I am runnign a T56 transmission with a 730 ECM. My question is what do I have to do to make the VSS work with that computer? what is the road speed constant that the T-56 is putting out? Can I just set teh constant higher in the PROM to compensate for it? thanks for any help

 

You'll need a VSS translator box. I'm running a '730 with a T56 and use the dakota digital box (www.dakotadigital.com) part SGI-5 with no problems.

The T56 VSS outputs 40K pulses, the 730 wants 4000 pulses with stock software and can be changed to 2000 pulses with GMEPro or TunerCat.

 

The T56 VSS outputs 40K pulses,

The T56 input is dependant upon gear ratio and tire height. There is no standard ratio.

the 730 wants 4000 pulses with stock software and can be changed to 2000 pulses with GMEPro or TunerCat.

You can change it to anything you want with a hex editor.

You can avoid buying the box by doing the following.

1. Figure out what rpm your car runs at in fourth gear (1:1) at 60 MPH. This is rev per min divided by 1 mile per min equals rev per mile.
2. Figure out how many teeth are on the reluctor in the trans. This is pulse per rev.

For example (3000 rpm / 1 mile per min )* 13 pulse per rev = 39000 pulse per mile.

3. BYTE $8015 in your prom should be changed to equal 921600/ pulse per mile

For example 921600/39000 = 23.63 or 24 with is $18 in hex.

Once you have done this your speedometer should work with no box.

HTH

John

 

Location $8015 is defined in TunerCat as "Road Constant" and yes you can adjust it to 40,000 Pulse/Mile (or even higher) and it will correct...but ONLY for the ECM, not your electronci speedometer if you have one.

There was a write-up by someone that found that altering the "Most Significant Bits" of $8014 will affect the ECM's output to the Instrument Panel/Speedo. It is too "coarse" to use for a speedo correction for a simple tire change or gear change. But it is supposedly "just right" for a change to a T56.

There are a couple of posts on this Board already, but you will need to find them. Do a search on "speedo" or "8014", and it should find them for you.

 

Well I don't have a factory electric speedo. I am using an Autometer. All I am worried about is the ECM. I can run the autometer off the tranny itself right?

 

T56 VSS has no input, only output.

T56 outputs 40K pulses per mile, how many rotations of the output shaft it takes to make one mile varies by gearing and tire diameter. T56 can not be adjusted to vary its output...there is exactly one output freq as stated above. You adjust PCM software to calculate MPH correctly for varying gear ratios.

As Glenn pointed out what you posted here was not quite correct either.

I posted exactly what I had done successfully, so as not to give the guy bad info.

 

T56 VSS has no input, only output.

Since we are talking about ECM's VSS is an input.

T56 outputs 40K pulses per mile, how many rotations of the output shaft it takes to make one mile varies by gearing and tire diameter. T56 can not be adjusted to vary its output...there is exactly one output freq as stated above.

There cannot be one frequency if rotation of the output shaft vary with gears and tire height. Think about it.

As Glenn pointed out what you posted here was not quite correct either.

I will admit that I was incorrect in my recollection of the control of the speedo output. It been a day or two since I did this and I should have review my notes more carefully before posting. I will try to do so in the future.

I posted exactly what I had done successfully,

I too am posting about a similar success.

so as not to give the guy bad info.

The info I posted was relevant and correct with the exception of the speedo output.

John

 

Well I don't have a factory electric speedo. I am using an Autometer. All I am worried about is the ECM. I can run the autometer off the tranny itself right?

Is it mechanical or electronic?

John

 

Come on guys, we ALL make mistakes or slightly confuse facts at one time or another. I like to think we are a little more politer to each other on this Board than some are on the other Boards.

 

Come on guys, we ALL make mistakes or slightly confuse facts at one time or another. I like to think we are a little more politer to each other on this Board than some are on the other Boards.

I agree. It was not my intent to start a flame war only to provide info. In my haste I misread the question and posted something I "knew" I remembered only to find I was incorrect. When it was pointed out I went back and found the answer that I should have posted to begin with.

Hopefully I will learn from this.

John

 

I have a Autometer electric speedo it should work if I connect it right into the tranny right?