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View Full Version : 4L80e input speed sensor siginal erattic when in second gear


ttls1
March 11th, 2026, 10:14 PM
Hi, I have a weird issue with my gearbox pressure which I think could also be causing the input speed sensor signal to go erratic or vice versa. I've replaced the input sensor and plug, checked the wiring and even tried a different PCM and its still does the exact same thing. The second gear change is brutal and when I log the pressure it appears to be going to full line pressure and then the speed signal starts going crazy. For the most part the signal is smooth as are the other gear shifts. Would anyone have any ideas on what I could check? My line pressures for 1-2 don't seem to far from stock. I've attached a screen shot of a section of the log and both the tune and log files. Any info would be much appreciated.

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ChipsByAl
March 12th, 2026, 03:05 AM
I would first check the proximity of the input speed sensor wiring to any EMI noise. Another possibility could be the input housing/sensor clearance causing the erratic issue. If the housing does not spin concentric in the transmission because of wear or balance problems.
Al
ttls1
March 12th, 2026, 09:53 AM
I was thinking the same however i can't think of anything that would cause EMI. The housing is tight and the old sensor had no signs of damage so the clearance should be still factory. Its definitely a weird one. I could try an amplifier and see if the signal cleans up.
ttls1
March 13th, 2026, 08:51 AM
All the amplifiers i could find online convert the vss sine wave to a square wave. I simulated a square wave at different voltage amplitudes and the PCM didnt register anything so it must require a sine wave. Ill run a dedicated shielded cable to the iss and see if that helps.
ChipsByAl
March 13th, 2026, 08:53 AM
Twisting the two wires from the sensor to the ECM will help too.
Al
ttls1
April 15th, 2026, 03:00 PM
Ok I have now finally got it sorted. What I had to do was run a separate shielded cable for the ISS. Not sure why the factory wiring was giving intermittent signal issues so ill just put it down to EMI noise as ChipsByAl mentioned. I did also bond the cable screen to chassis earth however I'm not sure if this was necessary. Anyway its now behaving perfectly and shifting a treat. Cheers.
ChipsByAl
April 15th, 2026, 03:06 PM
Glad you sorted it out.
Al