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VSS Issue???

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Speedo no shift issue

I apologize if I am in the wrong spot or asking something not allowed. It is not specifically tuning but just looking for an answer to a problem i am having. Having an issue with my drag car. Issue does not occur every pass but more often than not. When i roll into the burnout box all is fine. I do my burnout, speedo working fine at this point, and roll out of the water. As i slowly roll forward to stage, the speedo quits working and reads 0 or nearly 0. Upon launch I have apparently lost vss signal and the trans will not shift. Sometimes I do not lose vss signal until I'm on the 2 step. Same result. After I lift a few seconds later the signal comes back, speedo works car shifts fine. I have changed vss and swapped out pigtail. No help. I have separated vss signal lead so it is nowhere near any other wires so as to avoid any interference. No help. I have also found that if it does work and shift I show 1-3 mph on my log while on transbrake and on the 2 step. I have found a completely unacceptable workaround and that is if I unplug my vss plug from the sensor and then re plug it back in between rounds, It works flawlessly. E38 ecu, t42 tcm 416ci ls3 with an RPM trans 4L60E with Rossler trans brake. PSI custom harness. Due to stupid covid shit I only got out 1 time this year but all worked perfect in 2019. Possible TCM issue, ECU issue, still some kind of feedback issue, mech issue or ??? Any thoughts would be appreciated. Log and tune attached

Attached Files
  • 080620-bir-tnt-1st-pass-didnt-shift-forgot-nos-activation.hpl
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  • 080620-bir-tnt-4th-pass-launched-good-didnt-shift.hpl
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

  • 2020-CURRENTLY-INSTALLEDadjust-for-29.5-slicks.hpt
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Sounds like the sensor wire may have a short or the sensor itself is getting weak(if magnatized).

my $0.02


Thanks for the reply. Will not discount either of your thoughts. Adding a bit more info. So I made 5 passes in day 1. Pass 1 and 2 it lost signal so those runs it didnt shift. For the 3rd pass i replaced vss and pigtail and obviously removed harness wires. Worked perfect. Pass no 4 failed. Figured out the "cure". Pass 5 was perfect. Made 5 more passes the next day each time removing sensor wires and had no issues. Still wondering about feedback. Possibly a ground. I do not have a ground specifically to the trans. Have plenty to engine, body and frame.

any one else have any thoughts or have dealt with a similar issue

You should test the signal of the sensor to the TCM or ECU (where it is plugged in) with an oscilloscope. If the signal is not good, retry at the sensor to eliminate the wiring / connection. If everything checks ok, could be a faulty TCM or ECU, which is unlikely.

Thank you for the comments. I will be doing just what you have suggested

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