vss help

[85burb]

I am having a hard time with my vss. I cant get it to read much speed. It really does not start reading tell 10-15 miles a hour. I have a dakota digital 8k pulse generator on my speedo cable port on the transfer case. I told the ecm guy that is what i have. He said he changed it in the ecm from 4k to 8k. So i figure even if he did not change it it would read to fast not to slow. At 60 mph my ecm reads 3 mph. Any ideas?

[Dustytrix]

Double check your wiring, at vss I had that problem.

[85burb]

I have looked at my wires. The vss has 2 wires and the pulse generator has 2. tryied it both ways and no change.

[68GMCCustom]

if the ecm is looking for an 8k signal but only gets a 4k wouldn't it read slow? and a 2k signal would read even slower?

[dayj1]

Originally Posted by 85burb

I am having a hard time with my vss. I cant get it to read much speed. It really does not start reading tell 10-15 miles a hour. I have a dakota digital 8k pulse generator on my speedo cable port on the transfer case. I told the ecm guy that is what i have. He said he changed it in the ecm from 4k to 8k. So i figure even if he did not change it it would read to fast not to slow. At 60 mph my ecm reads 3 mph. Any ideas?

4K pulses per mile is the VSS output for a modern GM speedometer. 8K is the value commonly used by Chrysler and Ford speedometers. You're probably not using the VSS out anyway since your speedometer is mechanical. In other words, if your tuner changed the output from 4k to 8k it didn't do anything for you.

What you're really interested in is the VSS input. Specifically, what you want to change in the ECM is the number of pulses per driveshaft revolution. I'm assuming that you have a manual transmission tune in your ECM, so the default value is 17. I can't tell you what to change it to without knowing your tire size and rear gear ratio, but here's an example for 4.10 gears and 30 inch tires:

1) You need your tire circumference. That's just the tire height times 3.14159. For 30 inch tires, that would be 30 x 3.14159 = 94.2477 inches.

2) Figure out how many times your tire turns in one mile. There are 63360 inches in a mile, so divide that by the circumference from step 1. For 30 inch tires, that would be 63360 / 94.2477 = 672 tire revolutions per mile.

3) Now we need to know how many times your driveshaft spins in a mile. That is just your rear gear ratio times the number of tire revolutions from step 2. For 4.10 gears and 30 inch tires, that would be 4.10 x 672 = 2755 driveshaft revolutions per mile.

4) Last, we can finally figure out how many pulses per driveshaft revolution that your setup is producing. From step 3, we know that for 4.10 gears and 30 inch tires, the driveshaft is spinning 2755 times per mile. We also know that the Dakota digital box puts out 8000 pulses per mile. So, 8000 / 2755 = 2.9 pulses per driveshaft revolution.

The 2.9 pulses per driveshaft revolution from the calculations above is the number that needs to replace the default 17 in the case of 4.11 gears and 30 inch tires. If you didn't change it, the ECM would be seeing about 17% of actual speed. In other words, the ECM would think you were going 10.2 MPH when you're really going 60 MPH.

I hope that helps. Just post your gear ratio and tire height if you need help with the math for your actual setup.

[85burb]

4.10 gears and 32.4 tall tires. Thanks.
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[dayj1]

Originally Posted by 85burb

4.10 gears and 32.4 tall tires. Thanks.
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That would work out to 3.13 pulses per driveshaft revolution in the ECM. Also, make sure that your tire height/gear ratio is correct in the ECM. Good luck.

[85burb]

Wow. Thanks
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[68GMCCustom]

very good info dayj1!

[85burb]

Well I got a hold of my harness guy and he is sending me a new ECM. The one he sent me with my harness has a 4l60 tune in it somehow. He is sending one with a manual tune. I told him about the pulse per revolution thing and I hope he fixed it. Hopefully my truck stops stalling every time I move around slow. I guess we will see. Thanks for everyone's help. I will get it dyno tuned in a few weeks so maybe they can make it exactly on the speed.
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[68GMCCustom]

there are places in the tune for idle that is tied to speed not just rpm. I wouldn't be surprised if this ecm swap (if it corrects your drivetrain issue) cures, or at least improves upon... your problem.

[dayj1]

Originally Posted by 85burb

Well I got a hold of my harness guy and he is sending me a new ECM. The one he sent me with my harness has a 4l60 tune in it somehow. He is sending one with a manual tune. I told him about the pulse per revolution thing and I hope he fixed it. Hopefully my truck stops stalling every time I move around slow. I guess we will see. Thanks for everyone's help. I will get it dyno tuned in a few weeks so maybe they can make it exactly on the speed.
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Maybe the ECM swap will help you. The default pulses per driveshaft revolution for an automatic tune is 40. So that would make the vss problems even worse.

[85burb]

That is what we are thinking. I have found a place to have it tuned so if not I am sure the tuner can figure it out.
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[85burb]

ECM got here today. Can't wait to test it out!
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[85burb]

The vs is exactly the same. I wish I knew of a tuner in Tyler that could look the tune over. I think my trans is letting go so I don't really like the idea of driving 45 miles to have a dyno tune done. All I want fixed is the stalling and idle speed.
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[68GMCCustom]

thats one of the nice thing about having hptuners...I can look at things and try to mess them up.

[85burb]

I would love to have a program to mess with things but all I am sure I would do is make it worse. I for sure would not mess with any fueling or timing tables.
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[68GMCCustom]

nice thing about it......you can save the file with diff names and just go back and load up the previous or original one if...well, things go bad.

and yes........I'm afraid of those tables too. I'll change the rpm and speed limiters, shift points, a/f ratio, things like that... and have considered getting into torque management and dialing it back a hair. There are LOTS of tables that are greek to me.

in reality...........I have it so I can log runs to send to my tuner so he can review/modify and send me a new tune to load up.

[85burb]

I was looking at those programs and they are not as pricy as it thought. I can get hp tuner for like 500 bucks. Really thinking about it.
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