for 15-20MPH slower reading, I admit...) Or maybe they swapped out the sensor and it's got extra resistance or whatever... IDK, electronics get weird fast...
Again, I do hope that helps and good luck!
Hope that helps!
What do you mean, it doesn't? I'm absolutely right! The output shaft sensor reads a reluctor/tone ring (whatever you wanna call it) on the output shaft and a computer somewhere then figures out how fast you're going based on pre-selected parameters. Which is why a tire size change results in a faster speedo, and that would have to be corrected in the computer.
So. If the sensor's bad, it could just not read anymore, or it may be intermittent. The only way to check it is either swap out the rather expensive part and see if that fixes it or get an oscilloscope hooked up to it directly and start spinning the driveshaft to get readings that way, counting expected pulse rates and seeing if there's gaps or weakness or whatever in the signal. A proper scan tool may can read the thing directly to you that way as well, much like a wheel sensor can be seen and diagnosed. Either way, you need fancy tools or just a new part and see what happens.
Also, there's a possibility the reluctor/tone ring is messed up inside the transmission (which would highly (siphon with a large amount of negative pressure)), or the sensor is possibly misaligned for whatever reason. And there's always wiring, either chafed/shorted wires or ground issues.
But... that's just the sensor. Mayhaps it's the computer is being fussy, or maybe you changed some other part like differential gears or tire sizes (but they'd have to be pretty tall sidewalls in comparison for 15-20MPH slower reading, I admit...) Or maybe they swapped out the sensor and it's got extra resistance or whatever... IDK, electronics get weird fast...
Again, I do hope that helps and good luck!
Believe it or not, that sensor was cheapest ordering from Home Depot, I've never ordered a car part from them before, so we'll see how that goes!
7/12/21, 08:27 AM
#6
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Thread Starter
Update: I called Ben Calimer this morning and he said I just need a tune revision. Turns out I was wrong about the gear ratios not changing. Going to send me the info to forward to my tuner. I guess I should have called him before spending the money to replace that sensor, but it is what it is.
Last edited by dj050894; 7/12/21 at 09:02 AM.
7/12/21, 12:57 PM
#7
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yep the gear ratios would be the next thing to check, sorry I'm a little slow to reply
7/23/21, 12:44 PM
#9
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Thread Starter
Just thought I would give you guys an update. Bama was able to get the speedometer dialed in, took them about 5 tries, but in the end it worked out.
P.S. I am well aware that Bama isn't the best tuner out there lol. I'm planning a supercharger in the near future and going to switch to Lund then.
P.S. I am well aware that Bama isn't the best tuner out there lol. I'm planning a supercharger in the near future and going to switch to Lund then.
9/24/21, 07:54 AM
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I too just installed my stage 3 and noticed the speedometer being off, reading slow, by about 75% (52mph = 70). The cruise control also won't set. It turns on but will not set. Before I go looking elsewhere for this issue, did yours have the same cruise control behavior? Did it resolve itself once the speedo was correct again? I'm guessing the car is seeing a huge difference between wheel rpm and transmission output shaft speed and not allowing it to turn on. Thanks man!
9/24/21, 10:41 AM
#11
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Does that "Stage 3" thing involve a tune?
If yes, I suspect that is where the problem lies; the tune probably has the wrong rear gear ratio and/or the wrong tire revolutions/mile
There was another thread about cruise control weirdness recently, I'm pretty sure it was in this forum but not positive, and I don't know if the mystery was every solved . . . try the search and maybe you can find it
If yes, I suspect that is where the problem lies; the tune probably has the wrong rear gear ratio and/or the wrong tire revolutions/mile
There was another thread about cruise control weirdness recently, I'm pretty sure it was in this forum but not positive, and I don't know if the mystery was every solved . . . try the search and maybe you can find it
9/24/21, 11:06 AM
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Does that "Stage 3" thing involve a tune?
If yes, I suspect that is where the problem lies; the tune probably has the wrong rear gear ratio and/or the wrong tire revolutions/mile
There was another thread about cruise control weirdness recently, I'm pretty sure it was in this forum but not positive, and I don't know if the mystery was every solved . . . try the search and maybe you can find it
If yes, I suspect that is where the problem lies; the tune probably has the wrong rear gear ratio and/or the wrong tire revolutions/mile
There was another thread about cruise control weirdness recently, I'm pretty sure it was in this forum but not positive, and I don't know if the mystery was every solved . . . try the search and maybe you can find it
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