4L80E Rear VSS Wiring

[dayj1]

Originally Posted by pilotXC

I'm trying to wire a 8k pulse generator to an NP208 transfer case so I have a rear VSS that will transmit a signal to the PCM. I'm having a 1-2 hard shift issue and can't seem to pinpoint what the problem is. I keep getting a 0730 fault code, incorrect gear ratio, which puts the transmission into limp mode and maxes line pressure. I get maybe 1 clean shift before it trips the fault again. So, there is either an issue where the PCM thinks there's a slip in the transmission because input shaft speed is not matching output shaft speed for what gear it is in, or the transmission is actually slipping. I had the transmission rebuilt and the fact that it shifts smooth before the fault trips leads me to believe there is a PCM communication problem, but can't pinpoint what it is as I stated. I originally wired the pulse generator to have grey wire to the purple from harness and black to the green from harness. Dakota Digital told me black should go to ground, grey to high speed sensor and low speed sensor should be bare. They said if it's not grounded it will send erratic signals. I rewired it that way and thought it would solve the problem, but seem to have not done so. The tune is adjusted for the pulse generator and I've had the tuner run multiple variations trying to tune it out, but no success. The person that rebuilt the transmission isn't sure if it's an internal problem or not without tearing the transmission apart again. I'm doing everything possible to make sure we know it's internal before I pull the transmission, but as I said, I lean towards PCM miscommunication since it does shift smooth when the fault is not there.

You probably know this, but there is a little math involved here. Get it wrong, and you get the symptoms that you describe. The sender that you linked is 8k ppm which is much lower resolution than you'd get with the stock 40 tooth reluctor wheel on a 4l80e output shaft (it'd be more along 100K ppm depending on exact gear ratio and tire size).

The factory ECM is going to want the number of pulses per driveshaft revolution. 20168 divided by your tire height (in inches) and then multiplied by your gear ratio (i.e. 3.08, 3.73, 4.11, etc) will give you the number of driveshaft revolutions in a mile for your vehicle.

Since you're generating 8k pulses with your aftermarket sensor in that same one mile distance, divide 8000 by the number of driveshaft revolutions you calculated to get pulses per revolution. It's going to be a lot less than the factory 40 in the tune (probably in the single digits).

That'll be the number you need in your tune and that's all assuming your speedo gears in the transfer case are correct for your gear ratio/tire size. If not, you'll need to adjust the calculations to account for that, too.

[pilotXC]

Originally Posted by dayj1

You probably know this, but there is a little math involved here. Get it wrong, and you get the symptoms that you describe. The sender that you linked is 8k ppm which is much lower resolution than you'd get with the stock 40 tooth reluctor wheel on a 4l80e output shaft (it'd be more along 100K ppm depending on exact gear ratio and tire size).

The factory ECM is going to want the number of pulses per driveshaft revolution. 20168 divided by your tire height (in inches) and then multiplied by your gear ratio (i.e. 3.08, 3.73, 4.11, etc) will give you the number of driveshaft revolutions in a mile for your vehicle.

Since you're generating 8k pulses with your aftermarket sensor in that same one mile distance, divide 8000 by the number of driveshaft revolutions you calculated to get pulses per revolution. It's going to be a lot less than the factory 40 in the tune (probably in the single digits).

That'll be the number you need in your tune and that's all assuming your speedo gears in the transfer case are correct for your gear ratio/tire size. If not, you'll need to adjust the calculations to account for that, too.

Thank you for the detailed post. The person who modified my harness has done a few of these 8k pulse generators and tuned it accordingly he has told me. My tires are 285/75/16, so 33" in diameter and gear ratio is 4.11. Doing the math, that gives me 3.184925 pulses per revolution...