drcook

Let's think this through.

I looked at the tables and as you say, they are all controlled by MPH not RPM. <<<===WRONG

correction some are RPM and some are MPH. RPM constraints within an automatic transmission would be more for protection of the trans itself, regardless of how fast you are going , yes/no ???

Will not the RPM values for any given MPH vary depending on the rear gear ratio, whether or not we have a computer controlled engine/transmission ? Unless I am wrong, that has always been my experience. That is why an overdrive unit drops the engine RPM. It doesn't change how many times the output shaft turns for any given MPH, it changes how many times the input side has to turn to get 1 turn of the output shaft.

So the computer, in order to calculate a mph, has to take the VSS signal from the output side of the transmission and with its built in algorithm multiple it by the differential gear ratio plus tire diameter to get MPH.

in the case of my car VSS signal x 3.07 x tire diameter = MPH (or something like this). Regardless of what internal gear the transmission is in or how many RPM's the engine is turning, it is still how many times the output shaft turns, factored in with the differential gear ratio with the tire diameter factored in.

so for very simple visual presentation VSS = 1 and tire diameter = 10 (very simplified, I realize there is more to it than just using tire diameter, it is actually the circumference of the tire that governs how much distance it travels per revolution and how many revolutions per hour or how many inches the vehicle travels / 12 (to get feet travelled) / 5280 to get miles factored into time to get miles per hour)

1 x 3.07 x 10 = 30.7

which really means 1 turn of the driveshaft factored in with the rear gear ratio factored in with tire diameter = MPH

now we change to 3.45 rear gear to fudge the VSS signal, would not the equation be

a x 3.45 x 10 = 30.7 where a is the modified VSS signal so the computer THINKS that for every a value, it is equating it to 1 turn of the output shaft/drive shaft/pinion gear combination

a x 34.5 = 30.7

30.7 / 34.5 = 0.8898

so by intercepting the VSS signal and stepping it down from a value of 1 per revolution to a value of .8898 the equation is solved out to the same MPH that the computer expects it to be, since it has the values of the rear differential gear ration and tire diameter programmed in ?

now to look at it from the other side engine RPM going through the transmission (regardless of gear) to get that same 1 turn of the driveshaft. will it not take the same RPM through whatever gear the trans is in to get that same 1 turn of the driveshaft regardless of the rear gear differential ?

In my mind, the ABBOTT (or similar box) is just fudging a value being presented to the computer in order to let it complete its calculations

Or, since I have been away from doing math for so long, I am looking at this wrong.

bjankuski

That is my point, it will shaft the same as before based on RPM because the RPM is directly related to speed and since the tune is not changed the speedometer will read fast in compariison to road speed but the shaft points will still be OK. If you add in the calibrater and actually change the speedometer the shifts will happen at higher RPM's but at the same speed shown on your speedometer as now and the speedometer will read correctly.

2LZ

Ok....let me think about this...
You're saying that it shifts normal now (with an inaccurate speedo) because it still think things are the same.
If I change the VSS input to the PCM (to correct the speedo, via Abbott box), it will hold the shift points higher on the RPM range now because it's now being fed with incorrect (adjusted) information?

Am i understanding your point correctly? now I'm REALLY curious to see how it acts.

drcook

Notice how the transmission is controlled. MPH in most tables, RPM in others. If you click on the picture, it will enlarge so you can see it. These tables are direct out of the Jet software.

drcook

Transmission normal mode shift point thresholds ARE governed by RPM regardless of MPH

drcook

In a lot of the tables, the TPS (throttle position sensor) in relationship to MPH is used to govern shift points.

What gears were in your car to begin with before you put in the 3.54's.

bjankuski

See above

bjankuski

Yes

bjankuski

Correct for WOT as shown, the shift table in in MPH, that is what I am referencing with higher shift RPM at crusing speed.

Kickdown means WOT

bjankuski

Correct, shift tables in MPH. As noted WOT in RPM and truth be told the program works by whatever paramenter is satisified first. AT WOT if you hit the MPH number first of the RPM number first, whichever one hits first will cause a shift.

2LZ

Wow! To think a little plastic box can create such good conversation, ;-)

It came with the Cadillac 2.59's.

drcook

Does not the computer calculate MPH based upon a formula of 1 turn of the driveshaft (output shaft of the trans) factored in with rear gear ratio and tire diameter ? It has to. In the past, it was via a gear, now it is via an electronic signal. In our cars, the speedometer reading is governed by a signal sent from the computer to the speedo, which is a calculation.

