Anyone know if this output is adjustable? I'd like to use it to recalibrate my speedo after a gear change. (speedo reads high).

I know i can adjust my PPM constant which will show the correct MPH in the log, but how do I recalibrate the ECM's vss output?

-- Joe

 

You can only divide the 4000PPM output by 1,2,3,4,5,6,7,8. These are controlled by the DIVx signals and will require data or code change. Roadspeed constant (default = 0) in $58 code.
Try those numbers for the correction. If you need more accuracy than that then you need to install a 10-bit counter or something like that to the 4000PPM output and divide down to feed the speedo.

Or you could do a software only change by driving an unused output with a new VSS signal from the ECM to the speedo. It will require you to assemble the new code, but does not require extra hardware like the counter mentioned above.

J

 

Have you seen this post yet?https://www.thirdgen.org/techbb2/sho...ht=vss+scaling

 

No, I haven't but I just looked at it quickly.
It says the same thing I said. It is talking about the DIVx signals with the divide from 1 through 8.

Your default is zero so it divides by one. It appears that you think changing the divide to something from 1 to 8 will work in your app. Try it them and see if one of the divides gives a "close enough" value. An added 10-bit counter will give you perfect in the sense the resolution will be smaller than you can detect.

First, try the different divides. If that doesn't seem close enough then you have two options.
1) If you do source code, then I would output a speedo VSS on an unused output
2) If you don't do source, then install an additional counter.

 

couldn't you adjust the vss constant?
i changed my ppm to match the vss signal from the T56, using numbers I found through searching here.
My speedometer reads within 1-2 mph as read by one of those "your speed is " radar signs the cops put up
I am using 43885.71 PPM for 3.27 gears and it works fine.
couldn't you simply adjust the 4000PPM up or down until it comes in?

 

Jay,
I am not sure what you are trying to say. What were the grears & tire size before you changed them.

 

I kept the same gears and tire size.
3.27 and stock 245/50/16
but, i switched from the 4000 PPM VSS in the 700R4
to a T56 VSS which has substantially higher output.

All I did to get the get the ECM to read the input correctly was go into TunerPro and change the vehicle speed sensor constant from 4000 to 43885.71.
I have a dana 44 rear with 4.10s that I am putting in later, and to compensate for the gear swap, all I have to do is change that constant to 54198.
Then the ECM sees the correct VSS and outputs the right signal to the speedometer.
(i switched from the mechanical to a 90-92 electric speedometer in my car )
Couldn't you just change the 4000 to say 3950 and see how close the speedometer is then.. and make adjustments accordingly.

Maybe I am just not understanding the original question.

 

I could be wrong. I have only worked with the TPI ECMS that have the ECM read/output speed which is independent of the speedo speed.

In a TPI ECM, the only way to change the speedo speed is dividing the VSS signal by 1-8. The ECM speed is divided by a software set variable.

I still think the best method is to use a buffer box though to get the hardware part right.

 

I am using a '730. My 88 IROC that I converted to speed density.

 

Anesthes,
What ECM are you planning to run? It makes a huge difference.
I looked at some older TBI ECMs schematics and they only used an optical VSS or a magnetic with buffer box.

Jay,
Changing the software constant in a 730 ECM you described only changes the speed value read via the ALDL. In terms of the cluster/speedo gauge it has no affect. Try changing it to a 'way wrong' number and you will see the speedo reads the same but the ALDL value is 'way wrong'.

 

leaving it at 4000 PPM the speedo reads just past a roll, then once the car starts moving at all, it just tops out, then goes back and forth until highway speeds where it just reads nothing.

Initially I had it set to 48505.26 (which works for 3.42 rear gears )
which was close but not right, i then changed it to 43885.71
and it reads correctly.

It changes the speedometer reading. How else would the ECM be able to compensate for the complete wrong VSS input signal, and make the speedometer read correctly? With my previous AUJP .bin the VSS signal was 4000 ppm, and the speedometer read accurately. The T56 uses a completely different setup, 17 tooth reluctor wheel with something like 50,000 pulses per mile.

Maybe I am not talking about the same thing, and am confused, which since I am new to burning my own proms is entirely possible, but I was able to change *something* and compensate so that the ECM output to the speedometer is now correct.

 

Here is a screen shot of what I changed, with comparison from the normal AUJP to the T56 version I have modified.

http://www.ct3gparts.com/vss.jpg

 

Can't be anywhere near that high, you're probably overflowing a buffer somehow. Factory is 4 thousand. Your math is off somewhere if you came up with 48 thousand and something. Ar eyou sure the T56 is using a 50 thousand pulse value?

 

Factory for the TPI magnetic VSS is 4000 yes, and that's what I was using originally. You can also use the optical instead which is 2000 PPM.
It would seem to me that all you are doing is telling the ECM this is how many pulses per mile will be seen on the VSS input. To be more specific, if you set it to one, then for ever pulse, the ECM think the car has traveled 1 mile, with the factory 4,000 setting, then every 4,000 pulses = 1 mile. Why does it matter what the number is? It would be a basic formula of X pulses = 1 mile. As long as the X pulses corresponds to the VSS signal, then the ECM bases everything off that right?
Like the initial spark advance, that just tells the ECM what the distrubutor is set to. Factory being 6* advance. If you set the distributor to 8*, you change the expected input constant to 8* otherwise it ends up being 2* off from what the ECM expects to see.
I don;t know the exact PPM on the T56, but it's somewhere around that. LT1/LS1 don't go by PPM they use a different formula, they go by pulse per driveshaft revolution if I remember right. Also when changing rear end gears the PCM needs to be reprogramed, since there are no gears to change inside the T56 to drive the VSS.

To be honest I don't know the formula to come up with the numbers. I wish I did. I only have numbers for 3 gear sets. 3.23, 3.42 and 4.10, all using stock tire size or close. I got the numbers through searching the forums here.