on my scan tool it reads a different mph than the dash. what would cause that? i swapped the speed sensorand it still read different readings? my tranny isn't shifting right at all and i think its because of this.

 

Does ur car have aftermarket gears?

 

How much of a difference is there?

I've got a scanmaster and sometimes the MPH will be 2-4 MPH off on the gauge.

 

your gauge could be off

 

well at 45 the scan tool says around 20. no its the stock gears..

 

isnt the speedo mechanically driven? if so, that would make sense then that your tranny isnt working right because your pcm is getting an incorrect signal.

 

they are electrical. and you didnt say what year your car is or what trans you have, but if it is a 700r4, the governor tells the trans when to shift, not the speedometer

 

did you mess with the tire size at all?

 

What year car is it?

 

Which one is off? when your driving it which speed seems closer to the right one? thats kindof an odd deal other than aftermarket trans, tires, or gears....

 

well this post is in the lt1 section?!?! its a 94 trans am with a 4l60e and its an electrical speed sensor which was already posted in the original post that i replaced it with another one so that would cancel out it being mechanically driven.. and its kind of odd that the people resonding to the post can't come to the conclusion of what it may be.. and how would a computer read a mechanical driven speedometer? the dash is reading what seems to be the correct speed and the scan tool is way off which led me to believe that is the cause of the shifting problem. the tps is working correctly according to the data on the scan tool. so how am i getting the wrong reading from the computer and not the dash? i haven't had a problem with it till a few days ago and the car just lost 3rd and 4th gear and the tranny is got like 200 miles on it. fluid level is fine.

 

First of all smart ***..next time you want a technical question specify the full detials on the Vehicle.. year, make, etc. Different years had different things in them.

Now..how long have you had the car, did you just buy it? You say something about it having 200 miles. What exactly has 200 miles on it, the car since you have had it, the Transmission..

Possibly the previous owner could have gotten a tune and they forgot to adjust the shiftpoints, or did adjust them too much, or threw in a setting inside the computer for the shiftpoints for a different gear ratio.

BTW.. there are plenty of adapters out there to go from mechanical to computer for the speedometer..

 

ok.. i am not seeing the significance between having changing tire size, gear ratios, tune, tranny or anything else that would make the computer and the speedometer not read the same thing.. the vss sends the computer the signal.. then the computer sends the dash the same thing.. so how does the computer not only not have the same reading as what it is sending to the speedometer but less than the speedometer. the car is properly tuned for every mod it has after they were all completed/installed.. the new tranny which has not given me a problem since i put it in 200 miles ago all of a sudden doesn't shift into 4th at 46 mph like it has since i put it in the car then soon after 3rd wasn't working. i hook up my scan tool and notice a huge difference in mph between the speedometer which is reading correctly and the computer which is telling me i am going nowhere near the speed in which the car is traveling which didn't exceed 25 mph and the car doing in the 55-60 range so there would be no need for 3rd gear at 25mph.. i know that there are adapters for mechanical speedos to electric and vice versa and that gear ratio and tire size changes would alter the speedo reading to what the car is actually travelling but at the end of the day they would be showing the same reading on the scan tool as the speedometer which has me questioning why you would ask these questions. yeah i left out the year and make but it is the lt1 forum and if i had an exotic setup i still don't see how it would matter as its not really a tranny problem.

 

What the VSS does is read the speed of the Tail Shaft on the Tranny, not necessarily how fast the Car is actually moving. In the PCM you have to tune in the Tire size and Rear Gear Ratio. Then from there you can change the Shift Points and RPM Limits.
Now if you change the Tire size and the Gears, it is going to change the speed of the Vehicle, not the setting in the PCM. Bigger Tires, faster the actual vehicle speed compared to the Speedometer. Bigger the rear Gear Ratio, slower the Vehicle Speed compared the the Speedometer. You have to adjust all of this in the PCM.
What is sounds like is you have the stock programming for the 2.73's, and you have 3.73's or 4.10's in it. Don't know why it would bug out all of the sudden though..

 

you are all looking over the fact that the its not a matter of whether either are right or wrong its that they don't match. gear ratio wrong or not which if you read my post the dash is what seems to be the correct speed but the computer reads somewhere around half the speedometer.

 

OK my take on this is that you've got two different ratios in the gear/tire section of the PCM. Now I don't know much about this but there is a parameter for transmission ratio and speedo ratio. Perhaps the two together equate for the correct speed shown on the gauge. Mine personally have some off the wall ratios but the scanner and the gauge show correct. Perhaps the issue lies there.

 

Also the pulse rate could be off, the way the PCM picks up the signal.

 

Well on the 4L60, it reads a different sensor. Could the 4L60E have 2 then? The VSS for the ECM and then the speedo sensor? That speedo sensor runs off a plastic gear, which can be changed out if you change the ratio of your rear end (be it from tires or gears). If the VSS is the same sensor, then my bad. If it is a different one, then the reluctor that the VSS is reading off of is damaged to be putting out 1/2 the speed you are actually doing. Also, it could mean that the one being gear driven would be reading a more accurate speed for the speedo, and then the VSS would be getting a faster signal which it can't interpret, due to it not being programed into the ECM. It's just math. X+Y=Z. If X is the same, and Y gets changed to W, it sure isn't going to = Z anymore.

Also, if you know so much about what things are, or are not, why haven't YOU figured it out. Stop being a jerk to everyone. Since you're new here, allow me to explain how things work: A person posts having a problem, sometimes no one knows right away, they post suggestions. More suggestions roll in, which trigger theories from other people. Eventually all the suggestions add up and someone makes a final suggestions based off of those, generally leading to the correct solution. So take a chill pill and let the suggestions roll. We're not "Car Whisperers", we don't know exactly wtf is going on with everyone's car.

 

The VSS wire goes to the PCM. Then it goes to the Speedo, then relays to the Cruise Control. That is what I remember from breaking down the Harness..

 

Yeah the 4l60e just has the reluctor and the magnetic pickup (sensor). I've had the 700r4 as well and it just as the plastic gear as mentioned but the reluctor replaces all of that on the E. Has the PCM been modded?