In my mind, the ABBOTT box is fudging that signal of 1 turn of the output shaft to a value that you figure out by GPS or other method. So instead of telling the computer to change its equation values, the VSS signal is being modified by the DIP switch settings and thus the signal being sent to the computer.

Since the box is wired in between the VSS and the computer. NOT AFTER the computer to the speedo, the calculations can be correctly satisfied.

The equation is being satisfied by modifying that 1 turn of the output shaft by (in very simple terms) the difference in the gear ratios. Thus the computer is able to calculate the true MPH and use it in its tables. All the values are still based upon the assumption how many times that 1 turn of the output/drive shaft has occurred in any given amount of time, the ABBOTT box is changing the signal to equate to that 1 revolution.

I am not following why you say that it causes the tables to not be applied correctly. I wrote enough software to understand how to fudge equations as long as the fudging was done before the needed values were obtained. As far as the RPM tables are concerned, engine input RPM to the transmission is going to be the same regardless of MPH. So if the transmission has an RPM limit set of say 5800 RPM, what does it care what MPH it is going ? It only cares that it has reached 5800 RPM and has to shift. That 5800 input RPM is the same whether it is obtained in 1st gear, 2nd or 3rd. At WOT if you hit 5800 RPM it is going to shift, or in the case of 4th, it is going to defuel to not exceed that 5800 RPM.

Had the ABBOTT box been wired in after the computer and before the speedometer, then yes the readings would be off and I would agree entirely.

Multi-dimensional tables were my forte. I learned how to program in 7 dimensions which is sort of hard to envision. To do a simple one dimension table lookup, you derive the 2 values. In the case of TPS -vs- MPH the engine knows, because of the sensors what the TPS is, and it knows the MPH because it calculated it via VSS in conjunction with gear ratio, tire circumference and time. Because the ABBOTT box is before the calculations are applied, the values can be fudged to achieve the same result to calculate MPH.

As I mentioned, because it is before the values are applied, it will calculate out correctly to be used for table lookup which is all that is happening in order to get another value to satisfy another calculation to plug in and use.

bjankuski

See above

I am not questioning your programming ability, I am just explaining how this works, there is a misunderstanding on how the VSS sensor output is being used.

drcook

Please explain, and I understand you are not questioning what I did, I am simply relaying how I understand computers and electronics -vs- calculations to work. To really explain it and see how it is being presented to the computer we would have to measure the pulses both before the ABBOTT box (after the VSS) and after the ABBOTT box in relation to time. I would postulate that it is either sending it by a voltage change or an on/off pulse and that the time it occurs happens in milliseconds (because electronic signals happen so fast).

We would have to know how the signal is being manipulated within the ABBOTT box. Is it changing the voltage or is it changing the time between signals. Changing time would be hard to predict, changing voltage easier.

So what I am saying is because it is manipulating the signal BEFORE the computer even sees it to use the VSS signal that the equations will remain constant because it is adjusting that signal. In a very simplified way to look at it and knowing how you have to present data for computations lets assume that 1 turn of the output shaft = a particular value sent to the computer. This is the VSS part of the equation. And that 1 turn -vs- the internal clock of the computer (the time portion) then factored in with the gear ratio, equates to 1 turn of the ring gear. All computers have an internal clock. That clock can be driven different ways, in your desktop it is probably sourced via a 2024 battery. That is why when the battery dies, it loses the date and time.

When you let a car sit too long and the battery dies, or disconnect the battery it loses some of its "learned" values but retains the values that are stored in the non-volatile memory area, otherwise it would lose all parameters given to it and not function at all.

Then we stick in a different gear ratio without telling the computer. The computer still thinks it needs to use the constant it was set for. so 1 x constant in relationship to time sets its calculation off of MPH, thus it is wrong. If the VSS signal is modified to x times the actual signal it is outputting, then the calculations are correct. So in very simple terms the output shaft has to turn x amount of times because we changed the gear ratio, it has to turn a different amount of times to get 1 turn of the ring gear, after all that is what the wheels see as input, the ring gear making one revolution.

The ABBOTT box has to be manipulating the signal to the computer. So instead of saying (in a simplified sort of way) I turned 4 times, it alters it and said I turned 2 and then the calculations would be correct based on the constants of what was originally programmed into the computer as gear ratio and tire size and it can use what it calculates as MPH correctly.

So with 3.07 gears, the driveshaft turns 3.07 times to get 1 turn of the axle. With 3.54 gears, 3.54 times. So without changing the values in the stored tables, the computer will think that when the driveshaft (or VSS signal) has turned 3.07 times the axle has turned once and uses that for its calculations setting the MPH values wrong it can't do its calculations correctly. The signal is being manipulated so that when the ring gear has made 1 revolution adjustments have been made.

If the computer knows that it has to send a signal to an electronic speedometer that it is going 55 mph, it already knows that it is using a 55 mph figure in its table lookups. It has to, otherwise the 55 mph shown on the speedometer would not match up to measuring the speed via GPS or clocked on radar or by laser.

I will also postulate that it is off by such a small value (because the signal is being intercepted and altered) that it won't make a practical difference.

Our cars were built back when chips were single processors. It becomes a different scenerio when you have multiprocessors handling computations, but you still have to have one main "guy" so to speak to say "unit that sends MPH signals to the speedo, I have a figure of 55 for you to send" and if the main guy already knows that the value is 55 it is what is being used for every MPH computation.

Not trying to insult you or doubt what you are saying, but if you intercept it and adjust the data before the computer gets it, everything can be made correct.

drcook

We also have to take into account how computers work. They only process 1 thing at a time. That is why chips are rated in x calculations per second. They are just doing things so extremely fast it looks like they are doing multiple things all at once.

So they can calculate up the signal to figure out that the driveshaft turned once, but it has to finish that calc, or put it on the stack to come back to before it can say, calculate how much to change what the injector is flowing or to use the signal the MAF is sending to calculate the airflow into the engine.

Or to calculate the MPH. It has to process the VSS signal and store it in a bucket then fetch the constant for the rear gear ratio and store it and then fetch the tire circumference and fetch the time parameter and then start processing the equation one step at a time until it gets the MPH to store it in bucket to move onto another task in the stack.

MatthewMiller

Here is the actual window in Jet DST that we would use to correct the speedometer error, from my 96 LT4 car with ZF6 and 3.45 gears:


All I did was start with a ratio of 2.59 and the assume a new rear gear ratio of 3.55. This meant that the speedo should read 37% too high. So at the bottom left under "Speed Error," I said that when the actual speed is 1mph it is reading 1.37mph.* Jet DST automatically did the math to fill in the boxes on the upper right under "New Calibration Values." You can see that it just divided the original "Speedometer Scalar Scantool" (35.1) by 0.73 to get the new "Speedometer Scalar Scantool" of 48.087. That's all the OP would have to do in order to correct the speedo calibration in the PCM.**

Or he could just plug in the tire size and new gear ratio in the "Drivetrain Information" box and hit "Calculate" and it would do the same correction to the "Speedometer Scalar Scantool."

I don't see how that correction in the actual PCM is any different than what the Abbott box is doing before the PCM sees the signal. It's all literally just a matter of applying the correct multiplier to the VSS input. Whether we change the multiplier in the box or in the PCM shouldn't matter. If parts were available, we could also do all of the speedometer correction by changing out the actual gears that drive the VSS. Again, if we did that I think the PCM would be completely happy with that.

If anything, the OP's car should be shifting at too low of a road speed at part-throttle, since it thinks it's going faster than it really is. Correcting that with either the PCM multiplier or the Abbott box should fix it. Of course, I could be missing something. So 2LZ is our guinea pig!

*You could multiply these numbers by whatever you want. This obviously assumes you're using a roller or GPS to measure actual speed vs indicated speed. I'm just using it to correct the actual difference in rear axle ratios instead. If the speedo was accurate before the gear change, this will accurately correct for the new gears.

**Please note that the "Speedometer Scalar Scantool" in this example is wrong for the OP's car. The number in the Current Calibration Values is for my 3.45 car, not his 2.59 car. I'm just giving an example of how he would change it for his new gears using the "Speed Error" tool.

drcook

Thank you for help clarifying what I have been trying to say.

I look at it from a programmer's perspective knowing how computers from that era worked. You are illustrating it from that perspective correcting it in the computer itself and I am educating myself the more I write about and read. I did a lot of reading this morning and last night to make sure I wasn't so full of crap that my eyes were browner than they usually are.

I did run into some words on the internet that more modern ABS systems are calculating MPH at the wheels and back feeding it, but at this point I would need to clarify that.

bjankuski

Guys, when you ask the program to correct the speedometer it pops up a warning asking you if you want the program to change the multiple items that are effected by the tune, you should answer yes because then it will also adjust all of the MPH in the shaft tables and the lock-up mph in the tune. It asks that question becuase if you do not adjust those items the car will behave incorrectly.

jgalfo

I bought a copy for my 95 a year ago when I had the gears installed.

bjankuski

1995 is obd1 they still sell that one. 1996 is obd2 and that you can no longer buy.